Final results of searches for particles decaying to photon pairs in 2015 data keep hopes alive for imminent ground-breaking discovery

On December 15th last year, as the physics coordinators of the ATLAS and CMS collaborations showcased the results of their new searches, particle physicists around the world held their breath. Both experiments showed preliminary results from the analysis of LHC data acquired during 2015 at 13 TeV. That unprecedented energy made the potential for new discoveries high.

Among a wealth of interesting results one in particular stood out, hinting at the possible existence of a new particle not predicted by the Standard Model. The signal of the tentative particle, reconstructed at a mass of about 750 GeV from observed decays to pairs of energetic photons (Fig.1), looked like a heavier version of the Higgs boson signal observed in 2012. And like the latter, it was seen by both collaborations in their independent data sets.

Figure 1: Event display of a 13 TeV proton-proton collision recorded by CMS in 2015. The two green lines show two photons generated by the collision. (Image: CMS)

The results shown last December raised huge interest in the theoretical physics community. Over the past six months more than 450 scientific articles describing new physics models have been published. In those articles, theorists attempt to use the apparent features of the new signal as a key to open new physics scenarios which include that particle, extending the Standard Model without causing manifest inconsistencies with known experimental constraints.

ATLAS and CMS have now finalized their analyses of the 2015 datasets. Results have been published on the same day, June 14th. This synchronization had been agreed upon by the spokespersons Charlton (ATLAS) and Camporesi (CMS) last December in order to avoid a time-ordering of the publications, in case these ended up containing a discovery claim. The articles focus on the search for new particles decaying to photon pairs.

A particle which decays into a pair of photons must be a boson, i.e. have an integer value of spin. The two simplest hypotheses are that it has spin zero, like the Higgs boson, or spin two, like an exotic particle called the Randall-Sundrum graviton. Another question concerns the intrinsic width of the particle, a fundamental property which determines the speed at which the decay takes place. The two new publications quantify how well the data support the hypothesis of a spin-0 or spin-2 particle, and what intrinsic width is compatible with the observations. No preference can be given to one of the two spin hypotheses; as for the width, the CMS results are better fit by a narrow resonance, while ATLAS data prefer a wider signal.

The two articles also include estimates of the statistical significance of the observed signals. That number answers the question "What are the odds of observing an effect at least this large, if there is no new particle?" Significance must by convention be larger than five in order for a credible discovery claim to be put forth. That corresponds to odds of less than one in three millions. For specific values of mass and natural width the significance reaches the value of 3.9 in the ATLAS analysis, and of 3.4 in the CMS analysis. However, once the collaborations duly account for the number of different ways that a signal could manifest in the data (the so-called "trials factor"), significances get sizably reduced.

In the end, ATLAS and CMS report significances of 2.1 and 1.6 standard deviations, respectively. These numbers are suggestive of a potential signal waiting to be confirmed. It is therefore only natural to look forward to this year's data. Already more than twice as many events as those produced in 2015 have been delivered by the LHC in the past two months. Preliminary new results will be shown at the beginning of August in Chicago, during the 38th International Conference of High-Energy Physics.

Will the 2016 data confirm the new boson signal? This is presently a much discussed question. As opposed to the enthusiasm demonstrated by theorists with their extensive studies, experimentalists show a sceptical attitude. The CMS Spokesperson Tiziano Camporesi, for instance, accepted a few months ago a fellow theorist's offer and bet six good bottles of wine against the genuine nature of the new particle signal, at six-to-one odds. Camporesi explains this does not entirely reflect his personal beliefs; it is rather a sort of "insurance bet". If the signal is confirmed he will lose six bottles, yet he will be extremely pleased by the new discovery. Otherwise, he will receive a good bottle as a consolation prize!

For more information:

ATLAS publication

CMS publication