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The great antimatter mystery
http://www.newscientist.com/article.ns?id=mg19826511.500&print=true
[Related material appended.]

11 April 2008
Helen Quinn
Yossi Nir

IT IS lucky for us that the infant universe did not behave the way
our best cosmic theories would have it. Nearly 14 billion years
ago, the big bang forged equal amounts of matter and its nemesis,
antimatter. These should have annihilated each other in bursts of
pure radiation, leaving a universe filled with light. Instead,
though, it is full of stars and planets and gas - something threw a
cosmic spanner in the works.

The stars and galaxies that light up the heavens would not exist
today if matter had not won out over antimatter at some very early
time in the evolution of the universe. How and when did this
happen? Why is there something rather than nothing? These questions
are at the root of our very existence, but as yet science has no
clear answers.

That's not to say we haven't made progress. As in any good
detective story physicists have picked up important clues, mainly
by creating antimatter and studying what it does. Other evidence
comes from neutrinos, those ghostly particles created in
radioactive decays. These clues have provided two very promising
lines of inquiry and thrown up some controversial results along the
way. With the advent of new experiments there is a chance we will
have answers very soon.

To create the universe we see today, a preference for matter must
have arisen in the early universe. It only needed a minute
imbalance, with as few as one extra particle of matter surviving
for every 30 million antimatter particles.

That couldn't occur by chance, though. Even this tiny excess is too
big to occur as a random fluctuation in the hot, early universe.
Nor is the universe likely to have started out with such a finely
tuned imbalance (see "Was the universe born lopsided?"). What's
more, it definitely doesn't seem to be hiding pockets of antimatter
today (see "Where are all the anti-galaxies?"). So how the excess
arose during the history of the universe must be encoded somewhere
in the basic laws of physics.

Russian physicist Andrei Sakharov was the first to take on this
puzzle in 1967. He showed that for there to be more matter than
antimatter, three conditions are needed. First, Sakharov argued
that no conservation law can forbid reactions which effectively
change the balance between particles and antiparticles. This was a
bold claim, as such reactions have never been seen experimentally.

To make this possible, Sakharov pointed out that the laws of
physics must be slightly different for matter and antimatter, as
had been revealed in experiments three years earlier with particles
called long-lived kaons. These showed that the weak force, which is
best known for its role in radioactive decay, does not act equally
on quarks and antiquarks.

Finally, there must have been a period early in the universe's
history when the various reactions going on between the different
particles and antiparticles and radiation in the primordial plasma
started to take place at different rates. This can only happen if
they are, for some reason, not in thermal equilibrium. Without
these conditions the universe would never have evolved from its
initial state of having equal amounts of matter and antimatter to
its present highly unbalanced state.

Fast forward to today and Sakharov's conditions remain as relevant
as ever. In the intervening years, they have acted as an important
guide for our theories of the early universe.

The standard models of cosmology and particle physics suggest that
when the universe was less than 10^-12 seconds old, particles and
their interactions were very different from what they are today.
All the fundamental particles were massless and the weak
interactions between them were more active. As the universe
expanded and cooled, it switched to a more favourable, lower-energy
state. Here the particles gained mass and the weak interactions
became less active.

This cooler state started off as a tiny bubble that expanded
rapidly throughout the early universe. As it did so, the bubble's
surface upset the thermal equilibrium of the universe and
interacted with the massless particles and antiparticles. Some of
them passed through and ended up inside the bubble, while others
bounced off.

Interactions at the bubble wall made it more likely for a quark to
break through the bubble wall than an antiquark, so inside the
bubble there was an excess of quarks, while the antiquarks outside
were removed by the more active processes. Today, the bubble is the
size of the universe, and because we live inside it we see the
excess of quarks as a dominance of matter over antimatter.

It's a lovely, neat picture. The only problem is, it doesn't give
the right numbers. When we use the standard model to calculate the
amount of matter and antimatter, we get far too small an excess.
This is one of the reasons why particle physicists think the
standard model is incomplete. Is there a way to fix it?

Perhaps. One of the most promising extensions of the standard model
is supersymmetry, which demands many as-yet-unknown particles
beyond the reach of existing experiments. As well as explaining the
antimatter imbalance, supersymmetry might tell us about the nature
of the dark matter that accounts for 90 per cent of the matter in
the universe, and why gravity is so puny compared with the other
forces.

While theorists embrace supersymmetry, so far we have found no
evidence for it in experiments. However, hints of a process that
does not fit the standard picture recently came to light. Last
month, a team of physicists in Italy, France and Switzerland known
as the UTfit collaboration analysed particles called B[s] mesons,
created in two experiments at the Tevatron accelerator at Fermilab
in Batavia, Illinois. Made of a "bottom" antiquark and a "strange"
quark, B[s] mesons are unstable and decay via the weak force into
particles made of lighter quarks and antiquarks.

The UTfit collaboration argues that when they combine all the B[s]
meson results, they find a small discrepancy that could be evidence
for a new interaction outside the standard model that acts
differently on quarks and antiquarks, and might possibly be a
reason for the excess of quarks in our universe (New Scientist, 18
March, p 10).

It is far too early to say whether this a first hint of
supersymmetry. More observations are needed to confirm the UTfit
group has indeed found something amiss, and we will still need to
discover some supersymmetric particles to provide proof. They might
turn up at the Large Hadron Collider (LHC), the world's most
powerful accelerator, due to switch on later this year at the CERN
laboratory near Geneva, Switzerland. Assuming supersymmetric
particles are detected there, we will be able to measure their
masses and some of their interactions, but even that won't be
enough. Additional experiments will be needed to tell if
supersymmetry generated the right excess of matter when the
universe was about 10^-11 seconds old.

Other planned experiments to study supersymmetry in detail include
the International Linear Collider, which will smash electrons and
positrons together (New Scientist, 25 August 2006, p 36) and an
experiment to study the electromagnetic properties of the neutron.

Neutrinos to the rescue

An alternative way to explain the mystery of the missing antimatter
emerged in the mid-1980s. Japanese physicists Masataka Fukugita and
Tsutomu Yanagida showed how the matter-antimatter imbalance might
have arisen in a scenario known as leptogenesis. If this idea is
correct, we owe our existence to neutrinos.

Neutrinos are the most elusive of all particles in the standard
model, and were long thought to be massless. However, a series of
beautiful experiments carried out over the past 40 years in the US,
Japan, Canada and elsewhere have established that the standard
model is wrong: neutrinos do have mass, albeit a very tiny one.

This means they could have played a role in the antimatter
imbalance. Adding neutrino masses into the theoretical picture
means adjusting the standard model, and the simplest way to do this
is to assume the existence of a new type of particle, a kind of
very heavy neutrino called a singlet neutrino. These neutrinos are
unlike any other particle we know because they do not interact with
other particles via the usual forces in nature, so they are
probably extremely difficult to detect. Like all fundamental
particles, they would have been produced in appreciable quantities
in the very early universe. But their interactions would have been
too feeble to keep them in thermal equilibrium with the rest of the
primordial plasma, in keeping with one of Sakharov's three
conditions.

According to the leptogenesis scenario, singlet neutrinos travel
freely across the universe until they decay into either neutrinos
or antineutrinos. Crucially, according to the theory more
antineutrinos can be produced than neutrinos, once again in line
with Sakharov's ideas.

Leptogenesis therefore leaves the very early universe with an
excess of antineutrinos. At this stage, the standard model predicts
that certain reactions could occur in the very high-temperature
conditions to convert antineutrinos into matter particles,
eventually producing protons and neutrons and leaving the universe
devoid of antimatter.

Testing leptogenesis will be tricky, as there is unlikely to be a
way to produce singlet neutrinos in the lab and measure their
decays. They are likely to be much too heavy and their interactions
are dramatically too feeble for us to be able to do that. However,
there are ways to test whether the idea is at least possible.

Leptogenesis predicts that the singlet neutrinos can interact with
normal neutrinos by swapping Higgs particles - the particles that
are thought to give mass to all matter and antimatter particles.
>   From what we know about normal neutrinos and the Higgs particle, we
can make inferences about singlet neutrinos. So far, their features
appear to match what is needed for leptogenesis, and this provides
some circumstantial evidence in support of the idea.

Another test concerns a property called the "lepton number".
Electrons and neutrinos belong to the family of particles called
leptons and are assigned a lepton number of 1. Their antimatter
counterparts have a value of -1. In all the reactions we have
measured so far, the lepton number before and after the reaction
has stayed the same.

However, the leptogenesis theory predicts that adding singlet
neutrinos to the mix makes it possible for regular neutrinos to
change into antineutrinos and vice versa. So it fails to conserve
lepton number. Particle physicists regularly check their
experiments for signs of lepton number violation because this would
directly prove Sakharov's first condition, that there is no
conservation law in nature protecting the matter-antimatter
balance.

So far, only one group claims to have seen a reaction that violates
lepton conservation. Hans Klapdor-Kleingrothaus at the Max Planck
Institute for Nuclear Physics in Heidelberg, Germany, says that his
group first saw lepton violation in 2001, in germanium-76 nuclei
(New Scientist, 4 September 2004, p 37).

They claim to have observed a reaction called neutrinoless double
beta decay. In normal beta decay, a neutron inside a nucleus
spontaneously transforms into a proton, producing an electron and
an antineutrino in the process. The lepton number is 0 before and
afterwards. A few rare radioactive elements go one better and
undergo double beta decay, where two neutrons inside the same
nucleus change at the same time, spitting out two electrons and two
antineutrinos. In the neutrinoless version of double beta decay,
there are no antineutrinos, only two electrons. Here the lepton
number changes from 0 before the reaction to 2 afterwards.

The Heidelberg group's findings are controversial, though, and
while several teams of physicists are attempting to replicate the
experiment, none has yet succeeded. Still, many physicists are
convinced that leptogenesis is the prime suspect for solving the
antimatter puzzle, and so the search for lepton number violation
goes on.

For now the mystery has at least two possible answers. It is down
to experiments to choose between them, or even eliminate both and
send theorists back to the drawing board. If supersymmetry provides
the answer we will eventually know it. But if leptogenesis is the
right answer, then it is likely to remain forever a plausible yet
unproven aspect of cosmology. Like it or not, the universe may
never reveal all its secrets.

Cosmology - Keep up with the latest ideas in our special report.

Quantum World - Learn more about a weird world in our comprehensive
special report.

Was the cosmos born lopsided?

How do we know that the universe did not just start out with an
imbalance of matter and antimatter? The more we understand the
early history of the universe the less it seems that this is
possible. First, if the asymmetry between matter and antimatter had
been an initial condition, it would have been a very peculiar one.
By studying the amounts of light elements forged in the very early
universe, we can work out that there must have been 30,000,001
matter particles for every 30,000,000 antimatter particles. It
seems very unlikely that such a fine-tuned situation appeared
accidentally.

Even if there had been an initial imbalance, it would have been
erased in a period of rapid expansion called inflation that diluted
the initial densities to minuscule proportions. There is growing
evidence from the cosmic microwave background that inflation did
indeed take place. It is almost certain that the excess of matter
was generated after inflation.

Where are all the anti-galaxies?

We have good reasons to think that all the structures in the
observable universe are made from matter and not antimatter. For a
start, space probes have touched down on the moon and other planets
without annihilating in a burst of radiation. Intergalactic space
is flooded by particles blasted out by active galaxies; any
antimatter galaxies around would radiate antiparticles which would
annihilate on meeting these particles, creating glowing surfaces
with characteristic energies. We don't see any of these as far out
as we can possibly observe.

It is also extremely unlikely that the hot, dense mixture in the
early universe sorted itself out into a few equally huge regions of
matter and antimatter. You would have to have very different laws
of physics for matter and antimatter to unmix like this. We know
the laws are in fact very similar, so we conclude that it is matter
which makes up the galaxies in our entire universe.

Related Articles

Flipping particle could explain missing
http://www.newscientist.com/article.ns?id=mg19726483.600
18 March 2008
Upcoming colliders: physics on the edge
http://www.newscientist.com/article.ns?id=mg19125661.300
25 August 2006
Sterile neutrinos: the cosmic controllers
http://www.newscientist.com/article.ns?id=mg19025561.800
15 June 2006
Trouble with antimatter
http://www.newscientist.com/article.ns?id=mg18624936.300
2 April 2005

Weblinks

Antimatter: mirror of the universe, CERN
http://livefromcern.web.cern.ch/livefromcern/antimatter/
The particle adventure
http://particleadventure.org/
The Large Hadron Collider
http://public.web.cern.ch/public/en/LHC/LHC-en.html
The Ultimate neutrino page
http://cupp.oulu.fi/neutrino/


Flipping particle could explain missing antimatter
http://www.newscientist.com/article.ns?id=mg19726483.600&print=true

18 March 2008
Valerie Jamieson

It is one the biggest mysteries in physics - where did all the
antimatter go? Now a team of physicists claims to have found the
first ever hint of an answer in experimental data. The findings
could signal a major crack in the standard model, the theoretical
edifice that describes nature's fundamental particles and forces.

In its early days, the cosmos was a cauldron of radiation and equal
amounts of matter and antimatter. As it cooled, all the antimatter
annihilated in collisions with matter - but for some reason the
proportions ended up lopsided, leaving some of the matter intact.

Physicists think the explanation for this lies with the weak
nuclear force, which differs from the other fundamental forces in
that it does not act equally on matter and antimatter. This
asymmetry, called CP violation, could have allowed the matter to
survive to form the elements, stars and galaxies we see today.

The standard model, our best effort to describe the universe's
structure, fails to fully explain CP violation. Many alternative
theories claim to have the answer, such as those incorporating
supersymmetry, extra dimensions and hitherto unseen forces.
However, they often invoke new particles, and experiments have yet
to turn up evidence of these.

Particle physicists have long thought that they might find such
evidence in a particle called the B[s] meson, which comprises a
bottom antiquark bound to a strange quark. The B[s] is one of a
handful of mesons that transforms into its own antiparticle and
back again 3 trillion times per second before decaying into other
particles (see Diagram). These oscillations between matter and
antimatter make it a good place to look for evidence that CP
violation goes beyond the standard model.

At the Tevatron particle accelerator at Fermilab in Batavia,
Illinois, two groups of scientists running the rival CDF and D-Zero
experiments have been studying several properties of B[s] mesons
and their oscillations by picking through the debris created when
protons and antiprotons collide. While each experiment on its own
has found faint hints of CP violation above and beyond the standard
model, the experimental uncertainties have been too large to make a
definitive claim, says Giovanni Punzi, a physicist at the
University of Pisa in Italy and one of the leaders of the B meson
physics group at CDF.

Now Luca Silvestrini at Italy's National Institute of Nuclear
Physics (INFN) in Rome and colleagues in Italy, France and
Switzerland have managed to reduce these uncertainties. By
combining the published results of the CDF and D-Zero teams, they
have shown there seems to be much more CP violation than the
standard model permits. "We can say with greater than 99.7 per cent
probability that CP violation is there," says Silvestrini
(www.arxiv.org/abs/08030659). In other words, new physics is at
work in the oscillations. His group cannot yet say what kind of new
physics - that will require others to test whether existing
theories explain the data.

"It is tantalisingly interesting at the moment," says Val Gibson,
an expert on B meson physics at the University of Cambridge. "If it
is true, it is earth-shattering."

Jacobo Konigsberg, who leads the CDF collaboration, says that
Tevatron researchers are "cautiously excited" about the analysis.
He points out that more data needs to be analysed to rule out a
statistical fluke, which has happened several times before in
particle physics.

The real proof could come later this year when the Large Hadron
Collider switches on at CERN, near Geneva, Switzerland. The LHC-b
experiment has been designed specifically to study mesons
containing bottom quarks. "LHC-b will make an unambiguous
measurement within two months," says Gibson.

Related Articles

Higgs boson: glimpses of the God particle
http://www.newscientist.com/article.ns?id=mg19325934.600
2 March 2007
The lopsided universe
http://www.newscientist.com/article.ns?id=mg16121724.800
6 February 1999
The hunt for the Un-universe
http://www.newscientist.com/article.ns?id=mg19726401.400
25 January 2008

Weblinks

Silvestrini and colleague's paper
http://arxiv.org/abs/0803.0659
D-Zero
http://www-d0.fnal.gov/public/index.html
CDF
http://www-cdf.fnal.gov/public/index.html


City songbirds are changing their tune
http://www.newscientist.com/article.ns?id=mg19726491.400&print=true

28 March 2008
Ed Yong

DAYBREAK in the city. The brief quiet of the night gives way to the
low rumble of cars, trucks and industry, but one sound is notable
by its absence. Gone is the familiar dawn chorus, with its rich mix
of enchanting melodies and calls. In its place is a strangely
depleted music - abrupt, high-pitched and sometimes ear-piercing.
Welcome to the urban soundscape of the future.

This is no dystopian vision. It is the prediction of scientists who
have been studying the way in which noise pollution affects urban
bird life. The growing clamour of cities and roads may be annoying
to us, but for many birds it can mean the difference between life
and death. Background noise can mask both the sounds of approaching
predators and the alarm calls that warn of danger. They can also
rob individuals of reproductive success by drowning out the songs
that male birds use to attract mates and demarcate their territory.

The impact of this noise is now becoming clear. Some species simply
are not able to make themselves heard above the ever-growing racket
and are finding themselves squeezed out of the city. Others are
beginning to change the way they communicate. In the long term, new
species may evolve. If noise levels continue to rise, it seems
inevitable that urban bird life will change dramatically.

You can already hear the changes, if you know what to listen out
for. One giveaway is birds unexpectedly singing outside their
traditional peak times of morning and evening. At these prime times
of day, wind noise and turbulence are at their lowest, so sound
carries further - but not if you factor in the impact of rush-hour
traffic. Richard Fuller of the University of Sheffield in the UK
has found that some local robins have abandoned the traditional
dawn chorus and taken to singing at night to avoid the daytime din
altogether. This shift had previously been attributed to the
confusing effects of light pollution, but Fuller's analysis reveals
that daytime noise has a much stronger effect: the parts of
Sheffield with nocturnal singers were an order of magnitude noisier
by day than other areas of the city (Biology Letters, vol 22, p
368).

It remains to be seen whether singing at night is a successful way
to combat noise pollution. It is not the only option, however.
Nightingales, when not singing at night, have opted for an approach
that seems at odds with their delicate melodies - they simply shout
louder. When Henrik Brumm at the University of St Andrews in the UK
recorded nightingales singing between 5 am and 10 am he found that
those in Berlin sang up to 14 decibels louder than their forest
counterparts, achieving volumes of up to 95 decibels - enough to
send humans reaching for ear protection. The loudness of their
vocals was proportional to the level of background noise, with
birds singing particularly loudly on weekday mornings (Journal of
Animal Ecology, vol 73, p 434).

Hitting the high notes

Changes in the timing or volume of songs are fairly obvious
solutions to the problem, but some songbirds have taken a more
subtle approach. Urban noise is particularly loud at low
frequencies - between about 1 and 3 kilohertz. By avoiding these
low notes, birds can make their songs more audible. Blackbirds,
song sparrows and house finches have all adapted in this way, but
the most well-studied practitioner is the great tit.

For the past five years, Hans Slabbekoorn of Leiden University in
the Netherlands has analysed the ways in which great tits deal with
noisy cities. He found that those inhabitating noisier parts of
Leiden sing melodies with higher minimum frequencies than those in
quieter areas of the city. When he looked at populations of great
tits in 10 European cities, including London, Paris and Amsterdam,
he found that every one of them sang higher-pitched tunes than
their forest-dwelling counterparts, raising the minimum frequency
by 200 hertz on average, to around 3500 hertz. Not only do urban
great tits sing in a higher key, but they also eschew the standard
riffs of their forest peers for more original ones (Current
Biology, vol 16, p 2326).

The ability to change one's tune is a valuable asset in the growing
urban hubbub. Unlike some birds that learn their entire repertoire
while in the nest, great tits, song sparrows and others regularly
modify their songs throughout their lifetime. They have far more
tunes than they require and select different songs depending on the
context. By monitoring which songs work best in a particular
situation, individuals can learn from experience and adapt to local
changes. Successful new singing strategies can spread as young
birds learn to sing by imitating the performances of their seasoned
neighbours. Alternatively, songs may become better adapted by
default: if younger birds cannot hear the low-frequency segments of
their tutors' songs, they may never learn tunes containing these
lower notes, which could then drop out of local repertoires
altogether.

"This plasticity provides adaptive value in natural conditions,"
says Slabbekoorn. Forests can vary greatly in how loud they are and
those birds that live near noisy areas like waterfalls and river
torrents also sing at higher frequencies, in a similar way to
urbanites. By chance, their flexible singing has put them in a good
position to cope with the artificially noisy conditions caused by
humans.

Behavioural flexibility is what distinguishes species coping with
noise pollution from those that are struggling. The relatively
recent rise of urban noise means that most of the vocal strategies
used by city birds are likely to be learned responses rather than
the result of evolution. In the long term, however, genetic changes
are likely to occur because of the role that songs play in survival
and reproduction.

Songs are primarily sexual traits that influence the mates females
choose and so the success of males. If females come to see the
ability to avoid acoustic masking as an indicator of mate quality,
they will prefer to mate with males that can do this and the trait
will be boosted by sexual selection. In addition, individuals whose
hearing is attuned to picking out the songs of other birds amid
urban noise are also at a selective advantage, which will
ultimately increase their proportion of the population.

If singing and hearing diverge enough, urban birds may be less
likely to find the vocals of rural birds attractive, or even to
recognise them as members of the same species. These changes could
serve to eventually split populations into genetically distinct
urban and rural species. Alternatively, different populations of
the same species might adopt differing strategies to cope with
urban noise, leading eventually to a species split occurring in
birds living in the same neighbourhood. "It would be absolutely
fascinating to see this kind of sympatric speciation," says Fuller.

This is not mere speculation. Some scientists believe that the
European blackbird has already diverged into separate urban and
rural subspecies, with different body shapes and life histories.
Slabbekoorn and Erwin Ripmeester, also at Leiden University, are
now investigating how urban noise is driving this separation. They
are using playback recordings to see if urban blackbirds respond
more strongly to urban songs than to rural ones. They have also
taken blood samples from urban and rural blackbirds and plan to
check for genetic divergence. "It's certain that we are seeing
parts of the process of speciation taking place," says Slabbekoorn,
"but we might not be there to see the end result."

While blackbirds, great tits and others are apparently taking
advantage of their adaptable songs, not all birds are flexible
enough to cope with the urban clamour. Amid the loud and
low-pitched noise, the biggest losers are those that rely heavily
or exclusively on low-frequency calls and are physically incapable
of switching to higher frequencies. Orioles, cuckoos, great reed
warblers and even the familiar house sparrow all fall into this
category. House sparrows were once frequent visitors to the UK's
parks and gardens, but populations are now falling, as they are in
mainland Europe. "We don't really understand why that is, but noise
may be a factor," says Slabbekoorn. "House sparrows use an
important low-frequency component in their calls."

In the Netherlands, the great reed warbler has suffered a similar
decline. Anecdotal evidence suggests that road noise may have
contributed to this: the construction of a road near a particular
reed bed reduced the number of warbler breeding pairs from around
10 to just two. When the road was closed for repairs for two years,
five more pairs moved into the area, although the subsequent return
of traffic drove them away again.

Loud noises can also have unexpected effects, including driving
otherwise faithful birds to adultery. Zebra finches, for example,
maintain monogamous relationships through a series of calls that
allow them to recognise and locate their mate. John Swaddle from
the College of William and Mary in Williamsburg, Virginia, found
that loud environmental noise prevents female finches from hearing
these calls. This erodes the otherwise strong bonds between
partners and leaves females showing no greater preference for their
chosen males than for strangers (Animal Behaviour, vol 74, p 363).

Even among species that seem to be adapting successfully to noise
pollution, there are signs that they are merely making the best of
a bad situation and that such flexibility is not without its costs.
House finches, for example, expend more energy on louder songs in
noisy cities, and their efforts are shorter as a result. Female
house finches prefer mates with longer songs, so males who
compensate for background noise could be left with fewer mating
opportunities. A great tit's natural inclination is to show off its
lower vocal register during the prime moments of the breeding
season. These notes take more effort to produce, says Slabbekoorn,
so they probably provide a good indicator of the singer's strength
and his potential as a mate. In an urban setting, however, these
dulcet tones may be lost in the hoo-ha, so urban males face a
compromise between singing a highly rated song, or simply one that
is audible (see Graph).

Noise is just one of the challenges facing urban birds. Until
recently, concern has centred on other forms of pollution,
including chemicals and light. Yet there is no doubt that noise has
already contributed to a decline in the diversity of bird species
around cities and major roads. Frank Rheindt at the University of
Würzburg, Germany, measured the diversity of different bird species
near a busy local highway and found a dearth of birds that sing at
lower frequencies.

The very fact that urban birds around the world are coming up with
strategies to deal with noise speaks volumes about the gravity of
the problem. "There are many factors that affect a bird's capacity
for breeding in cities but noise has been the most neglected one,"
says Slabbekoorn. Just how much it will change the familiar dawn
chorus remains to be seen.

Evolution - Learn more about the struggle to survive in our
comprehensive special report.

Endangered species - Learn more about the conservation battle in
our comprehensive special report.

Related Articles

Birdsong goes out of fashion too
http://www.newscientist.com/article.ns?id=mg19526115.000
05 July 2007
Birds tune in to keep their songs note perfect
http://www.newscientist.com/article.ns?id=dn10112
19 September 2006
Urban songbirds raise their voice to be heard
http://www.newscientist.com/article.ns?id=dn10720
04 December 2006

Weblinks

Ed Yong's blog
http;//scienceblogs.com/notrocketscience/
Sound recordings of urban and forest great tits, made by Slabbekoorn, 
Current Biology
http://www.current-biology.com/cgi/content/full/16/23/2326/DC1/
The Royal Society for the Protection of Birds
http://www.rspb.org.uk/
Hans Slabbekoorn, Leiden University
http://www.biology.leidenuniv.nl/ibl/S8/S8peopleinfo.php?PeopleID=143


Higgs boson: Glimpses of the God particle
http://www.newscientist.com/article.ns?id=mg19325934.600&print=true
02 March 2007
Anil Ananthaswamy

If the blips in the debris of the Tevatron particle smasher really
are signs of the Higgs boson then it's not what we expected. It
might mean that it's time to replace the standard model with a more
complex picture of the universe

On 9 December last year, as John Conway looked at the results of
his experiment, a chill ran down his neck. For 20 years he has been
searching for one of the most elusive things in the universe, the
Higgs boson - aka the God particle - which gives everything in the
cosmos its mass. And here, buried in the debris generated by the
world's largest particle smasher, were a few tantalising hints of
its existence.

Conway first revealed the news of his experiment earlier this year
in a blog. Experimental particle physicists are sceptics by nature,
loath to claim the discovery of any new particle, let alone a
particle of the Higgs's stature, and in his blog Conway dismissed
hints of its existence as an aberration, just as many other
supposed signs of the elusive particle have proved to be after
closer examination. The tiny blips in Conway's data have so far
simply refused to go away.

What's more, using data made public last week in a second blog,
another team of researchers has independently seen hints of a new
particle with similar mass. Both results may yet be dismissed, but
the coincidence is striking, and is one that is getting physicists
excited. If they have found evidence of a Higgs particle, then it
points towards the existence of a universe in which each and every
particle we know of has a heavier "super-partner", an arrangement
of the cosmos known as supersymmetry.

The Higgs boson is infamous as the only particle predicted by the
standard model of physics that remains undetected. In theory, every
other particle in the universe gets its mass by interacting with an
all-pervading field created by Higgs bosons. If the Higgs is
discovered, the standard model could justifiably claim to be the
theory that unifies everything except gravity.

But the model is creaking. Take the Higgs itself. The standard
model tightly links the masses of the Higgs, the W boson (the
carrier of the weak nuclear force), and the top quark (one of the
fundamental constituents of matter). Experiments at the Large
Electron-Positron (LEP) collider at CERN, near Geneva, in the late
1990s, and at the Tevatron, Fermilab's 6.3-kilometre-long particle
accelerator at Batavia, Illinois, where Conway detected his blips,
have homed in on the mass of the W boson and the top quark. If you
use these measurements to calculate the mass range of the Higgs,
and compare it with the standard model's predictions, you run into
trouble. "The best measurements of the W and top quark mass don't
agree well with the standard model," says Conway, who is based at
the University of California, Davis (see Diagram).

Physicists such as John March-Russell of the University of Oxford
go further. "If you ask most theorists about the Higgs, they will
say it is very unlikely that we'll see just the standard model
Higgs," he says. And that is what makes the hints of new particles
seen by Conway and others so intriguing.

Super-partners

With the help of the Collision Detector at Fermilab (CDF) Conway's
team has been searching for a more complex version of the Higgs
than the standard model predicts - one that might support the
supersymmetry model of the universe.

In supersymmetry, an electron has a heavier partner called the
selectron, while quarks have squarks, and so on. Although none has
yet been found, supersymmetry solves some niggling questions raised
by the standard model. For instance, when particle physicists take
the measured strengths of the electromagnetic and the weak and
strong nuclear forces, and extrapolate them to the ultra-high
energies of the early universe, they are supposed to unify. The
idea is that in the early universe these forces were the same. To
get the forces to unify at this grand unified theory (GUT) scale,
the parameters of the standard model have to be tuned to an
astounding precision of 1 part in 10^32.

This extreme fine-tuning makes many theorists uneasy. Why should
the properties of the early universe have to be so exact to give
rise to the universe we have today? "It is like creating in a
straitjacket," says March-Russell.

Supersymmetry, specifically a version called the minimal
supersymmetric model, achieves this grand unification more
naturally, with far less fine-tuning. The theory predicts five
Higgs bosons of different masses, which makes the process by which
the universe gets its mass more complicated than that laid out by
the standard model with its single Higgs. "But very often, in the
history of science, nature likes simple concepts, but it has quite
complicated realisations," says March-Russell.

It's a manifestation of this complex reality that Conway's team has
been probing. They are after one of the five Higgs predicted by
minimal supersymmetry. Such a Higgs could be produced by the
collision of protons and antiprotons at the Tevatron and some would
decay into two tau leptons, which are heavier cousins of the
electron. The taus decay immediately into other particles, and it
is this debris the team was sifting through. Essentially, they were
creating a plot which showed the mass of the particles that could
give rise to two tau leptons on the x-axis, and the number of such
particles on the y-axis.

Conway admits they only expected to see known particles decaying
into tau leptons. But then, on that Saturday morning before
Christmas, the CDF team saw the blip in their plot: signs that the
Tevatron had produced a small number of some unknown particle with
a mass of 160 gigaelectronvolts (GeV), which had promptly decayed
to two tau leptons. "I thought maybe, just maybe, this could be the
beginning of something," says Conway.

Convinced by their analysis, the entire CDF experiment team
approved the data on 4 January and Conway presented it at a
conference in Aspen, Colorado, a few days later. The team had found
a signal which, in particle physics lingo, had a 2-sigma
significance - a 1 in 50 chance of being a random fluctuation.
Normally, to merit new particle status a signal must be significant
to 5-sigma - where there's only a 1 in 10 million chance of it
being a fluctuation.

"People were excited to see this," says Conway. But why was there
so much excitement if the signal was statistically insignificant?
That's because a supersymmetric Higgs at this mass is extremely
plausible. "This kind of [Higgs] mass of 160 GeV is on the lower
end of what we were expecting, but we are comfortable with it, in
the context of supersymmetric models," says Jack Gunion, a
theoretical physicist at the University of California, Davis.

He has been advocating another version of supersymmetry called
next-to-minimal supersymmetry. When Gunion saw Conway's graph
showing a possible Higgs with a mass of 160 GeV, he realised he
only had to tune the parameters of his theory by about 1 part in 10
to explain it - an amount most physicists are willing to accept.
"You can only do that in next-to-minimal supersymmetry," says
Gunion. To make the minimal supersymmetry model of the universe
fit, you would have to tune it to levels that would make many
physicists uncomfortable, he says.

This is not the first time Gunion has used next-to-minimal
supersymmetry to explain an anomalous signal. In the late 1990s,
the LEP collider at CERN, which smashed electrons and positrons
head-on, saw what seemed to be a new particle with a mass of 100
GeV. Again, the significance of the signal was about 2-sigma, not
enough to claim a discovery. Because the signal did not sit well
with a standard model Higgs, it was mostly ignored, and the LEP
shut down in 2000, making it impossible to check the signal
further. "It is still a big deal," says Gunion, because nobody
could explain it."

But Gunion's next-to-minimal model could and does. "I claim that
the model provides a simple explanation, namely that there is a
Higgs at 100 GeV, and that it decayed in some extra ways that
weren't expected."

That means the LEP data from the 1990s and Conway's latest findings
from the CDF experiment could point to two of the five
supersymmetric Higgs particles, one with a mass of 100 GeV and the
other with a mass of 160 GeV. Gunion, for one, says that it is not
such a stretch to think so. "These are very naturally explained in
next-to-minimal supersymmetry."

First find the lepton

The story doesn't end there, however. Conway's initial analysis had
given them an approximate mass for the Higgs, but there was a more
accurate way to determine it.

Conway looked specifically for those tau leptons that were moving
in the so-called transverse plane, which is perpendicular to the
Tevatron's beam of protons and antiprotons. In particle
interactions in a collider, energy should be conserved, but some
energy can be emitted as neutrinos which cannot be detected
directly. In the transverse plane, the detector can indirectly
account for the missing energy of neutrinos with great precision.
So by limiting themselves to interactions in the transverse plane,
the researchers were able to accurately calculate the mass of the
heavy particles that gave rise to the tau pairs, and put those
heavy particles into bins of different masses. In each bin, they
could explain, from known physics, what gave rise to the tau pairs.
"Except in one bin," says Gunion. "And guess where that one bin
is?"

It turns out that the bin is at about 160 GeV. It shows the merest
hint of a new particle. "There are few events out there, right at
the place where we might expect a bump," says Conway. "It is so
preliminary, but it is there."

Conway's team is intrigued enough to pursue their signal. "We have
got data pouring in now," says Conway. "We are going to take it to
the next step." This involves doubling the statistics, increasing
the sensitivity of the instruments, and even searching in other
channels besides looking for tau-lepton pairs.

While increasing statistics could help verify the veracity of the
signal, one particular analysis could nail the identity of the
mystery particle. A supersymmetric Higgs should turn up with
b-quarks, also known as bottom quarks, one of the six types of
quarks. "If we see a Higgs being produced in association with
b-quarks, that's a dead giveaway," says Conway. "That's the
analysis we have been working towards for six to seven years now."

Meanwhile, another team led by Tommaso Dorigo of the University of
Padua, Italy, has been independently analysing an entirely
different set of particle interactions seen by the CDF experiment
and it too has found hints of some unknown particle at 160 GeV.
While the team is far from convinced that the signal is real, the
coincidences are intriguing (see "Sticking with the standard
model").

Markus Schumacher of the University of Siegen in Germany is also
highly sceptical that the Tevatron has seen anything new. "If you
look back in the history of particle physics, we have had a lot of
2-sigma effects," says Schumacher. "You have to wait until the
Fermilab experiment analyses more of the data." Dorigo agrees that
any claims of supersymmetry, based on the CDF data so far, are
premature. "I have seen hints of new physics beyond the standard
model coming and going, coming and going," he says.

Conway also remains cautious, expecting his team's own 2-sigma
signal to be a fluctuation and "evaporate". If that is the case,
then at least he has proved that the Tevatron collider is sensitive
enough to catch glimpses of a host of other theoretical particles
(see "Race you to the gluino").

But if the two teams have glimpsed a supersymmetric Higgs, then the
doors to the unknown are wide open. "It's like the first few pages
of a thriller," says March-Russell. "You get the first little hint
that something strange is happening and that things are not quite
what they seem. Then the evidence accumulates. We are turning the
first few pages of this very interesting story."

Race you to the gluino

The chill felt by John Conway in December could be a foretaste of
things to come. The 160-gigaelectronvolt (GeV) signal seen at the
Tevatron particle collider suggests that it is capable of testing
the supersymmetry model of the universe by searching for the
"super-partners" of some of the known particles, and means that the
race to find new particles between the Tevatron and CERN's
27-kilometre-long Large Hadron Collider (LHC), which is due to
start up later this year, enters new territory.

The Tevatron is scheduled to run at full throttle until 2009,
collecting data faster than ever before. By 2009, the LHC is
expected to have enough data to start searching for supersymmetry.
"If we were to make a discovery before the LHC after all these
years and billions of dollars, that would be really amazing," says
Conway.

Markus Schumacher of the University of Siegen in Germany, who works
on the ATLAS detector for the LHC, knows only too well that the
Tevatron could find new particles with undisputed certainty before
the LHC. "There was always a race between the Tevatron and the
LHC," he says. "It might well be that the Tevatron will be the
first collider to see something."

That something could be not just Higgs particles, but other
supersymmetric partners as well. Of course, it depends on whether
next-to-minimal supersymmetry, with its modest fine-tuning, is the
right description of reality. In that model, the masses of some of
the super-partners should be in the range of about 300 to 400 GeV.
That puts such particles in the sights of both the Tevatron and the
LHC. Specifically, partners for the top quark and the gluon, namely
the stop and the gluino, would be up for grabs.

Sticking with the standard model

Tomasso Dorigo of the University of Padua in Italy has put his
money where his mouth is. A believer in the standard model of
particle physics, Dorigo has bet his theorist friends a cool $1000
that it's the right description of reality. There's a small chance,
however, that his own experiment will lose him that bet.

Last week, Dorigo's team announced the results from the CDF
experiment looking at how Z bosons decay to b-quarks, a process
described by the standard model of the universe. However, his team
has seen, just as John Conway's team did last month, a few
anomalous events at a mass of about 160 gigaelectronvolts.

If this is indeed a supersymmetric Higgs boson, then theory
predicts the researchers should have recorded 100 such events based
on the amount of data they have collected. According to Dorigo, the
possibility that they have already done so cannot be ruled out.
"There is an upward fluctuation of the data right at about that
mass value, of the size one would expect from minimal
supersymmetry," he says.

However, he still firmly believes that the signals his team has
picked up are just noise in the data, and he's far from conceding
his bet. "Extraordinary claims need extraordinary evidence," he
says. "After thirty years of incredibly precise confirmations of
the standard model we need a huge signal of new physics before I
get convinced there is something beyond."


Upcoming colliders: Physics on the edge
http://www.newscientist.com/article.ns?id=mg19125661.300&print=true
25 August 2006
Stephen Battersby

The machine will stretch for 50 kilometres, a giant string of
supercooled, superconducting cavities that will pump energy into
two colliding beams of matter and antimatter. Physicists have
already spent years designing this amazing machine, called the
International Linear Collider. If it ever gets built, it will be
the most ambitious accelerator of its kind, costing many billions
of dollars.

And yet, the linear collider will be a mere baby in terms of the
energy levels it can reach, compared with another high-profile
accelerator, the Large Hadron Collider now nearing completion at
the CERN laboratory near Geneva. From next year the LHC will be the
world's most powerful particle accelerator. So why are physicists
so keen to build the linear collider?

Part of the answer is that it is likely to be built in the US, and
without it American particle physicists face a bleak future, with
no big accelerators to play with on home ground and no fresh data
to work on. Are several thousand physicists clutching at one very
long straw - or is there really something special about this
machine?

The ILC's supporters have a tough sell ahead. On the face of it, it
sounds like yet another expensive hole in the ground, uncomfortably
reminiscent of the superconducting supercollider (SSC), which was
scrapped in 1993 after $2 billion had already been spent on the
preliminary stages of construction - and without the SSC's selling
point of reaching unexplored realms of energy. But look a little
closer, and the potential rewards from the ILC are dazzling. It
could be the tool to finally take us beyond the standard model of
particle physics, not merely opening up a new world of exotic
particles, but telling us why those particles exist and behave as
they do. The ILC could examine the origins of mass, dissect dark
matter, reveal secret symmetries of the universe, perhaps even
discover extra dimensions of space.

To do all that, the linear collider will use the traditional
technique of high-energy physics - bang two things together hard
and see what comes out (see "Go with a bang").

Enter the terascale

The same method is used at the most powerful accelerator in
operation today, the Tevatron at Fermilab in Chicago, which can
inject a total of 2 teraelectronvolts (TeV, or 10^12 eV) into
collisions between protons and antiprotons. That's 2000 times the
energy locked up in the mass of a proton, or about a million times
the punch in a particle of radioactivity. But the Tevatron is due
to shut down in 2010, and in the US, particle physics is looking
queasy. "From an American perspective it's a little frightening:
when you turn off the Tevatron, you don't get data coming in for a
long time," says James Rosenzweig of the University of California
at Los Angeles, who works on future accelerator technologies.

The LHC should power up at CERN next year, eventually reaching
collision energies of 14 TeV and giving Europe the edge in the
high-energy physics stakes. Researchers hope that such energies
will be enough to start making particles from the next layer of
reality. They are confident that something new will emerge because
at these energies the standard model of particle physics breaks
down.

The standard model has been a great success over the past 30 years,
building up the universe out of six quarks, six leptons (three
electron-like particles and three types of neutrino) and four
particles carrying the electromagnetic, weak and strong forces that
glue particles together. It has matched the results of almost every
experiment going, but the model is not perfect. It does not
describe all forces in the same framework, and gravity is missing
altogether. In the standard model neutrinos have zero mass; in
nature, they don't. And worst of all, when you use the equations
and particles of the standard model to work out what happens in
high-energy collisions above about 1 TeV, it starts predicting
nonsense, saying that some outcomes occur with more than 100 per
cent probability.

One solution to this problem is the notorious Higgs boson, a
hypothetical particle that could simply slot into the standard
model. The Higgs is a manifestation of an energy field that would
give mass to most subatomic particles, and explain why the
electromagnetic and the weak nuclear force are so different, even
though they spring from the same source. Higgses would also clear
up the high-energy problem, making the equations balance and
produce sensible answers, so that things don't happen more than all
of the time.

The Higgs is something of a smokescreen, however. The simplest
version of the theory, with only one kind of Higgs particle, has
serious problems of its own: the Higgs mass ought to be infinite,
an obvious impossibility. "Nobody believes that it's just the
standard Higgs," says ILC theorist JoAnne Hewett, of the Stanford
Linear Accelerator Center in Menlo Park, California.

Many physicists are instead pinning their hopes on the theory
called supersymmetry, which posits many new particles, heavy
siblings of the known quarks and leptons, with several Higgs-like
particles among them. Then there are other theories in which space
has more than the three dimensions we see, with new heavy particles
inhabiting these extra dimensions.

Evidence from deep space suggests that other new particles are
within reach of the LHC. The rapid motions of stars and galaxies
point to some kind of exotic dark matter scattered throughout the
universe. Some cosmologists believe that this is made of weakly
interacting massive particles, known as WIMPs, and they calculate
that WIMPs should have masses of a few hundred gigaelectronvolts
(GeV, 10^9 eV). In any case, say the theorists, something will turn
up - and probably at a low enough energy for the LHC to reach.

So why do we still need the ILC? Its maximum collision energy will
be 1 TeV, only a few per cent of the mighty LHC and less even than
the Tevatron. But brute strength isn't everything. The Tevatron and
the LHC collide protons, each made of three quarks floating in a
bag of force-carrying gluons and short-lived quark pairs. And
protons make blunt instruments. "It's like banging together two
grand pianos - you get all sorts of complicated stuff flying out,"
says Brian Foster of the University of Oxford and head of the ILC's
European design team. That can make it hard to tell what new
particles have been produced.

Instead, the ILC would collide electrons with their antiparticles,
positrons. These are much sharper instruments, single point-like
particles. They carry the full advertised energy of the machine,
whereas at a proton collider, the energy available for making new
particles is only the small share carried by two individual quarks
or gluons.

A hatful of particles

Even more important, while the state of the colliding quarks and
gluons in the LHC is a mystery, in the ILC the exact energy and
other properties of each electron and positron will be known in
advance. This makes it possible to work out the precise properties
of whatever exotica are flung from the collision. So the hope is
that the LHC will discover a hatful of new particles, and then the
ILC will examine them closely so that physicists can uncover the
principles of nature that lie beneath.

For example, heavy particles inhabiting extra dimensions should be
versions of familiar particles, boosted to high enough energy to
oscillate in the tight confines of the extra dimensions. These
"Kaluza-Klein" states, named after two theoretical physicists of
the early 20th century who proposed extra dimensions to space-time,
would have a distinctive spectrum of energy and other properties.
The ILC could measure them so precisely that we would find out not
just that extra dimensions exist, but how many there are, how large
they are, and how they are curved and connected.

Supersymmetry too has its signature. And although the LHC might be
able to reveal it, the ILC would be far better suited to the task.
Even if there did only seem to be a single Higgs, the ILC could
zero in on it and find out if it is indeed alone, or is influenced
by much more massive cousins.

And the ILC would be unrivalled as a dark-matter laboratory. The
LHC might create WIMPs, and other experiments around the world
might detect them passing through the Earth, but that won't prove
that WIMPs form a significant fraction of dark matter. The ILC
could measure the exact interaction strength of WIMPs, which would
tell cosmologists how much of the stuff should have been created in
the big bang.

Particle physicists generally agree that building the linear
collider is the next logical step in advancing our understanding of
the universe, a view endorsed earlier this year by the US National
Academies. Even so, some physicists have reservations about the ILC
design. Last December, the 60-strong design team agreed that the
best approach would be to accelerate the electrons and positrons
using about 16,000 superconducting cavities made from niobium.

The trouble is that this design has inherent limits. Turning up the
power too high destroys the superconducting nature of the niobium
and the cavity becomes useless, no matter how cunningly designed.
"It's an inelegant solution," says Rosenzweig. "You can rely on
this stuff, but you're not going to get more than a factor of two
beyond current accelerating gradient."

Will that matter? It depends on what's out there. If the LHC does
indeed discover a set of mysterious particles with energies of just
a few hundred gigaelectronvolts, the ILC will be well suited to
examining them. But what if all the interesting stuff is at higher
energies? "Then there's a question of whether you'd want the ILC at
all," says CERN's chief theorist, John Ellis.

It's not that the ILC would be useless, even then. Its precision
would give the accelerator a surprisingly long reach, being able to
sniff out hints of particles far beyond its energy range. A more
powerful lepton collider might make more sense, however. Another
machine is on the table, called the compact linear collider (CLIC),
which could reach up to 5 TeV using a less well tested technology.
"As an insurance policy, CERN is trying to complete the research
and development on CLIC," says Ellis.

Then again, the current design for the ILC might turn out to be too
powerful. "It could be that at lowish energy there's some amazingly
interesting thing - then you might be able to build a cheaper
machine," says CERN physicist John Swain.

So why are physicists designing the ILC now, before we know what to
expect? "We don't have the time to wait," says Rosenzweig. "What
you build is very expensive and takes a long time to complete.
Waiting longer would harm high-energy physics. You would lose a lot
of scientific momentum and intellectual infrastructure." If the
accelerator designers do lose interest and find other jobs, it will
be very difficult to rebuild their expertise. "You can't put those
brains on the shelf," says Swain.

Cagey about costs

So for now it's full steam ahead with the ILC design project. If
President Bush's 2007 budget request to Congress is approved, the
ILC's funding will double to $60 million a year. That's just for
the research and development, and only the US part of it; European
and Asian teams are also at work.

The design team is anxious to avoid the ballooning budget of the
SSC, which rose from $4 to $10 billion before the thing was
scrapped, so it is being cagey about cost estimates at the moment.
By the end of this year, however, it will need to publish a firm
figure. An earlier estimate put a tag of $5 billion on the first
phase, and it's likely to turn out more expensive, putting the ILC
at the high-cost end of big science.

The ILC will almost certainly have to be truly international if it
is to scrape together enough cash. And that raises a tricky
question: where to put it? The US government will probably be
crucial in funding it, and might not warm to such an expensive
project unless it is sited on US soil.

The ILC team are understandably bullish about their machine. "There
is a strong case to build the linear collider, no matter what you
find at the LHC - even if it is nothing," says Foster. If the LHC
sees no new particles at all, then high-energy particle
interactions must be somehow subtly different from the standard
model's impossible predictions. The ILC might be able to find out
exactly how.

No actual decision will be made until the LHC provides a sketch map
of the new territory. Realistically, the ILC's prospects depend on
what's out there. "We need input from the LHC to tell us what new
physics appears at what energy scale," says Ellis. "I'm sure the
politicians will need it too before they decide whether to entrust
us with another few billion."

Go with a bang

Hit it hard and see what comes out. The deceptively simple essence
of particle physics is not as destructive as it sounds. The forces
between colliding particles actually create new matter, as long as
there is enough energy available.

According to that most famous of equations, E = mc^2, energy and
mass are equivalent, and it takes more energy to create a heavy
particle than a light one. In fact, particle physicists measure
mass by a unit of energy: the electronvolt (eV) is the kinetic
energy that a single electron would gain by pinging from the
negative to the positive terminal of a 1-volt battery. A proton has
a rest mass of 938 million electronvolts; the lightweight electron
merely 511,000 eV.

The Z particle, which helps to carry the weak nuclear force, has a
mass of 91.2 billion electronvolts (GeV), with a single particle
being heavier than five molecules of water. To make Z particles at
the Stanford Linear Accelerator Center, beams of electrons and
positrons were given exactly 46.6 GeV of kinetic energy. When they
collided head on, there was just enough energy available to make a
Z.

Other collisions can be much messier, with hundreds of particles
flying out, so particle accelerators have huge and complex
detectors wrapped around the collision zone. These trace the paths
of all the shrapnel and reconstruct the explosions, performing a
forensic investigation to spot any interesting interactions or
short-lived new objects that can only be recognised from the
particles they leave behind when they decay.

One aim of the linear collider will be to spot particles of dark
matter. They would slip out without registering in the detector,
but should betray themselves by stealing a lot of energy from the
collision.

End of the rings

The International Linear Collider (ILC) would be the biggest
electron gun ever built. Colliding the electrons head-on with
positrons, both accelerated to 500 GeV, would give a total
collision energy of 1 TeV. That's five times what was achieved at
the Large Electron-Positron collider (LEP), the accelerator that
once occupied the 27-kilometre tunnel at CERN that is now being
used for the LHC. Electrons and positrons circled LEP, gradually
being boosted to energies as high as 100 GeV.

To reach higher energies still, a ring is impractical, because
circling electrons rapidly lose energy by emitting photons in a
process called synchrotron radiation. The faster they go, the
faster their energy leaks away. That's where the "linear" bit of
the ILC comes in - make the path straight and there are no losses.
So the plan is to have two separate accelerators pointed head to
head.

That raises its own problems, however. In a ring, you can make the
counter-rotating electron and positron beams cross as often as you
want, so the particles have many chances to hit each other. In a
linear collider, they've only got one chance. The solution is to
focus the beams down to just a few nanometres across. That way,
each electron is running into a dense mass of positrons (and vice
versa) so plenty of collisions should take place. It's difficult,
but the necessary techniques have been developed at the Stanford
Linear Accelerator Center and at the KEK laboratory in Tsukuba,
Japan.

Sterile neutrinos: The cosmic controllers
http://www.newscientist.com/article.ns?id=mg19025561.800&print=true
15 June 2006

WHEN all else fails, invent a new particle - after all isn't that
what physicists do best? If there's mysterious stuff lurking
somewhere in outer space, you can be sure someone will dream up a
new particle to fit the bill. So it is no surprise to see yet
another made-up particle on the block. This time, it is the
"sterile neutrino", a ghostly particle so slippery that it might
never be detected on Earth.

Nonetheless, this elusive hypothetical particle is getting a
surprisingly good reception. Most physicists seem rather keen on
sterile neutrinos, which might just have played a cameo role in the
universe's history billions of years ago. Equally, they could be
centre-stage today, roaming space and healing all cosmology's woes.

For instance, sterile neutrinos might account for all the invisible
dark matter in space. They could explain why stars lit up the young
cosmos so quickly, and why giant black holes emerged hot on their
heels. "It's as if the pieces of the puzzle are suddenly coming
together, and for the first time, things fit," says Peter Biermann
from the Max Planck Institute for Radio Astronomy in Bonn, Germany.

It's an impressive track record for a particle that seems to be
little more than a figment of the imagination. Historically, many
particles that physicists dreamed up to solve theoretical riddles
were detected many years later. These include the ordinary
neutrinos that flood out from nuclear reactions inside stars.
Although neutrinos were first predicted in 1931, they were not
detected for another 25 years.

And now physicists say they have good reasons to think the ordinary
neutrinos have invisible cousins called sterile neutrinos. These
reasons stem from the surprise discovery in 1998 that the normal
neutrinos have mass. Before then, scientists were not sure if the
three types or "flavours" of neutrinos, dubbed electron, muon and
tau, had any mass at all. Understanding neutrinos is especially
tough because they are notoriously difficult to detect. They don't
feel the strong or the electromagnetic force, so normally fly
through anything, including people, stars and planets.

Only very occasionally do neutrinos interact with an atom, via the
weak force. Billions of neutrinos flood through the Earth all the
time, but researchers have to build giant detectors weighing
thousands of tonnes just to capture a tiny fraction of them.

Since 1998 several experiments in the US and Japan have confirmed
that the three neutrinos can "mix" or flip from one type into
another in the course of their travels - something which can only
happen if the neutrinos have mass. The experiments have allowed
researchers to pin down the weight of neutrinos to no more than
about 0.3 electronvolts, less than a millionth of the mass of an
electron.

Standard theories of particle physics sit comfortably with a
massless neutrino. Having accepted that they do have mass,
physicists now have a new puzzle to solve. Why are the masses of
neutrinos so much tinier than those of other particles like
electrons or quarks? One popular idea is that it is because they
have undetected partners - enter the "sterile neutrinos". These
particles would be sterile in the sense that they don't interact
with normal matter at all, except through gravity.

The idea is that neutrinos have a normal mass on average, except
that it is shared between heavier sterile ones we can't detect and
the familiar ordinary light neutrinos. "Most theorists believe
that, given the experimental fact that neutrinos have masses, it is
likely that sterile neutrinos do exist," says Boris Kayser, a
neutrino theorist at Fermilab in Illinois. Another popular feature
of sterile neutrinos is that they can help explain why matter
dominates antimatter in our universe (New Scientist, 4 September
2004, p 37).

Still, there's one thing that leaves scientists at odds. The role
sterile neutrinos play in our universe depends crucially on their
mass, and theory puts no limits on that at all. It could be
anything. Nor does theory suggest how many types of sterile
neutrinos can exist. Some scientists plump for three - just because
particles seem to crop up in threes - but there could be any
number.

According to Janet Conrad, a neutrino experimenter at Fermilab,
that has inspired a dismissive quip in neutrino circles: "Sterile
neutrinos are like cockroaches - once you get one in your theory,
there's no stopping them."

For now, however, scientists are free to play with any number of
sterile neutrinos with any mass they fancy, although it seems to
have settled out into a three-horse race. One group, the particle
theory buffs, tend to favour superheavy sterile neutrinos as
massive as bacteria (see "Heavyweight bout"). Some other scientists
extol the virtues of a much lighter sterile neutrino, which may
have left its mark in an experiment in New Mexico (see "Lure of the
lightweights").

Many astronomers and cosmologists have their eye on something in
between - a middleweight sterile neutrino with a mass about a
hundredth that of the electron. It is this middleweight sterile
neutrino that can neatly resolve a host of cosmology's problems.

The first of these problems is the nature of the curious dark
matter in the universe. Around 90 per cent of the matter in the
universe is in some strange, invisible form. It clumps into big
dark balls centred on galaxies and we know it is there because its
gravity tugs on stars and galaxies.

In 1993 Scott Dodelson from Fermilab and his colleague Lawrence
Widrow pointed out that the dark matter could be made up of
middleweight sterile neutrinos. If copious numbers of sterile
neutrinos were churned out in the hot big bang 14 billion years
ago, theory predicts that they would still be hanging around today.

Unlike normal neutrinos from the sun that zip around close to the
speed of light, these middleweight sterile neutrinos would be
relatively sluggish. Hence, they could easily clump together under
their mutual gravity to form the dark balls of matter that glue
galaxies together. This fits with observations of dwarf galaxies in
which the dark matter seems to be made of "warm" particles milling
around at speeds of several kilometres per second (New Scientist,
11 February 2006, p 7).

Dodelson simply chose the mass of the sterile neutrinos he believed
would best solve the dark matter problem. Now Alexander Kusenko of
the University of California, Los Angeles has shown that these
middleweight tailor-made particles have an uncanny knack of solving
other mysteries as well. "We're surprised to find that the same
neutrinos explain some other astrophysical puzzles," he says.
"There are several coincidences that make me very excited about
this idea."

Earlier this year, for instance, Kusenko and Biermann reported that
the sterile middleweights can explain why the first stars in the
universe formed so quickly. The latest observations by NASA's
Wilkinson Microwave Anisotropy Probe (WMAP) suggest stars must have
been burning just 400 million years after the big bang. However,
star formation theories have struggled to explain how gas could
have clumped together and shrunk into stars so quickly.

Middleweight sterile neutrinos come to the rescue. While most of
the ones churned out just after the big bang would still be around
today, their lifetime, like that of a radioactive nucleus, isn't
fixed. A tiny fraction would have decayed into normal neutrinos
during the first few million years after the big bang. As they did
so, they would have released X-rays that then ionised hydrogen
atoms. That would have encouraged them to bind to other hydrogen
atoms to form molecular hydrogen.

All that molecular hydrogen is just the ticket for star formation,
because the molecules efficiently radiate away heat, allowing hot
clouds of gas and dust to cool and lose pressure. Only then can
they stop fighting and collapse under gravity into dense stars.
Biermann and Kusenko calculate that the decays of middleweight
sterile neutrinos should indeed have allowed stars to form by 400
million years after the big bang, exactly as WMAP measures.

Seeding black holes

Biermann and his colleague Faustin Munyaneza have shown that
middleweight sterile neutrinos can also help build black holes
quickly by clumping happily together into little nuggets of dark
matter at the centres of galaxies, ready to "seed" the supermassive
black holes that inhabit the cores of galaxies. Observations reveal
that black holes millions of times the mass of the sun formed just
800 million years after the big bang.

Sterile neutrinos help this happen naturally because they clump
together easily into superdense matter. Because they don't emit any
light, they wouldn't generate bright radiation that would resist
the collapse. "Suddenly, it becomes obvious that the sterile
neutrino is a good candidate to solve all kind of problems," says
Biermann.

Kusenko has noticed that middleweight sterile neutrinos have
another talent. In 1996, he and his colleague Gino Segrè showed
that they could explain why neutron stars move so fast. A neutron
star forms when a massive star explodes at the end of its life,
leaving behind a superdense ball of neutrons about the size of a
city. These weird stars have solid crusts of iron nuclei, and some
have been spotted barrelling through our galaxy at speeds of more
than 1000 kilometres per second.

Why so fast? As the neutron star forms, some kind of "kick" must
accelerate it up to that enormous speed. Kusenko and Segrè argue
that middleweight sterile neutrinos could do this very efficiently.
A newborn neutron star is incredibly hot, with a temperature of
about 10^12 kelvin. Energetic neutrons and protons undergo several
chain reactions that produce neutrinos. For instance, a single
neutron can decay to make a proton, an electron and an electron
neutrino.

Neutron stars have superstrong magnetic fields too. And the
neutrons, protons and electrons involved in neutrino generation all
have their spins aligned with the magnetic field. The upshot is
that the newly created neutrinos preferentially speed out towards
the star's south pole.

These normal neutrinos don't get far because they constantly bump
into neutrons, scatter and fly off in random directions. However,
if a small fraction suddenly changes into sterile neutrinos, which
don't interact with matter at all, they maintain their initial
direction and whoosh out from the neutron star's south pole, giving
it an almighty kick. "They transfer momentum to the neutron star
just like a rocket," says Kusenko. His calculations suggest sterile
neutrinos are quite capable of providing that giant kick.

As astronomical trouble-shooters, middleweight sterile neutrinos
have a lot going for them, according to Conrad. "I think it's
wonderful science, it's really a very nice marriage of the things
that go on in particle physics, nuclear physics and astrophysics,"
she says. "I think there's room for this model to make sense."

What's lacking is experimental proof, which will be fiendishly
difficult to find given that sterile neutrinos are totally
indifferent to the real world. No one can imagine any way to detect
them directly, though there are possibilities for seeing them
indirectly (see Graphic).

Many theorists find that a turn-off. "It is bad," says Joe Lykken,
a Fermilab theorist, "because you will never truly believe a
particle-physics explanation of astrophysical puzzles until you
observe the relevant particle in the laboratory."

Conrad argues that someone might think up a way to detect sterile
neutrinos directly in future. "People are very smart and if they
think they're onto something, they sniff around until they finally
figure out how to get there," she says.

Kusenko adds that the night sky might hold the answer. If
middleweight sterile neutrinos do make up dark matter, their decays
today will produce X-rays at a telltale wavelength in a pattern
that mirrors the distribution of dark matter in the universe.

That signal could be detected by NASA's Constellation-X project, a
fleet of X-ray telescopes that could launch a decade from now if
NASA budgets allow. "Constellation-X would be fantastic, it would
really do a very good job searching for these sterile neutrinos,"
says Kusenko. He is also discussing with other astronomers the
prospects of piggybacking a tailor-made, cheap X-ray detector onto
an earlier space mission.

One way or the other, we may be entering new territory for physics.
With some ingenious thinking, it might be possible to prove that
the imperceptible cousins of ordinary neutrinos shaped the cosmos
profoundly. Then again, sterile neutrinos are not the first made-up
particles dreamed up to plaster over cosmological cracks, and they
probably won't be the last.

Heavyweight bout

In the world of sterile neutrinos, size matters. Boris Kayser, a
neutrino theorist at Fermilab in Illinois, says that most of the
theoretical clan favours the idea that the masses of sterile
neutrinos are enormous - about 10^20 electronvolts, or about 10^11
times the proton mass. That's as heavy as an E. coli bacterium.

Theorists favour such heavy sterile neutrinos because they think
they neatly explain why the three normal neutrinos are so light.
Neutrinos, like other matter particles, may acquire their mass by
interacting with as-yet-undetected particles called Higgs bosons.
That process would give the neutrinos a mass that's fairly normal
compared with other subatomic particles.

However, that mass would be shared out between normal neutrinos and
their hefty sterile cousins that only live for a fleeting instant.
Making the sterile partners superheavy most easily explains why the
normal neutrinos are so very light.

Lure of the lightweights

Wearing a gold wedding ring? Pricey, was it? Some scientists
suspect it would have cost a good deal more if it weren't for
sterile neutrinos.

Very light sterile neutrinos could help to account for large
amounts of heavy elements, like gold and uranium, in the universe.
These elements mainly come from supernova explosions at the end of
huge stars' lives. The crushing pressures in supernovae are thought
to force neutrons to gang together into heavy nuclei.

However, the explosions also create zillions of electron neutrinos.
Electron neutrinos tend to kill off neutrons, by interacting with
them to make protons and electrons. So much so that there shouldn't
be enough neutrons left to build all the heavy elements we see
around us.

George Fuller from the University of California in San Diego has
suggested that a light sterile neutrino with a mass of only 1
electronvolt - just a little heavier than its normal cousins -
could come to the rescue. If a small fraction of electron neutrinos
inside supernovae flip into these light sterile neutrinos and
escape for good, that could explain why more neutrons survive to
form copious heavy nuclei like gold.

What's more, there's a whiff of experimental evidence for this
light sterile neutrino. The results come from an experiment called
the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos
National Laboratory in New Mexico.

During the 1990s, this experiment measured muon neutrinos flipping
into electron neutrinos, but it measured more conversions than
expected, if results from other experiments are also correct. One
possible reason was that at least one light sterile neutrino with a
mass of about 1 electronvolt was joining in with the three familiar
neutrinos.

The LSND result is controversial and has never been confirmed.
Things might change soon, however. The MiniBoone experiment at
Fermilab is gathering data that will either confirm or reject the
LSND results once and for all. Janet Conrad of Fermilab, a
spokeswoman for MiniBoone, says her team will announce the results
during the next few months.


Trouble with antimatter
http://www.newscientist.com/article.ns?id=mg18624936.300&print=true
02 April 2005
James Gillies

DAN BROWN's novel Angels and Demons begins with the big bold word
"FACT". It continues: "The world's largest scientific research
facility - Switzerland's Conseil Européen pour la Recherche
Nucléaire (CERN) - recently succeeded in producing the first
particles of antimatter." That much is true: CERN really does
exist. It is the European laboratory for particle physics near
Geneva, and it really does make antimatter. The story is about a
plot to destroy the Vatican using antimatter stolen from CERN. And
much of what follows is pure fiction.

That CERN exists at all probably came as news to a lot of Brown's
readers, many of whom have subsequently taken the time to find out
what this mysterious place really is. What they have discovered is
that while much of the science in the book is pure invention, the
real science is every bit as fascinating.

When Angels and Demons first appeared in 2000, it provoked a ripple
of interest in CERN. But when Brown's next book, The Da Vinci Code,
came out a few years later and sold in the millions, people started
buying Angels and Demons in droves. Interest in CERN soared, and
the centre soon put up a website to respond to the enormous demand
for information about antimatter that resulted. In its first month,
the site received nearly 70,000 hits - not bad for a physics lab.

Dan Brown visited CERN in the 1990s while researching the book.
CERN's press officer then, a Scotsman who now lives in California,
dimly remembers showing a wannabe American author around the
laboratory. At that time, not many Americans had heard of CERN.
Today, when he tells his neighbours that he once worked at CERN,
the reaction is "Wow! That's cool!". Thanks Dan!

The CERN described in Angels and Demons is undoubtedly cool. It
looks like an Ivy League university. The director-general is a
larger-than-life character worthy of a James Bond movie. The lab
owns a private jet capable of crossing the Atlantic in just an
hour. Would that it were true. There is little that is correct
about the science in Angels and Demons. To some scientists this is
an outrage, to others it's just amusing. To me, it really doesn't
matter. The whole of the book is so clearly fiction that few people
take the science at face value. Instead, they turn to CERN to find
out what antimatter is really about.

In the book, just a gram of antimatter is stolen from CERN. As
Brown correctly points out, when antimatter meets matter, the mass
of the two is converted into pure energy in a process vastly more
efficient than nuclear fission. This, he claims, could be used for
destructive purposes, which makes for a gripping thriller, or for
peaceful purposes, as a new source of energy. But there's a flaw in
the logic. To generate energy, you would need a source of
antimatter, which simply doesn't exist in nature. You can't go out
and mine it. You have to make it, and that is a very
energy-inefficient process.

Brown is quite right that CERN makes antimatter. We've been making
it for decades to help us understand, for example, why there
appears to be no naturally occurring antimatter in the universe.
But in all that time, we've made less than a billionth of the
quantity stolen in Angels and Demons. If we could gather it all
together and annihilate it with matter, the energy released would
light a single light bulb for just a few minutes. Even if a gram of
antimatter did exist, transporting it would not be a simple matter
of picking up a bottle and walking off. The kind of antimatter Dan
Brown's villain takes to Rome cannot be contained.

So, sadly, antimatter will never solve the world's energy problems,
and happily it will never be used to make a bomb either. That's not
to say that it can't be used at all. Apart from its importance in
research, antimatter is used in hospitals on a daily basis. It is
matter-antimatter annihilation that allows positron emission
tomography (PET) scanners to produce images that are vital in some
forms of medical diagnosis, frequently for cancer. PET works by
detecting the two photons that are produced when an electron in the
body annihilates with its antimatter counterpart, a positron, which
is released inside the patient by a radioactive isotope.

Some forms of cancer treatment already rely on bombarding tumours
with particles to kill cancer cells. Today, these techniques use
protons or carbon ions. In the future, antimatter may have a role
to play. A recent experiment at CERN has taken the first steps
towards testing whether antiprotons can kill cancer cells.

CERN is a very open laboratory. Anyone who wants to find out about
antimatter can visit www.cern.ch and click the "Spotlight on"
button. And the lab has an on-site exhibition and a visitor
programme that takes in the antimatter facility.

By telling people that CERN exists, Dan Brown has provided us with
the opportunity to share the excitement of fundamental research
with a whole new audience. And in the case of antimatter, the truth
is every bit as strange and exciting as the fiction.

E-mail me if you have problems getting the referenced articles.