By Edwin Cartlidge of Nature magazine
This year's hunt for the Higgs boson is drawing to a close. On October 30, the Large Hadron Collider (LHC) at CERN, Europe's particle-physics laboratory near Geneva, ended its 2011 run of the proton-proton collisions that search for the elusive particle, thought to give other fundamental particles their mass.
But physicists believe that the collider will have collected enough data by the end of 2012, after experiments resume in March, to say whether the Higgs exists. In fact, they say, strong hints of whether or not it exists may be present in data already collected. "We are entering the golden year for the Higgs search," says Guido Tonelli, spokesman for the LHC's CMS experiment, one of the detectors that is used in the search for the Higgs boson, "and in the next few months we may be able to give important messages."
There have been tantalizing hints already. In July, the ATLAS and CMS experiments at CERN reported a few "excess events" suggestive of the Higgs boson within the debris generated by the proton-proton collisions (see Hint of Higgs, but little more). But by August, with more data to analyze, the probability of these events being due to the Higgs, rather than simply statistical fluctuations in the data, had fallen from a significance of 2.8 sigma (corresponding to a more than 99 percent chance of being "real") to 2 sigma (around 95 percent chance of being real), the opposite direction to what would be hoped (see Higgs signal sinks from view).
With double the amount of data now than they had in August, Tommaso Dorigo, a particle physicist at the University of Padua in Italy and a member of the CMS team, says he is "willing to bet a few bucks" that the small excess of events indicating that the Higgs boson has a mass of around 120 gigaelectronvolts, reported in July, really are due to the Higgs. And he estimates that, if is he right, then by the time the 2011 data are analyzed early next year, the significance of the excess will have grown to about 3 sigma (a significance value of 5 sigma is the minimum at which discovery of the Higgs boson could definitely be claimed). "That would not be enough to claim a discovery," he says, "but it would be enough to convince most physicists that the effect is real."
Vivek Sharma of the University of California, San Diego, who heads the search for the Higgs at the CMS, points out that results from both CMS and ATLAS have already ruled out, with a confidence of 2 sigma, a Higgs mass of between about 145 and 400 gigaelectronvolts, and that the LHC's predecessor, the Large Electron Positron collider, ruled out a Higgs mass below about 114 gigaelectronvolts. So the Higgs, if it exists, almost certainly lies in the gap between the two. According to Sharma, the extra data to be collected in 2012 once proton-proton collisions resume in March will allow the CERN scientists to "either find the Higgs in this mass range, or wipe it out."
Sticking to the standard
Apart from the Higgs' no-show, results from the LHC so far have not pointed to any new physics beyond the "standard model." The decay patterns of particles known as Bs mesons, which consist of one "bottom" and one "strange" quark, remain in line with the predictions of the standard model and not with its extensions such as "supersymmetry"--a class of theories that suggest that each known particle has an exotic and as yet unseen partner. In fact, physicists now believe that the simplest, and most popular, version of supersymmetry is probably not correct (see Beautiful theory collides with smashing particle data). "The data are less exciting than people had hoped," admits Adam Falkowski, a particle-physics theorist at the University of Paris-South in Orsay, France.
A positive result in the Higgs hunt would perhaps be a more obvious public-relations success for CERN, but for Falkowski a negative outcome would be more exciting, as it would, he says, "definitely mean new physics". He explains that it is not clear what that new physics might be, pointing out that a particle such as the Higgs may still exist but that other as-yet undiscovered particles cause it to decay in a unexpected way. Another, more radical possibility, he says, is that the breaking of the symmetry between the weak and the electromagnetic forces (for which the Higgs is believed to be responsible) could instead be carried out by a completely new set of very strong forces described by a theory called "technicolor."
"If the standard model is true then we wouldn't have much to do as theorists, but if the Higgs isn't found then a whole new world of possibilities opens up," Falkowski says.