The two experiments that discovered the Higgs boson in 2012 have sensed an intriguing if very preliminary whiff of a possible new elementary particle. Both collaborations announced their observations on 15 December, as they released their first significant results since completing a major upgrade earlier this year.
The results largely matched a rumour that has circulated on social media and blogs for several days: that both the CMS and ATLAS detectors at the Large Hadron Collider (LHC) outside Geneva, Switzerland, have seen in the debris of proton-proton collisions an unexpected excess of pairs of photons carrying around 750 giga electronvolts (GeV) of energy combined. This could be a tell-tale sign of a new particle — also a boson, but not necessarily similar to the Higgs — decaying into two photons of equal mass. It would be about four times more massive than the next heaviest particle discovered so far, the top quark, and six times more massive than the Higgs.
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In their talks at CERN, the laboratory that hosts the LHC, the two speakers took turns in surveying the results of the higher-energy ‘Run 2’ of their experiments, which began in June and was suspended in early November. Both speakers left the results on photon pairs for the end of their talks.
In each case, the statistical significance was very low: Marumi Kado of the Linear Accelerator Laboratory at the University of Paris-Sud said that his experiment, ATLAS, saw about 40 pairs of photons above the numbers expected from the standard model of particle physics; Jim Olsen of Princeton University in New Jersey reported that CMS saw merely ten. Neither would have even mentioned the excesses if the other experiment had not seen an almost identical hint.
“It is a little intriguing,” says ATLAS spokesperson Dave Charlton of the University of Birmingham, UK. “But it can happen by coincidence.”
In particle physics, statistical bumps such as this come and go all the time. If this one turned out to be a real particle, it would be “a total game changer,” says Gian Francesco Giudice, a CERN theorist who is not a member of either ATLAS or CMS. Experimenters have spent decades validating the standard model, and the Higgs was the last missing piece in that picture. A much heavier particle would open an entire new chapter in the field, Giudice says: “The Higgs boson pales in comparison, in terms of novelty.”
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Tiziano Camporesi, a CERN physicist who is the spokesperson for CMS, says that his team does not know what to make of the data so far. The bump appeared as the CMS team was searching for an unrelated particle called a graviton.
Maxim Perelstein, a theoretical particle physicist at Cornell University in Ithaca, New York, says that although the 750 GeV boson is not among the particles that LHC physicists had been searching for, theorists would not necessarily regard it as exotic. For example, it could be a particle similar to the Higgs, only heavier, he says. “I would not find it a big surprise if this turns out to be real.”
Meanwhile, searches for particles predicted by supersymmetry, physicists’ favourite extension of the standard model, continue to come up empty-handed. To theoretical physicist Michael Peskin of the SLAC National Accelerator Laboratory in Menlo Park, California, the most relevant part of the talks concerned the failure to find a supersymmetric particle called the gluino in the range of possible masses up to 1,600 GeV (much farther than the 1,300-GeV limit of Run 1). This pushes supersymmetry closer to the point where many physicists might give up on it, Peskin says.
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As for the two photons, Camporesi says that in 2016 the LHC should conclusively establish whether the bump was just another bump or a new particle. It will be a top priority for the next round of data taking, due to start in March, he says. “If there is an actual natural phenomenon behind these fluctuations, we will know.”