Physicists weigh up Higgs signals
Dec 14, 2011 43 comments
Particle physicists are today digesting the news from yesterday's special seminar at CERN, where potential glimpses of the Higgs boson were revealed. Based on nearly all the data collected so far – the debris of some 500 trillion proton–proton collisions – the Large Hadron Collider's ATLAS and CMS experiments have now excluded the Higgs at pretty much all masses outside the narrow region between 115–130 GeV/c2 and 115–127 GeV/c2, respectively. The analyses represent a major reduction in the particle's possible hiding places compared with the situation just a few months ago.
"We would like the Higgs to be in this range, but it's too early to draw conclusions," ATLAS spokesperson Fabiola Gianotti told the several hundred staff packed into CERN's main auditorium. The Higgs is the observable upshot of a mathematical trick that preserves symmetry in the equations of fundamental particles as coded by the Standard Model of particle physics. Higgs bosons heavier than about 600 GeV/c2 have not yet been ruled out, but are disfavoured by indirect fits based on precision Standard Model measurements.
The two experimental collaborations also revealed what could be direct signs of the Standard Model Higgs lurking in the remaining low-mass region, which is where theory suggests it should appear. Although physicists stressed that the data are not sufficient to provide a conclusive statement either way on the existence of the Higgs, researchers could barely conceal their excitement about the tiny bumps in their plots. So is this finally the real thing, or are physicists being teased once again by the notorious, so far still imaginary, boson?
Hints and hopes
"It's too early to be sure," says theorist Matt Strassler of Rutgers University in the US. "But not too early to be hopeful." Indeed, history would advocate caution. After all, in late 2000 the CERN auditorium was also the venue for a packed seminar on potential sightings of the Higgs from the forerunner to the LHC – the Large Electron–Positron (LEP) collider. Tantalizing hints were presented by LEP physicists of a Higgs at a mass of about 115 GeV/c2, but on further analysis of the data these eventually proved less significant and left physicists with the current lower bound on the Higgs mass.
Several potential sightings of the Higgs at other colliders have since come and gone. In 2007, for instance, the Tevatron proton–antiproton collider in Fermilab, Chicago, hinted at a Higgs with a mass of 160 GeV/c2, only for this and neighbouring masses to be ruled out a couple of years later by the same collider. In July this year, meanwhile, ATLAS and CMS both reported possible sightings of a roughly 140 GeV/c2 Higgs, but within weeks those hints had faded once more data were analysed, and yesterday the Large Hadron Collider (LHC) killed off any remaining hopes of finding a Higgs at this mass.
The latest round of Higgs mania – some physicists had camped out in the auditorium from 8 a.m. to make sure they got a seat and security guards were turning away disgruntled staff more than two hours before the afternoon event began – concerns an excess of LHC collision "events" consistent with the production and decay of a Higgs with a mass of about 125 GeV/c2. "In the past we have seen similar fluctuations, but now it is different because the experiments are entering a region where the sensitivity is very high," CMS spokesperson Guido Tonelli told reporters.
The statistical significance of the latest Higgs hints is not too different to that of previous sightings: around the 2σ mark, which means there is a probability of a few per cent that such bumps in the data would appear anyway in that mass region because of fluctuations in background processes. Less-conservative estimates – 3.6σ in the case of ATLAS – are obtained if one quotes the "local probability", which represents the chance of getting such a fluctuation at a particular mass value. But there is good reason why many physicists think the latest bumps may grow rather than fade once collisions resume in the spring after the LHC's scheduled winter shutdown.
Reasons to be cheerful
The first reason is that both experiments, which are physically independent and use different analysis techniques, see hints at similar masses: roughly 126 and 124 GeV/c2 for ATLAS and CMS, respectively. The second is that the experiments see an excess of events in more than one of the Higgs' independent decay channels, which dictate the handful of ways in which a Higgs would reveal itself inside the detectors. The most striking signals at about 125 GeV/c2 come from events where the Higgs is presumed to have decayed into a pair of photons, but ATLAS also sees the effect in the channel where a Higgs decays into two Z bosons, which subsequently decay into leptons such as electrons.
Veteran Higgs hunter Sau Lan Wu from the ATLAS collaboration says that the fact that there is a potential signal in two channels is "most intriguing", especially given that CMS also sees something at a similar mass. "I am excited and optimistic that there is a low-mass Higgs at about 126/125 GeV/c2, but the final judgement will come only in the second half of 2012," she says. Indeed, having so far analysed more decay channels than ATLAS with the full 2011 dataset, CMS researchers say that the data across all channels provide a consistent picture. "We are observing exactly the significance that we would expect if we were to inject a Higgs with a mass of 124 GeV/c2 into the data," Vivek Sharma of the CMS collaboration told physicsworld.com.
However, CMS also sees a small excess at about 119 GeV/c2 in the ZZ channel, and the fact that the main CMS and ATLAS bumps are not at precisely at the same masses does not sit well with some physicists. "The picture is somewhat confused," explains Patrick Janot of the CMS experiment, who has been chasing the Higgs for the past 20 years. "We see something at 119, 126 and 124 GeV/c2, but everything is compatible with anything. People are being a bit too enthusiastic. These are not strong hints – LEP had a bigger Higgs significance. Let's wait for more data."
More work needed
Unlike LEP in 2000, when Higgs fever took hold just as the machine was scheduled to be shut down, the LHC is now at the very beginning of its life and is certain to either find the Standard Model Higgs or rule it out. If the Higgs does exist, it will take considerable time to understand precisely what it is, for example by measuring the many ways in which it interacts with other particles. Only then would physicists know whether the particle was the basic Higgs required to complete the Standard Model or something more complicated that could open the door to a more general theory beyond the Standard Model that, for instance, addresses the mystery of dark matter.
"If the Higgs is light as the data now suggest, then this is very important theoretically because it means there must be new physics," Gigi Rolandi of CMS told physicsworld.com. "If the Higgs is lighter than 127 GeV/c2, the Standard Model breaks at [energies of] about 108 GeV/c2."
In the spotlight
Another difference in today’s Higgs search compared with a decade ago is the huge interest from outside CERN – more than 110 000 people hooked up to the webcast of yesterday's seminar. "Journalists are picking up on the chase and reporting real science in the making," CERN's head of communications James Gillies told physicsworld.com.
Few physicists who spoke to physicsworld.com were convinced, however, by CERN's decision to invite the media to an interim report on the Higgs search yesterday, although most accepted that it was inevitable given the amount of speculation on physics blogs in recent weeks. Although those blogs proved fairly accurate, Dave Charlton of ATLAS says that the rumours were "not helpful", arguing that they could have harmed the scientific process by alerting ATLAS or CMS to each others' search results before independent conclusions have been reached. Dave Barney of CMS goes further. "The only people talking up the significance of these latest Higgs hints were the [bloggers]," he told physicworld.com. "CERN had no choice [but to hold a press conference to clarify the situation]."
One thing is certain: yesterday will not be the last time physicists pack into CERN's main auditorium to hear a special seminar on the Higgs. "The CERN seminar has a very positive effect on the young physicists here," adds Wu. " More than ever, they are excited about their work hunting the Higgs."
About the author
Matthew Chalmers reports from CERN. He is a science writer based in the UK