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Prospective particle disappears in new LHC data

5 August 2016
It’s back to the drawing board as hints of a 750 GeV particle vanish in this year’s run.

Well, it was exciting while it lasted.

The existence of a particle with a rest-mass energy of 750 GeV, which seemed tantalizingly probable following last year’s run of the Large Hadron Collider (LHC), is now all but ruled out. Unlike in 2015, no significant excess of pairs of photons at or around 750 GeV has shown up in the LHC’s data-rich 2016 run. The news was announced by scientists from the CMS and ATLAS experiments on 5 August at the International Conference on High Energy Physics (ICHEP) in Chicago.

CMS tracked this pair of jets in May 2016. Hopes that dijets would lead to the discovery of a new particle were dashed at the International Conference on High Energy Physics on 5 August. Credit: CMS Collaboration, CERN

CMS tracked this pair of jets in May 2016. Hopes that dijets would lead to the discovery of a new particle were dashed at the International Conference on High Energy Physics on 5 August. Credit: CMS collaboration, CERN

The null result severely dims hopes that the LHC will offer up a quick and easy road map for extending the standard model. “The standard model continues to be the best game in town,” says Matthew Buckley, a theoretical physicist at Rutgers University in New Brunswick, New Jersey, who is not involved with the experiments.

The disappointing news caps a months-long saga that began in December, when CMS and ATLAS experimenters spotted a modest excess of photon pairs in data from 2015, the LHC’s first year running at 13 TeV. The statistical significance of the bump at around 750 GeV was 2.6 σ for CMS and 3.6 σ for ATLAS. Hundreds of theorists jumped on the finding and devised ingenious explanations to fit the particle into a revised standard model.

But even in December there were warning signs that the excitement would be short-lived. Some physicists pointed out that the evidence was far weaker using a stricter statistical measure: Look for something interesting over some 2 TeV worth of masses, they argued, and it’s not surprising that a fluctuation would show up somewhere. Then earlier this year a CMS paper revealed no excess of photons and Z bosons, a pairing that should also have emerged from the hypothetical particle’s decay. Rumors over the past month or so hinted at the discouraging result, though CMS and ATLAS physicists stressed that they were still crunching their data right up until the ICHEP presentations.

The CMS results, which were mistakenly posted a day early on the CERN document server, reveal no noticeable deviation at 750 GeV in the nearly 13 fb-1 of 2016 data. (An inverse femtobarn is equal to roughly 100 trillion proton–proton collisions.) That’s nearly four times as much data as was collected all of last year. If there were a new particle, the statistical significance would have shot up to more than 4 σ, Buckley says.

No significant excess of photon pairs appeared in 12.2 fb-1 of 2016 ATLAS data either, ATLAS physicist Bruno Lenzi announced in a 5 August presentation. Even without the fresh collisions, he said, a reprocessing of 2015 ATLAS data exposed a less robust signal than was originally reported.

Fresh data from the ATLAS (left) and CMS collaborations found no significant excess of photons corresponding to a particle with a rest-mass energy of 750 GeV. Credit: ATLAS and CMS collaborations, CERN

Fresh data from the ATLAS (left) and CMS collaborations found no significant excess of photons corresponding to a particle with a rest-mass energy of 750 GeV. Credit: ATLAS and CMS collaborations, CERN

The not-that-surprising news was still hard to take. The potential 750 GeV particle was particularly appealing because it had showed up as a beautiful, clean bump on a relatively simple plot. Now the window for receiving a neatly wrapped gift from nature is closing. It will likely take a much deeper dig into the data to uncover new physics.

Adding to the particle’s appeal was that it would have thrown a wrench in the simplest proposed standard model extensions. Theorists had to work hard to fit the 750 GeV particle into a consistent particle-physics picture—an enticing challenge that perhaps spurred some researchers to overlook the preliminary nature of the 2015 data.

Fortunately, physicists have plenty of new data to dig into, much of it presented at ICHEP. Not only is the LHC producing data at a record rate; it is also surpassing its designed luminosity. Meeting attendees are buzzing about new, robust measurements of the Higgs boson and top quark—although the results so far do not deviate far from the standard model.

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