Extending LHC Coverage to Light Pseudoscalar Mediators and Coy Dark Sectors

TL;DR

This paper explores the potential for the LHC to detect light pseudoscalar mediators related to dark matter models, especially in the 20-80 GeV mass range, by analyzing associated production with bottom quarks without relying on missing energy signatures.

Contribution

It proposes new search strategies for light pseudoscalars at the LHC in the 20-80 GeV range, extending beyond current experimental capabilities and covering models explaining the Galactic Center gamma-ray excess.

Findings

LHC can probe pseudoscalars in the 20-80 GeV range via associated production with bottom quarks.

A detailed analysis of backgrounds and trigger efficiencies supports the feasibility of these searches.

Significant parameter space can be tested with 100 fb$^{-1}$ at 13 TeV, encouraging experimental extension.

Abstract

Many dark matter models involving weakly interacting massive particles (WIMPs) feature new, relatively light pseudoscalars that mediate dark matter pair annihilation into Standard Model fermions. In particular, simple models of this type can explain the gamma ray excess originating in the Galactic Center as observed by the Fermi Large Area Telescope. In many cases the pseudoscalar's branching ratio into WIMPs is suppressed, making these states challenging to detect at colliders through standard dark matter searches. Here, we study the prospects for observing these light mediator states at the LHC without exploiting missing energy techniques. While existing searches effectively probe pseudoscalars with masses between 5 - 14 GeV and above 90 GeV, the LHC reach can be extended to cover much of the interesting parameter space in the intermediate 20 - 80 GeV mass range in which the mediator can have appreciable Yukawa-like couplings to Standard Model fermions but would have escaped detection by LEP and other experiments. Models explaining the Galactic Center excess via a light pseudoscalar mediator can give rise to a promising signal in this regime through the associated production of the mediator with bottom quarks while satisfying all other existing constraints. We perform an analysis of the backgrounds and trigger efficiencies, detailing the cuts that can be used to extract the signal. A significant portion of the otherwise unconstrained parameter space of these models can be conclusively tested at the 13 TeV LHC with 100 fb$^{-1}$, and we encourage the ATLAS and CMS collaborations to extend their existing searches to this mass range.