Search for Higgs boson decays to beyond-the-Standard-Model light bosons in four-lepton events with the ATLAS detector at $\sqrt{s}=13$ TeV

TL;DR

This paper searches for signs of new light bosons in Higgs decays to four leptons at the LHC, setting limits on their possible existence and decay rates, but finds no significant evidence for them.

Contribution

It introduces a novel search for beyond-the-Standard-Model light bosons in Higgs decays using ATLAS data, establishing new upper limits on their production and decay.

Findings

No significant excess observed above background

Set 95% CL upper limits on decay branching ratios

Constrained models with light vector and pseudoscalar bosons

Abstract

A search is conducted for a new beyond-the-Standard-Model boson using events where a Higgs boson with mass 125 GeV decays to four leptons ($\ell =$ $e$ or $\mu$). This decay is presumed to occur via an intermediate state which contains one or two on-shell, promptly decaying bosons: $H\rightarrow ZX/XX \rightarrow 4\ell$, where $X$ is a new vector boson $Z_d$ or pseudoscalar $a$ with mass between 1 and 60 GeV. The search uses $pp$ collision data collected with the ATLAS detector at the LHC with an integrated luminosity of 36.1 fb$^{-1}$ at a centre-of-mass energy $\sqrt{s}=13$ TeV. No significant excess of events above Standard Model background predictions is observed; therefore, upper limits at 95% confidence level are set on model-independent fiducial cross-sections, and on the Higgs boson decay branching ratios to vector and pseudoscalar bosons in two benchmark models.