Abstract

A search for weakly interacting massive dark-matter particles produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and missing transverse momentum are considered. The analysis uses 36.1 ; mathrm {fb}^{-1} of proton–proton collision data recorded by the ATLAS experiment at sqrt{s}=13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are interpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour-neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross-section of 300 times the predicted rate for mediators with masses between 10 and 50; hbox {GeV} and assuming a dark-matter mass of 1 ;hbox {GeV} and unitary coupling. Constraints on colour-charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35; hbox {GeV}, mediator particles with mass below 1.1; hbox {TeV} are excluded for couplings yielding a dark-matter relic density consistent with measurements.

Highlights

  • Astrophysical observations have provided compelling evidence for the existence of a non-baryonic dark component of the universe: dark matter (DM) [1,2]

  • The searches presented in this paper focus on the case of a fermionic DM particle produced through the exchange of a spin-0 mediator, which can be either a colour-neutral scalar or pseudoscalar particle or a colour-charged scalar mediator

  • The background-only fit to the control regions described in Sect. 5 is compared to the predictions based on the Monte Carlo (MC) normalisation

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Summary

Introduction

Astrophysical observations have provided compelling evidence for the existence of a non-baryonic dark component of the universe: dark matter (DM) [1,2]. At the large hadron collider (LHC), one can search for WIMP DM (χ ) pair production in pp collisions. The SM backgrounds contributing to each of the five SRs are estimated with the aid of the MC simulation and using control regions (CRs) constructed to enhance a particular background and to be kinematically similar but orthogonal to the SRs. The expected background is determined separately in each SR through a profile likelihood fit based on the HistFitter package [96]. The number of background events predicted by simulation in the SRs is normalised according to the results of the fit. The background estimates in the SRs are validated by extrapolating the results of the likelihood fit in the CRs to dedicated validation regions (VRs), which are designed to be orthogonal to both the signal and control regions. In all CRs and VRs used in this analysis the signal contamination was found to be negligible

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