Search for neutral long-lived particles in $pp$ collisions at $\sqrt{s}=13$ TeV that decay into displaced hadronic jets in the ATLAS calorimeter

TL;DR

This paper reports a search for neutral long-lived particles decaying into displaced hadronic jets in the ATLAS calorimeter, setting new limits on their production and decay properties based on 139 fb$^{-1}$ of LHC data.

Contribution

It introduces specialized reconstruction techniques for displaced jets and provides the first limits on long-lived particles decaying in the ATLAS calorimeter at 13 TeV.

Findings

No significant excess observed over background.

Excludes Higgs boson branching ratios above 10% for certain lifetimes.

Sets upper limits on cross-section times branching ratio for various scalar masses.

Abstract

A search for decays of pair-produced neutral long-lived particles (LLPs) is presented using 139 fb$^{-1}$ of proton-proton collision data collected by the ATLAS detector at the LHC in 2015-2018 at a centre-of-mass energy of 13 TeV. Dedicated techniques were developed for the reconstruction of displaced jets produced by LLPs decaying hadronically in the ATLAS hadronic calorimeter. Two search regions are defined for different LLP kinematic regimes. The observed numbers of events are consistent with the expected background, and limits for several benchmark signals are determined. For a SM Higgs boson with a mass of 125 GeV, branching ratios above 10% are excluded at 95% confidence level for values of $c$ times LLP mean proper lifetime in the range between 20 mm and 10 m depending on the model. Upper limits are also set on the cross-section times branching ratio for scalars with a mass of 60 GeV and for masses between 200 GeV and 1 TeV.