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Accelerators and detectors

Accelerators and detectors

And so to bed for the 750 GeV bump

05 Aug 2016 Tushna Commissariat
the combined 2015 nd 2016 ATLAS diphoton data

By Tushna Commissariat

After months of rumours, speculation and some 500 papers posted to the arXiv in an attempt to explain it, the ATLAS and CMS collaborations have confirmed that the small excess of diphoton events, or “bump”, at 750 GeV detected in their preliminary data is a mere statistical fluctuation that has disappeared in the light of more data. Most folks in the particle-physics community will have been unsurprised if a bit disappointed by today’s announcement at the International Conference on High Energy Physics (ICHEP) 2016, currently taking place in Chicago.

The story began around this time last year, soon after the LHC was rebooted and began its impressive 13 TeV run, when the ATLAS collaboration saw more events than expected around the 750 GeV mass window. This bump immediately caught the interest of physicists the world over, simply because there was a sniff of “new physics” around it, meaning that the Standard Model of particle physics did not predict the existence of a particle at that energy. But also, it was the first interesting data to emerge from the LHC after its momentous discovery of the Higgs boson in 2012 and if it had held, would have been one of the most exciting discoveries in modern particle physics.

According to ATLAS, “Last year’s result triggered lively discussions in the scientific communities about possible explanations in terms of new physics and the possible production of a new, beyond-Standard-Model particle decaying to two photons. However, with the modest statistical significance from 2015, only more data could give a conclusive answer.”

And that is precisely what both ATLAS and CMS did, by analysing the 2016 dataset that is nearly four times larger than that of last year. Sadly, both years’ data taken together reveal that the excess is not large enough to be an actual particle. “The compatibility of the 2015 and 2016 datasets, assuming a signal with mass and width given by the largest 2015 excess, is on the level of 2.7 sigma. This suggests that the observation in the 2015 data was an upward statistical fluctuation.” The CMS statement is succinctly similar: “No significant excess is observed over the Standard Model predictions.”

 CMS diphoton data

Tommaso Dorigo, blogger and CMS collaboration member, tells me that it is wisest to “never completely believe in a new physics signal until the data are confirmed over a long time” – preferably by multiple experiments. More interestingly, he tells me that the 750 Gev bump data seemed to be a “similar signal” to the early Higgs-to-gamma-gamma data the LHC physicists saw in 2011, when they were still chasing the particle. In much the same way, more data were obtained and the Higgs “bump” went on to be an official discovery. With the 750 GeV bump, the opposite is true. “Any new physics requires really really strong evidence to be believed because your belief in the Standard Model is so high and you have seen so many fluctuations go away,” says Dorigo.

And this is precisely what Colombia University’s Peter Woit – who blogs at Not Even Wrong told me in March this year when I asked him how he thought the bump would play out. Woit pointed out that particle physics has a long history of “bumps” that may look intriguing at first glance, but will most likely be nothing. “If I had to guess, this will disappear,” he said, adding that the real surprise for him was that “there aren’t more bumps” considering how good the LHC team is at analysing its data and teasing out any possibilities.

It may be fair to wonder just why so many theorists decided to work with the unconfirmed data from last year and look for a possible explanation of what kind of particle it may have been and indeed, Dorigo says that “theorists should have known better”. But on the flip-side, the Standard Model predicted many a particle long before it was eventually discovered and so it is easy to see why many were keen to come up with the perfect new model.

Despite the hype and the eventual letdown, Dorigo is glad that this bump has got folks talking about high-energy physics. “It doesn’t matter even if it fizzles out; it’s important to keep asking ourselves these questions,” he says. The main reason for this, Dorigo explains, is that “we are at a very special junction in particle physics as we decide what new machine to build” and some input from current colliders is necessary. “Right now there is no clear direction,” he says. In light of the fact that there has been no new physics (or any hint of supersymmetry) from the LHC to date, the most likely future devices would be an electron–positron collider or, in the long term, a muon collider. But a much clearer indication is necessary before these choices are made and for now, much more data are needed.

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