[tt] Has new physics been found at the ageing Tevatron? - space - 03 November 2008 - New Scientist Space

[Brian Atkins]

http://space.newscientist.com/article/dn15115-has-new-physics-been-found-at-the-ageing-tevatron.html?feedId=online-news_rss20

While engineers at the Large Hadron Collider (LHC) race to fix its teething 
problems and start looking for new particles, its ageing predecessor is refusing 
go silently into the night.

Last week, physicists announced that the Tevatron particle accelerator at 
Fermilab in Batavia, Illinois, has produced particles that they are unable to 
explain. Could it be a sign of new physics?

The Collider Detector at Fermilab (CDF) monitors the particles that spew from 
collisions between protons and anti-protons, which are accelerated and smashed 
head-on by the Tevatron. The collision occurs inside the 1.5-centimetre-wide 
"beam pipe" that confines the protons and anti-protons, and the particles 
created are tracked by surrounding layers of electronics.

In this instance, the CDF was looking at bottom quarks and bottom anti-quarks 
that decay into, among other things, at least two charged particles called muons.

The team was in for a big surprise. First, they saw far more muons coming from 
the collisions than expected. But crucially, some of these muons seemed to have 
been created outside of the beam pipe: they had left no trace in the innermost 
layer of the detector.

The CDF team says it is unable to explain such muons using the standard model of 
particle physics, or from what they know of their detector.
Unknown particle

However, "we haven't ruled out a mundane explanation for this, and I want to 
make that very clear", says CDF spokesperson Jacobo Konigsberg, who adds that it 
is important that other experiments verify the effect.

While the CDF team is circumspect, theoreticians are more willing to speculate. 
If the signal is not spurious, this means that some unknown particle with a 
lifetime of about 20 picoseconds was produced in the collision, travelled about 
1 centimetre, through the side of the beam pipe, and then decayed into muons.

"A centimetre is a long way for most kinds of particles to make it before 
decaying," says Dan Hooper of Fermilab. "It's too early to say much about this. 
That being said, if it turns out that a new 'long-lived' particle exists, it 
would be a very big deal."
Dark matter?

Neal Weiner of New York University agrees. "If this is right, it is just 
incredibly exciting," he says. "It would be an indication of physics perhaps 
even more interesting than we have been guessing beforehand."

So what could it be? As it happens, Weiner and Nima Arkani-Hamed of the 
Institute for Advanced Study in Princeton, New Jersey, and colleagues have 
developed a theory of dark matter – the enigmatic stuff thought to make up a 
large proportion of the universe – to explain recent observations of radiation 
and anti-particles from the Milky Way.

Their model posits dark matter particles that interact among themselves by 
exchanging "force-carrying" particles with a mass of about 1 gigaelectronvolts.

The CDF muons appear to have come from the decay of a particle with a mass of 
about 1 GeV. So could they be a signature of dark matter? "We are trying to 
figure that out," says Weiner. "But I would be excited by the CDF data regardless."

-- 
Brian Atkins
Singularity Institute for Artificial Intelligence
http://www.singinst.org/