The plot is thickening for the still-hidden Higgs boson. Two US-based experiments report new, hopeful hints of the slippery particle, but one of the two main detectors at the Large Hadron Collider (LHC) says an existing signal has started to fade away.
The announcements, all made today at the Recontres de Moriond meeting in La Thuile, Italy, come from the four experiments that stand the best chance of finding the Higgs: CDF and Dzero, based at the now-defunct Tevatron particle collider in Batavia, Illinois; and ATLAS and CMS, based at the LHC at CERN in Geneva, Switzerland. Together they dim our understanding of what the true mass of the Higgs is – if it exists at all.
The CDF and DZero researchers, who are still combing through the data produced by the Tevatron, say they see hints of the Higgs boson at approximately the same mass as two tantalising signals reported by ATLAS and CMS in December.
ATLAS reports that the statistical significance of its December signal has weakened on closer inspection – but not disappeared. CMS reports no change.
Losing the scent
The Higgs is the final missing piece in the jigsaw puzzle that is the standard model, physicists’ most successful theory of how particles and forces interact. The mysterious particle gives all other particles mass, but the standard model can’t predict what the Higgs itself weighs.
In December, CMS and ATLAS reported that they had each seen a hint of a Higgs weighing about 125 gigaelectronvolts (GeV), or about 133 times the mass of a proton. ATLAS narrowed the range where the Higgs can still hide to between 115 and 131 GeV, and CMS reduced it to 115 and 127 GeV. Now, with more data, ATLAS seems to have lost the scent.
The Higgs can show up briefly when particles crash together at high speeds, but it quickly decays into smaller, lighter particles. Like archaeologists piecing together shards of pottery, physicists use these smaller, lighter particles to reconstruct the Higgs. Physicists also know how often certain particles should pop up in the detritus from other processes, and they look for suspicious excesses as evidence of something new.
The signal ATLAS presented in December was based on only two of the five possible decay routes, called channels, available to the Higgs – decays into four leptons and decays into two gamma-ray photons. That signal had a statistical significance of 2.2 sigma, meaning it had a 2 per cent chance of being due to random fluctuation. It was seen as a hint of the Higgs – rather than a discovery (particle physicists generally wait until they have significance levels of at least 5 sigma before declaring a discovery – and for 3 sigma to declare evidence).
The results ATLAS presented today involve the remaining three channels – the Higgs decaying into two W particles, two tau particles, or a bottom quark and a bottom antiquark – and are empty of excesses, says Sandra Kortner of the Max Planck Institute for Physics in Munich, Germany, who gave the ATLAS presentation.
“In the remaining channels, we don’t see such an excess,” she says. “The probability that we see something has decreased a little bit.”
The signal still hasn’t gone completely. “If one combines all the channels together, we still see an excess around 126 GeV,” says Kortner. “It could go either way. We can’t say anything conclusively yet. We still need more data to really tell.”
Blogger and theorist Matthew Strassler of Rutgers University in Piscataway, New Jersey, agrees that the disappearing signal might not be cause for alarm. He points out that the three empty channels carry less information about the Higgs and therefore are less significant.
When the Higgs decays into two photons or four leptons, nothing is lost – every pottery shard is on hand, so physicists can reconstruct the entire particle. By contrast, when it decays to two W bosons, two neutrinos flit out of the detector and are lost forever.
“The reason they focused on the two they presented [in December] are those are the ones that really can show you a nice peak when they get enough data,” he says. “They’re the ones that will be really convincing over time.”
Whether the Higgs is at 125 GeV or not, it’s running out of room to hide. ATLAS’s new results further squeeze the range where it could still lurk, ruling out everything from 110 to 117.5 GeV, 118.5 to 122.5 GeV, and above 129 GeV.
For its part, CMS, ATLAS’s twin detector, sees nothing significantly different than it reported in December. It saw an excess at about 125 GeV with a significance of 2.1 sigma. “The general conclusion does not change,” says Marco Pieri of the University of California, San Diego, who presented the results at the Moriond meeting.
More optimistically, the two main experiments from the Tevatron, reported that they do see hints of the Higgs almost exactly where the LHC experiments see it.
The Tevatron’s detectors, CDF and DZero, were only sensitive to a Higgs decaying into a bottom quark and bottom antiquark, which are “messier” to interpret, Strassler says. That mess prevents the detector from finding sharp signals of a Higgs at one specific mass, but it does allow researchers to see excesses over a range of masses.
Both saw signals that could be from a Higgs between 115 and 135 GeV at a level of 2.2 sigma, which is consistent with the LHC’s results. “But until we get to at least 3 sigma, we don’t use the word ‘evidence’,” says CDF spokesperson Rob Roser at Fermilab.
The LHC should have enough data to confirm or rule out the existence of the Higgs boson by the end of this year. But these searches are only for the most basic type of Higgs that the standard model allows. If the elusive particle never shows up, it could open the door to more exotic kinds of physics, including extra particles, extra forces and maybe even extra Higgs bosons.
Some physicists are hoping that’s the case, if only because it would make things more interesting. “One Higgs would be very sad,” says Nobel laureate Steven Weinberg, a key architect of the standard model in the 1960s.
Physicists have been writing theories based on the assumption that there is only one Higgs boson for decades, he says. But the standard model is already incomplete: it doesn’t account for gravity or dark matter, for instance. “We’ve been living in a world with a Higgs in our theories for a long time, and we’ve gotten stuck. We don’t see clearly what the road ahead is,” he says. “If all we see at the LHC is one Higgs, it’s not going to help us. We’ll still be stuck.”
More on these topics: