[tt] NS: Has dark matter's telltale signature been spotted?

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Has dark matter's telltale signature been spotted?
http://www.newscientist.com/article.ns?id=mg19926702.600&print=true

* 25 August 2008
* Anil Ananthaswamy

"THE 70-year-old dark matter puzzle is close to resolution," says
Michael Turner of the University of Chicago. And he's not alone in
thinking so. Rumour has it that a European space experiment has
discovered a telltale signature of the dark matter that makes up 90
per cent of the mass in the universe.

Earlier this month, the team working on the experiment, the Payload
for Antimatter Matter Exploration and Light-nuclei Astrophysics,
released preliminary data at the International Conference on High
Energy Physics in Philadelphia, Pennsylvania. PAMELA has spotted
more antimatter than expected in our galaxy - one of the signs that
dark matter particles are being annihilated. Though the team does
not claim to have discovered dark matter, and will not discuss the
results any further before they are published, other physicists are
deeply intrigued, as they add to a growing list of satellite and
balloon experiments that have found hints of a similar signature.

Dark matter has frustrated cosmologists ever since the 1930s when
they discovered that clusters of galaxies contain much more mass
than can be found in stars, gas and dust combined. Computer
simulations of the large-scale structure of the universe suggest
that dark matter is most likely made of weakly interacting massive
particles (WIMPs), whose mass can range from tens to thousands of
gigaelectronvolts (GeV). These particles should accumulate at the
centre of our galaxy, sucked in by the gravity of the supermassive
black hole at its heart. If so, they would annihilate each other,
spewing out other particles, including electrons, positrons and
gamma rays.

The earliest sightings of such emissions came from NASA's Energetic
Gamma Ray Experiment Telescope, which flew from 1991 to 2000. EGRET
saw more gamma rays than expected in the energy range of 1 to 10
GeV. Their source was near the galactic centre and some interpreted
them as products of dark matter annihilation. But, "it is a very
fuzzy, blurry picture", says astrophysicist Dan Hooper at Fermilab
in Batavia, Illinois.

Then came the High Energy Antimatter Telescope carried aloft by
balloons in 1994, 1995 and 2000. HEAT was looking for positrons.
When cosmic rays travel through space, they can smash into
interstellar dust and generate positrons and antiprotons. For
energies in the range of about 10 GeV, HEAT saw more positrons than
could be explained from the action of cosmic rays alone. It was a
strong hint that dark matter was being annihilated. But again, "it
wasn't a very detailed picture", says Hooper. "We needed better data
to know more."

More recently, the International Gamma-Ray Astrophysics Laboratory
(INTEGRAL), launched in 2002, saw very bright emissions of photons
at energies of 511 kiloelectronvolts (KeV). You expect to find such
photons when electrons and positrons annihilate each other.
Calculations showed that every second about 3 × 10^42 positrons were
being injected into the inner regions of the Milky Way - far more
than is possible from cosmic ray interactions. Again, dark matter
was suspected, although the mass of these hypothetical particles
would have to be in the megaelectronvolt (MeV) range, much lighter
than the WIMPs favoured by theory. "But we haven't found any
conclusive test to confirm or refute the dark matter hypothesis,"
says Hooper.

NASA's WMAP satellite, which was launched in 2001 and is busy
measuring the cosmic microwave background, the radiation left over
from the big bang, has also spotted similar signals. To help the
WMAP team to see the faint CMB signals, Douglas Finkbeiner of
Harvard University studied the known sources of microwaves from our
galaxy, such as emissions from hot dust and synchrotron radiation
from high-energy electrons spiralling around galactic magnetic
fields. The idea was to subtract these signals to reveal the CMB.
"But I couldn't get things to fit," he says. "I kept coming up with
too many microwaves in the centre of the galaxy."

Finkbeiner realised that the electrons and positrons produced by the
annihilation of dark matter would also emit synchrotron radiation,
and this could give rise to what he has dubbed the WMAP haze. "To me
that was a compelling coincidence," he says.

These experiments have primed physicists for PAMELA, one of the most
sensitive instruments to be sent into space. "We expect to have 10
times better statistics than all the HEAT flights," says Mirko
Boezio of the National Institute of Nuclear Physics in Trieste,
Italy, who presented the first results in Philadelphia.

Graciela Gelmini of the University of California, Los Angeles,
attended the sneak preview. According to Gelmini, PAMELA has also
seen more positrons in the energy range of 10 to 60 GeV than can be
explained by cosmic rays. For energies of 40 to 60 GeV, the excess
signal was as high as 10 per cent. This in line with data from HEAT,
and is statistically more significant. The excess of positrons
points towards dark matter annihilation. Gelmini is cautiously
optimistic. "Cosmic rays are extremely difficult to predict," she
says. "This indication [of dark matter] has to be taken with a grain
of salt."

Hooper, however, is more excited. "This may be the thing that we
have been contemplating for all these years, namely dark matter
annihilations." Finkbeiner agrees: "This is potentially some of the
best evidence we have seen in many years that dark matter actually
is a WIMP and is annihilating."

Physicists are waiting for PAMELA's full results, which will be
published in a few months, says Boezio. The experiment can detect
positrons with energies up to 270 GeV. If dark matter is responsible
for the excess of positrons, then you would expect this excess to
fall sharply at some higher energy. This cut-off point is related to
the mass of the WIMP since there can be no more energy in the
daughter particles than there is in original WIMPs. So no positrons
in excess of those generated by cosmic rays should be seen above
this energy. From the fuzzy HEAT data it appears that this cut-off
lies at about 238 GeV. If PAMELA shows a clear fall-off of positrons
at this energy, then it would be the clearest indication yet of the
mass of dark matter particles. "It is a very important next step for
PAMELA to actually show where this [excess] finishes," says Gelmini.

However, PAMELA is not sensitive to the direction of positrons and
cannot tell if they are coming from the centre of the galaxy. So all
eyes are on the Gamma-Ray Large Area Space Telescope, which was
launched on 11 June. Like EGRET, GLAST will be looking for an excess
of gamma rays from the centre of the Milky Way, and could soon
confirm PAMELA's findings.

To settle the issue once and for all, Turner would like a triple
strike: PAMELA and GLAST would look for indirect signs of dark
matter while underground experiments would hunt for particles of
dark matter (see "Where on Earth is the dark matter?). And the Large
Hadron Collider, which is due to start operating next month near
Geneva, Switzerland, should be able to produce dark matter. "That
way we can convince even the most sceptical that most of the matter
in the universe is not made of the star stuff that we are: a truly
extraordinary and humbling fact," he says.

Where on Earth is the Dark Matter?

In April, the Dark Matter (DAMA) collaboration reported an increase
in the energy of particles hitting their detectors inside the Gran
Sasso Mountain in Italy every June compared with December for the
past 11 years. This annual fluctuation, they said, is a sign that
the Earth is moving through the sea of dark matter particles in our
galaxy (New Scientist, 25 April, p 14).

But there's intense scepticism about the results. "I would like to
see an annual modulation experiment run from a different location,
maybe using a different detector material," says Dan Hooper of
Fermilab in Batavia, Illinois. "Otherwise, I am not likely to
believe that the DAMA signal was created by dark matter."

The scepticism stems from the fact that the DAMA results hint at
dark matter particles with a mass that's less than 100
gigaelectronvolts.

But none of the many other direct detection experiments, such as the
Cryogenic Dark Matter Search in Soudan, Minnesota, the Chicagoland
Observatory for Underground Particle Physics near Chicago and
XENON-10 in Gran Sasso, have seen anything in that energy range.
These experiments are looking for evidence that dark matter
particles have smashed into the nuclei of their detector material,
such as xenon.

It is likely that dark matter particles will not interact well with
the nuclei of normal matter. So it's possible that the direct
detection experiments may not yet be sensitive enough. But as they
improve, "it's very plausible that dark matter will show up in the
next few years", says Hooper.

Related Articles

* Physicists respond to dark matter 'discovery'
* http://www.newscientist.com/article.ns?id=mg19826534.300
* 25 April 2008
* Is dark matter mystery about to be solved?
* http://www.newscientist.com/article.ns?id=mg19726461.500
* 10 March 2008
* Pamela satellite begins searching for antimatter
* http://www.newscientist.com/article.ns?id=mg19025574.400
* 24 June 2006

Weblinks

* PAMELA official website
* http://pamela.roma2.infn.it/index.php
* GLAST official website
* http://glast.gsfc.nasa.gov/
* DAMA official website
* http://people.roma2.infn.it/~dama/web/home.html
* CDMS overview
* http://cdms.berkeley.edu/experiment.html

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