Search for dark matter produced in association with bottom or top quarks in $$\sqrt{s}=13$$s=13 TeV pp collisions with the ATLAS detector

Abstract

A search for weakly interacting massive darkmatter 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 fb −1 of proton-proton collision data recorded by the ATLAS experiment at √ 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 darkmatter 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 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour-charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements. IntroductionAstrophysical observations have provided compelling evidence for the existence of a non-baryonic dark component of the universe: dark matter (DM) [1,2]. The currently most accurate, although somewhat indirect, determination of DM abundance comes from global fits of cosmological parameters to a variety of observations [3,4], while the nature of DM remains largely unknown. One of the candidates for a DM particle is a weakly interacting massive particle (WIMP) [5]. e-mail: atlas.publications@cern.ch At the large hadron collider (LHC), one can search for WIMP DM (χ ) pair production in pp collisions. WIMP DM would not be detected and its production leads to signatures with missing transverse momentum. Searches for the production of DM in association with Standard Model (SM) particles have been performed at the LHC [6][7][8][9][10][11][12].Recently proposed simplified benchmark models for DM production assume the existence of a mediator particle which couples both to the SM and to the dark sector [13][14][15]. 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 (denoted by φ or a, respectively) or a colour-charged scalar mediator (φ b ). The couplings of the mediator to the SM fermions are severely restricted by precision flavour measurements. An ansatz that automatically relaxes these constraints is Minimal Flavour Violation [16]. This assumption implies that the interaction between any new neutral spin-0 state and SM matter is proportional to the fermion masses via Yukawa-type couplings.1 It follows that colour-neutral mediators would be sizeably produced through lo...

Searching for production of dark matter in association with top quarks at the LHC

“…A reliable estimate of these backgrounds depends on a detailed simulation of the detector effect beyond the scope of this paper. This simplification is motivated by the results of the ATLAS analysis [42], which addresses kinematic configurations similar to the ones targeted in the present study, and finds that the background from non-prompt leptons does not exceed the level of 15% of the total background. The backgrounds from tt [43], tW [44], WW, WZ and ZZ production [45,46] were all generate at NLO with POWHEG BOX [47].…”

“…Since the amount of E miss T is typically larger for the signal due to the undetected DM and the m T 2 observable [56,57] has a sharp edge at M W in the case of tt production, these two variables provide the main handles to suppress the leading SM backgrounds. An analysis based on E miss T and m T 2 has been shown in [10,42] to achieve a good separation of the tt + E miss T signal from the tt background, whereas a part of ttZ is irreducible. Applying the same strategy to tW + E miss T production would also allow to separate the signal from the tt background, but, due to the lower cross section, it would be difficult to extract the tW + E miss T signal from the irreducible ttZ background.…”

Simplified dark matter models with loop effects in direct detection and the constraints from indirect detection and collider search *

“…The Large Hadron Collider (LHC) performed the search for DM in association with energetic jet [45,46] or the third generation quarks [47][48][49] for simplified DM models with spin-0 mediator at √ s = 13 TeV collisions. The most severe limits are from final states with tt and missing transverse momentum [50].…”

The pursuit of dark matter at colliders—an overview

“…The background processes are estimated using signal depleted control regions in data and MC simulations. Because the missing transverse momentum provides less discrimination power in heavy quark searches, distributions such as the opening angle between the b-quarks, or the angle between E / T and b-quarks are used [138]. The CMS collaboration also uses a kinematic fit to the top quark and W bosons masses to reconstruct jet momenta, energy, and resolutions.…”

“…[21,145,146] they are implemented in EFTs and simplified models. Most [42], represented by the blue contour, and the ATLAS search for direct sbottom pair production [142], represented by the red contour [138]. simplified mono-H models either invoke a Z type vector mediator in which the Z radiates the SM-like Higgs boson or a Z − 2HDM model where the dark matter couples to a heavy pseudo-scalar particle in an extended Higgs boson sector.…”