Hunt for the Higgs enters endgame
Aug 22, 2011 29 comments
Tantalizing hints that the Higgs boson is rearing its head at CERN's Large Hadron Collider (LHC) have become slightly less thrilling than was previously thought, reported physicists on the opening day of the Lepton Photon 2011 conference taking place in Mumbai, India, this week. Possible sightings of the famous particle had caused a stir at last month's European Physical Society meeting in Grenoble, when data presented from both the ATLAS and CMS experiments showed a small excess of events consistent with the production and decay of Higgs bosons with a relatively low mass of about 144 GeV.
Now, having almost doubled their datasets since the Grenoble meeting, the researchers continue to see a small excess in the low-mass region, but it is one with a lower statistical significance (about 2–2.5σ compared with 2.8σ). If the excess really is the genuine signature of a new particle, rather than a statistical fluctuation of similar-looking background events, physicists would have expected its significance to grow – not to shrink – as more proton–proton collisions were analysed.
"The fact that we're introducing data collected up until two weeks ago is scary and wonderful," Vivek Sharma, who presented the results of the CMS experiment, told physicsworld.com. "We don't know if the excess is a statistical fluctuation as it seems to persist, but the picture will become much clearer when we add data collected during the next two months."
CMS spokesperson Guido Tonelli cautions that a real Higgs signal could become weaker, despite extra data being included. "Some people got a bit too excited about the Grenoble excess so this latest snapshot of the data may therefore appear a let-down, but it's simply too early to say," he told physicsworld.com on Friday. "This is a historical time for particle physics and we have to be absolutely sure before we draw any conclusions."
Keep calm and carry on
As well as using more data, the new Higgs results are based on improved analysis routines, says deputy ATLAS physics coordinator Richard Hawkings. "With more time, we've done a better job of handling the background, which gives us increased sensitivity," Hawkings told physicsworld.com. "There's still plenty of room for the Higgs to hide at lower masses – we just need more data."
Some people are starting to think "What if the Higgs isn't there?" James Gillies, CERN communications chief
Discovering the Higgs boson would complete the Standard Model of particle physics, providing an explanation for how electroweak symmetry broke a fraction of a second after the Big Bang to leave certain elementary particles with the property of mass. Not discovering the Higgs, or something else that performs this symmetry-breaking role, would leave a major hole in physicists' understanding of nature's fundamental constituents.
"Some people are starting to think 'What if the Higgs isn't there?'," CERN's head of communications James Gillies admitted to physicsworld.com. "Our job is to stay calm and to get the message out that a non-discovery of the Higgs, if that plays out, is a big scientific discovery in itself."
Narrowing the range
Apart from a couple of narrow windows at mid-range masses, the LHC has now pretty much excluded Higgs bosons with masses between 145–466 GeV and finds no significant excess of events across the region 110–600 GeV. Direct searches at CERN's previous Large Electron Positron (LEP) collider, which shut down in 2000, excluded a Higgs lighter than 114 GeV, while fits to precision measurements of electroweak Standard Model parameters disfavour a Higgs heavier than 180 GeV.
Meanwhile, the latest results from Higgs searches at the Tevatron collider at Fermilab near Chicago, which is due to close down at the end of September, that were also shown at the Mumbai meeting exclude the regions 100–109 GeV and 156–177 GeV.
"The mass regions in which to search for the Higgs boson are narrowing," says Aleandro Nisati, who presented the ATLAS results. "I'm an Higgs enthusiast and I'm getting very excited by this!"
With the LHC delivering data faster than the researchers can analyse them, physicists have decided against presenting an official combination of the ATLAS and CMS Higgs results until the end of this year's data-taking. The LHC is due to cease proton–proton collisions in early November, switching to heavy-ion collisions for a month before closing down until early 2012.
I'm not particularly fond of the Higgs hypothesis, which seems ad hoc; so if we don't find the Higgs, I'd be quite happy Vivek Sharma, head of CMS Higgs group
"As head of the CMS Higgs group I can't hold 'religious' views on whether or not the Higgs exists," says Sharma. "But I'm not particularly fond of the Higgs hypothesis, which seems ad hoc; so if we don't find the Higgs, I'd be quite happy."
CERN theorist John Ellis says there is still everything to play for. "The region that is currently surviving the LHC's onslaught is precisely the favoured region for the Higgs based on previous electroweak fits," he explains from a sofa in the CERN theory department's common room. "With just two inverse femtobarns of data [more than 20 trillion collisions] recorded by each of CMS and ATLAS, a Standard Model Higgs boson has been excluded at the 95% confidence level between 130 and 600 GeV, demonstrating the need for new physics in the electroweak symmetry-breaking sector."
About the author
Matthew Chalmers is a science writer based in the UK. He was reporting from CERN