Authors: CMS Collaboration
Reference: CERN Document Server (CMS-PAS-EXO-16-027, presented at ICHEP)
In the early morning hours of Friday, August 5th, the particle physics community let our a collective, exasperated sigh. What some knew, others feared, and everyone gossiped about was announced publicly at the 38th International Conference on High Energy Physics: the 750 GeV bump had vanished.
Had it endured, the now-defunct “diphoton bump” would have been the highlight of ICHEP, a biennial meeting of the high energy physics community currently being held in Chicago, Illinois. In light of this, the scheduling of the announcements for a parallel session in the middle of the conference – rather than a specially arranged plenary session – said anything that the rumors had not already: there would be no need for champagne or press releases.
While the exact statistical significance depends upon the width and spin of the resonance in question, meaning that the paper presents multiple p-value plots corresponding to different signal hypotheses, the following plot is a good representative.
We hoped that the 2016 LHC dataset would bring confirmation that the excess seen during 2015 was evidence of a new particle, but instead, the 2016 data has assured us that 2015 was merely a statistical fluctuation. When combining the data currently available from 8 TeV and 13 TeV, the excess at 750 GeV is reduced to <2σ local significance. The channel with the largest, which was 3.4σ local significance excess before the addition of 2016 data, has now been reduced to 1.9 sigma local significance, and other channels have seen analogous drops. As a result, CMS reports that “no significant excess” is observed over the Standard Model predictions.
The excess disappearing was clearly the less-desirable of the two possible outcomes, but is there a silver lining here? This CMS result puts the most stringent limits to date on the production of Randall-Sundrum (RS) gravitons, and the excitement generated by the diphoton bump sparked a flurry of activity within the theory community. A discovery would have been preferred to exclusion limits, and the papers published concerned a signal that has subsequently disappeared, but I would argue that both of these help our field move forward.
However, as we continue to push exclusion limits further across all manner of search for new physics, particle physicists become understandably antsy. Across all manner of searches for supersymmetry and exotica, the jump in energy from 8 to 13 TeV has allowed us to place more stringent exclusion limits. This is great news, but it is not the flurry of discoveries that some hoped the increase in energy during Run II would bring. It seems that there was no new physics ready to jump out and surprise us at the outset of Run II, so if we are to discover new physics at the LHC in the coming years, we will need to pick it out of the mountains of background. New physics may be lurking nearby, but if we want it, we will have to work harder to get it.
References and Further Reading
- CERN Press, “Chicago sees floods of LHC data and new results at the ICHEP 2016 Conference” (link)
- Alessandro Strumia, “Interpreting the 750 GeV digamma excess: a review” (arXiv:1605.09401)
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