Search for displaced photons produced in exotic decays of the Higgs boson using 13 TeV $pp$ collisions with the ATLAS detector

TL;DR

This paper searches for displaced photons from long-lived particle decays in Higgs boson events using 13 TeV proton-proton collision data from the ATLAS detector, setting limits on exotic Higgs decays and displaced photon production.

Contribution

It introduces a novel analysis exploiting ATLAS calorimeter timing to detect displaced photons from exotic Higgs decays with no significant excess found.

Findings

No significant excess of displaced photons observed.

Upper limits set on Higgs exotic decay branching ratios.

Model-independent limits on displaced photon production.

Abstract

A search is performed for delayed and nonpointing photons originating from the displaced decay of a neutral long-lived particle (LLP). The analysis uses the full Run 2 data set of proton-proton collisions delivered by the LHC at a center-of-mass energy of $\sqrt{s}=13$ TeV between 2015 and 2018 and recorded by the ATLAS detector, corresponding to an integrated luminosity of 139 fb$^{-1}$. The capabilities of the ATLAS electromagnetic calorimeter are exploited to precisely measure the arrival times and trajectories of photons. The results are interpreted in a scenario where the LLPs are pair-produced in exotic decays of the 125 GeV Higgs boson, and each LLP subsequently decays into a photon and a particle that escapes direct detection, giving rise to missing transverse momentum. No significant excess is observed above the expectation due to Standard Model background processes. The results are used to set upper limits on the branching ratio of the exotic decay of the Higgs boson. A model-independent limit is also set on the production of photons with large values of displacement and time delay.