[新聞] Higgs signal sinks from view
http://www.nature.com/news/2011/110822/full/news.2011.495.html
Early hints of the boson grow weaker with fresh data.
Geoff Brumfiel
The Higgs boson, the most sought-after particle in all of physics, is proving
tougher to find than physicists had hoped.
Last month, a flurry of 'excess events' hinted that the Higgs could be popping
up inside the Large Hadron Collider (LHC), the world's most powerful particle
accelerator located at CERN, Europe's high-energy physics lab near Geneva,
Switzerland. But new data presented today at the Lepton Photon conference in
Mumbai, India, show the signal fading. It means that "this excess is probably
just a statistical fluctuation", says Adam Falkowski, a theorist at the
University of Paris-South in Orsay, France.
For almost 50 years, scientists have been chasing after the Higgs. The boson
is thought to be a key part of the mechanism that endows other particles with
mass. Just as importantly for physicists, it allows for the unification of
electricity and magnetism with the weak nuclear force into a single
'electroweak' force. Most believe that the particle is the last key part of
the standard model of particle physics.
Many physicists hoped that the LHC might be able to catch the particle early
on in its run, and there was cause for optimism. Last month in Grenoble,
France, scientists announced evidence for what seemed to be enticing hints of
a Higgs boson inside their detectors. The faint signal, at around 144
gigaelectronvolts (GeV), was seen independently by both the ATLAS and the
Compact Muon Solenoid (CMS) detectors, the two largest detectors at the
collider.
Moreover, the events appeared as an excess of W bosons, particles that
moderate the weak nuclear force. Higher-than-expected numbers of W bosons
were predicted by many theorists to be an early indicator of a Higgs boson
(see 'Collider sees tantalizing hints of Higgs').
Higgs hopes fade
But the latest results, which use about twice the data, seem to show that
signal weakening. Both detectors now report that the significance of the find
has dropped from around 2.8 sigma to 2 sigma. That means the odds of it being
the real Higgs have fallen, from more than 99% to 95%, the opposite of what
researchers would hope with additional data.
The drop in confidence came because of that additional data, and because
physicists have improved their understanding of the other processes that can
make W bosons, according to Richard Hawkings, deputy physics coordinator at
ATLAS. It is not entirely unexpected, adds Joe Incandela, deputy spokesperson
for the CMS. "The veterans who have been in the trenches know that in the
early goings there are often things that are unclear," he says.
What researchers can say for sure is where the Higgs isn't. The CMS
experiment has ruled out the boson's existence for energies between 145 and
400 GeV; ATLAS has eliminated several large patches between 146 and 466 GeV.
"There's still certainly plenty of room for a Higgs to be hiding in there,"
says Hawkings. Many physicists believe that the Higgs, if it exists, is likely
to be at the lower mass end of the energy spectrum, perhaps between about 120
and 140 GeV. Those lower energy ranges will require more data to find a signal.
It may be possible to rule out the Higgs in these regions by the end of the
year, but confirmation of a discovery, which requires more data, will probably
have to wait until the end of 2012.
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