Bayes factor of the ATLAS diphoton excess: Using Bayes factors to understand anomalies at the LHC

As an example of using Bayesian statistics in searches for new physics at the LHC, we present a calculation of Bayes factors for the erstwhile digamma resonance (Ϝ) versus the SM in light of ATLAS 8 TeV 20.3/fb, 13 TeV 3.2/fb and 13 TeV 15.4/fb data. We matched, wherever possible, parameterisations in the ATLAS analysis. We calculated that the plausibility of the Ϝversus the Standard Model increased by about eight in light of the 8 TeV 20.3/fb and 13 TeV 3.2/fb ATLAS data, somewhat justifying interest in Ϝ models. All told, however, in light of 15.4/fb data, the Ϝ was disfavoured by about 0.7. We argue that Bayes factors could play an important role in future anomalies by clarifying the plausibility of new physics and sidestepping look-elsewhere effects.