Search for an invisibly decaying Higgs boson or dark matter candidates produced in association with a $Z$ boson in $pp$ collisions at $\sqrt{s} =$ 13 TeV with the ATLAS detector

TL;DR

This paper reports a search for invisibly decaying Higgs bosons or dark matter particles produced with a Z boson at 13 TeV, setting upper limits on their production and decay rates based on ATLAS data.

Contribution

It provides the first limits on invisible Higgs decays and dark matter production in association with a Z boson at 13 TeV using ATLAS data.

Findings

No significant excess over Standard Model backgrounds.

Upper limit of 67% on Higgs invisible decay branching ratio.

Exclusion limits on dark matter and mediator masses.

Abstract

A search for an invisibly decaying Higgs boson or dark matter candidates produced in association with a leptonically decaying $Z$ boson in proton--proton collisions at $\sqrt{s} =$ 13 TeV is presented. This search uses 36.1 fb$^{-1}$ of data collected by the ATLAS experiment at the Large Hadron Collider. No significant deviation from the expectation of the Standard Model backgrounds is observed. Assuming the Standard Model $ZH$ production cross-section, an observed (expected) upper limit of 67% (39%) at the 95% confidence level is set on the branching ratio of invisible decays of the Higgs boson with mass $m_H = $ 125 GeV. The corresponding limits on the production cross-section of the $ZH$ process with the invisible Higgs boson decays are also presented. Furthermore, exclusion limits on the dark matter candidate and mediator masses are reported in the framework of simplified dark matter models.