Search for light neutral particles decaying promptly into collimated pairs of electrons or muons in pp collisions at SQUARE ROOTs = 13 TeV with the ATLAS detector

Abstract

A search for a dark photon, a new light neutral particle,which decays promptly into collimated pairs of electronsor muons is presented. The search targets dark photonsresulting from the exotic decay of the StandardModel Higgsboson, assuming its production via the dominant gluon-gluonfusion mode. The analysis is based on 140 fb-1 of datacollected with the ATLAS detector at the Large HadronCollider from proton-proton collisions at a center-of-massenergy of 13 TeV. Events with collimated pairs of electronsor muons are analysed and background contributions are estimatedusing data-driven techniques. No significant excess inthe data above the Standard Model background is observed.Upper limits are set at 95% confidence level on the branchingratio of theHiggs boson decay into dark photons between0.001% and 5%, depending on the assumed dark photon mass and signal model.