On the significance of the excesses in the ATLAS diphoton and four lepton decay channels

TL;DR

This paper evaluates the statistical significance of excesses in ATLAS diphoton and four lepton channels for Higgs searches, emphasizing the look elsewhere effect and comparing different analysis methods.

Contribution

It provides a detailed significance assessment of ATLAS Higgs search channels, including profile likelihood and alternative methods like scan statistics, highlighting the impact of analysis choices.

Findings

Global p-value of excesses: 0.033 or 0.013

Local and global significances align with ATLAS reports

Alternative methods can increase significance to 3.3 sigma

Abstract

This note describes an assessment of the statistical significance of the recently released ATLAS data regarding the Higgs search in the decay channels especially suited for the low mass region, in particular the diphoton and four lepton decay channels. Besides providing the significance evaluation of the considered channels, alone or combined, in the low mass range from 110 to 146 GeV, some details of the profile likelihood procedure employed for the calculations are described, too. Special emphasis is given to the look elsewhere effect, which arises when the search of a new signal is carried out over a broad mass range, therefore specifying separately local and global significances. When combined together, the global p-value of the localized excesses detected in the two channels is found equal to either 0.033 or 0.013, corresponding, respectively, to a significance of 1.8 sigma and 2.2 sigma depending upon the assumption of mutual independence or dependence of the amplitudes of the expected signal in each channel. Among the other obtained results, the local and global significances of the two channels individually taken are computed in good agreement with those reported by the Collaboration, while only a minor discrepancy is found for their combined local significance. Finally, some considerations are illustrated related to the low statistics four lepton decay channel, showing that a search strategy different from the profile likelihood method, e.g. scan statistics, can result in a substantially different significance, enhanced to a 3.3 sigma global effect.