GENEVA—The two largest collaborations of physicists in history Tuesday presented intriguing but tentative clues to the existence of the Higgs boson, the elementary particle thought to endow ordinary matter with mass.
Representing the 6,000 physicists who work on two separate detectors at the Large Hadron Collider (LHC), called CMS and ATLAS, two spokespersons said that both experiments seemed to agree, as both their data sets suggested that the Higgs has a mass close to that of about 125 hydrogen atoms. The LHC is an international facility hosted by CERN, the European particle physics laboratory outside Geneva.
"We are talking of intriguing, tantalizing hints," said CMS spokesperson Guido Tonelli, speaking to a room filled with dozens of journalists and TV crews. "It's not evidence."
The experiments, in which protons traveling at nearly the speed of light collide head-on, cannot directly detect the Higgs, because the boson would decay within a fraction of a nanosecond into other particles. Instead, physicists must search through the debris of many different types of particle decay to find precise combinations of by-products that the Higgs would produce—and different chains of particle decays may well have the same signatures. A particular combination that appears more often than expected from other, "background" processes may signal the presence of the Higgs. But if it does not appear often enough compared with the expected background, it could just be a statistical fluctuation. Today, neither CMS nor ATLAS could claim to have the "3-sigma" statistical significance needed to claim evidence for a new particle—let alone 5 sigma for the accepted standard to claim a discovery. (A 3-sigma result implies a fraction of a 1 percent chance of a statistical fluke.) Instead, so far each experiment could only claim a statistical significance of around 2 sigma.
Both the detectors and the LHC accelerator itself, however, have been performing better than expected; so all the ducks are now in a row for settling the question soon, according to the researchers. "The nice thing to know is that by the end of 2012—sooner if we are lucky—we should be able to say the final word," Fabiola Gianotti, the ATLAS spokesperson, said at the press conference.
"I find it fantastic that we have the first results on the search for the Higgs, but keep in mind that these are preliminary results. And keep in mind that we have small numbers," said CERN Director General Rolf-Dieter Heuer in summarizing presentations that both Tonelli and Gianotti gave during a CERN seminar earlier that day.
"I think the evidence is very encouraging, though it's still too early to be sure," comments Steven Weinberg, a leading theoretical physicist at the University of Texas at Austin and a winner of the Nobel Prize in Physics.
A generation of high-energy physicists came of age studying and testing the Standard Model of particle physics, a theory devised in the 1970s that has withstood all experimental challenges. One final piece is missing, though, and it is one without which the whole model could fall. Without the Higgs boson, physicists cannot explain how other particles have mass. The Higgs itself has mass, and going by exclusion, researchers from the LHC and from its predecessor particle colliders were able narrow down the range of its value to between 115 and 140 giga–electron volts, or GeV. (One GeV is roughly the mass of a hydrogen atom.)
Together, the LHC detectors have now reduced the allowed range further: Tonelli said that according to CMS data its mass cannot be greater than 127 GeV. That was not for lack of data—in fact, quite the opposite. "We were not able to exclude the range below 127 GeV because of excesses," or more of certain particle by-products than would be expected in the absence of the Higgs, he remarked during his seminar talk—which was an understated way of saying that the CMS experiment had actually seen hints of a Higgs existing and having a mass of 124 GeV or so. ATLAS saw excesses in a similar range of energies, although the graphs did not quite line up—the ATLAS data favor a Higgs around 126 GeV.