Tuesday, December 13, 2011

Higgs Boson: ‘Tantalizing Hints’ but No Direct Proof in Particle Search

Via New York Times -

Two teams of scientists sifting debris from high-energy proton collisions in the Large Hadron Collider at CERN, the European Center for Nuclear Research, said Tuesday that they had recorded “tantalizing hints” — but only hints — of a long-sought subatomic particle known as the Higgs boson, whose existence is a key to explaining why there is mass in the universe. It is likely to be another year, however, before they have enough data to say whether the elusive particle really exists, the scientists said.

The putative particle weighs in at about 125 billion electron volts, about 125 times heavier than a proton and 500,000 times heavier than an electron, according to one team of 3,000 physicists, known as Atlas, for the name of their particle detector. The other equally large team, known as C.M.S. — for their detector, the Compact Muon Solenoid — found bumps in their data corresponding to a mass of about 126 billion electron volts.

If the particle does exist at all, it must lie within the range of 115 to 127 billion electron volts, according to the combined measurements. “We cannot conclude anything at this stage,” said Fabiola Gianotti, the Atlas spokeswoman, adding, “Given the outstanding performance of the L.H.C. this year, we will not need to wait long for enough data and can look forward to resolving this puzzle in 2012.”

Over the last 20 years, suspicious bumps that might have been the Higgs have come and gone, and scientists cautioned that the same thing could happen again, but the fact that two rival teams using two different mammoth particle detectors had recorded similar results was considered to be good news. Physicists expect to have enough data to make the final call by the summer.

The Atlas result has a chance of less than one part in 5,000 of being due to a lucky background noise, which is impressive but far short of the standard for a “discovery,” which requires one in 3.5 million odds of being a random fluctuation. Showing off one striking bump in the data, Ms. Gianotti said, “If we are just being lucky, it will take a lot of data to kill it.”


CERN Press Release
The main conclusion is that the Standard Model Higgs boson, if it exists, is most likely to have a mass constrained to the range 116-130 GeV by the ATLAS experiment, and 115-127 GeV by CMS. Tantalising hints have been seen by both experiments in this mass region, but these are not yet strong enough to claim a discovery.

Higgs bosons, if they exist, are very short lived and can decay in many different ways. Discovery relies on observing the particles they decay into rather than the Higgs itself. Both ATLAS and CMS have analysed several decay channels, and the experiments see small excesses in the low mass region that has not yet been excluded.

Taken individually, none of these excesses is any more statistically significant than rolling a die and coming up with two sixes in a row. What is interesting is that there are multiple independent measurements pointing to the region of 124 to 126 GeV. It's far too early to say whether ATLAS and CMS have discovered the Higgs boson, but these updated results are generating a lot of interest in the particle physics community.

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