Tuesday, August 23, 2011

Higgs boson is running out of places to hide 

The elusive Higgs boson, thought to give all other particles mass, is running out of places to hide. A new deluge of data from the Large Hadron Collider's twin particle detectors has ruled out a wide range of possible masses for the theoretical particle.

The Higgs is the last undiscovered particle in the standard model of physics, the leading theory for how particles and forces interact.

Data from the LHC's ATLAS and CMS detectors have now ruled out – with a confidence level of 95 per cent – all masses for the Higgs between 145 and 466 gigaelectronvolts (GeV). That covers the bulk of the mass range that is easiest for physicists to explore.

"We're ruling out the easy bits, and we're leaving the harder bits still to study," says James Gillies, spokesman for CERN, the particle physics lab in Switzerland where the LHC is based. "There's still a lot of space left, but it's the space that's harder for us to analyse."

Hiding out

The announcement, made on Monday at the biennial Lepton-Photon conference in Mumbai, India, came on the heels of a hopeful whiff of the Higgs announced at the Europhysics Conference in Grenoble, France, in July. The LHC has collected twice as much data in the month since the Grenoble conference.

That intriguing signal was found around 140 GeV, which is still in the allowed range, albeit barely. But physicists at the time warned that the signal could just be due to statistical fluctuations. With the extra data, the significance of the signal has indeed gone down, CERN says.

The Higgs is most likely hiding out between 115 and 145 GeV – at the low end of what the LHC can detect, Gillies said. The LHC's predecessor, the Large Electron Positron Collider (LEP), ruled out masses below 115 GeV before it shut off in 2000.

'End game'

"We are definitely approaching the end game in looking for the Higgs," Gillies says. "But we don't want to give the impression that the answer is just around the corner." Teams at the LHC hope to double the amount of data they currently have by the end of this year, but they will need to double that again to say for sure if the Higgs is there or not. Collecting that much data will probably take the better part of a year if nothing goes wrong, Gillies says.

Physicists are not panicking about the fate of the Higgs yet, but they may be getting frustrated. If the Higgs is indeed hiding in the low mass range, "a malicious deity has carefully chosen the Higgs mass to make it as hard as possible for physicists to study it", said Peter Woit of Columbia University on his blog, Not Even Wrong.

They're also starting to worry about what it means for physics if the Higgs isn't there, Gillies said. If it doesn't exist, physicists will have to come up with another explanation for why matter has mass.

And even if the LHC comes up Higgs-less at the end of next year, there could be other, stranger versions of the Higgs that do not fit into the standard model – and thus could point to entirely new physics.

"[That] would be really cool," says Gillies, adding that these bizarro versions could potentially lie in the mass range that has just been excluded for the "garden-variety" Higgs. "That's what everyone wants – something that goes beyond the standard model."

Source: http://goo.gl/QFMc9

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