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.
Not everyone is impressed with the new findings. The data are
"unconvincing," says Matt Strassler, a theoretical physicist at Rutgers University who was visiting CERN for the occasion. "I was a little disappointed," he adds, that the results did not live up to the expectations and the rumors—some called it a "Higgsteria"—that had circulated in the run-up to the announcement. On the other hand, he grants, no one expected to have a discovery at this stage—the experiments have not yet amassed enough data.
Vivek Sharma, Higgs search coordinator at the CMS collaboration, agrees that the two experiments have a small discrepancy on what the supposed Higgs mass would be, and that tantalizing hints of new physics from other experiments have often turned out to be statistical anomalies. "People should curb their enthusiasm," he cautions.
Joe Lykken, a theoretical physicist at Fermi National Accelerator Laboratory in Batavia, Ill., who is a member of the CMS collaboration, is more optimistic about the discrepancy. "Even though we are only seeing hints of the Higgs boson, it is encouraging that the ATLAS and CMS hints seem to be consistent with each other," he says.
A Higgs with a mass of 125 GeV would fit with a hypothesized extension of the Standard Model called supersymmetry, which posits that every known particle has a heavier, as-yet-undiscovered partner. "The low-mass Higgs is not so bad for supersymmetry, to say it diplomatically," CERN's Heuer said.
The LHC first fired up in September 2008, but within a week it was crippled by a serious accident that put it out of order for more than a year. "It was a big setback," says Lyn Evans, a CERN accelerator physicist who oversaw the construction and commissioning of the LHC from 1994 until his retirement a year ago. After repairs, however, the machine restarted in 2009 and has delivered more collisions than predicted, enabling the ATLAS and CMS collaborations to amass data five times faster than expected.
As recently as a year ago, one would not have thought that the LHC would make so much progress in its Higgs search by the end of 2011, observes Dmitri Denisov, spokesperson for the DZero experiment, one of the detectors at Fermilab's recently retired Tevatron collider. "It performed better than anyone expected," Denisov says.
If the Higgs really exists, it will answer the long-standing question of how matter gets its mass. It will also reveal the nature of the connection between two fundamental forces, the weak nuclear force and the electromagnetic force—a relationship termed the electroweak interaction. The two forces were unified for the first instants of our universe, but now they behave differently. Weinberg says the new results suggest that "it should be possible to reach a definite decision about whether this is the particle associated with the breakdown of the symmetries of the electroweak theory. I'll bet that it is."