The Significance of the 750 GeV Fluctuation in the ATLAS Run 2 Diphoton Data

We investigate the robustness of the resonance like feature centred at around a 750 GeV invariant mass in the 13 TeV diphoton data, recently released by the ATLAS collaboration. We focus on the choice of empirical function used to model the continuum diphoton background in order to quantify the uncertainties in the analysis due to this choice. We extend the function chosen by the ATLAS collaboration to one with two components. By performing a profile likelihood analysis we find that the local significance of a resonance drops from $3.9\sigma$ using the ATLAS background function, and a freely-varying width, to only $2\sigma$ with our own function. We argue that the latter significance is more realistic, since the former was derived using a function which is fit almost entirely to the low-energy data, while underfitting in the region around the resonance.