Detecting Dark Matter with Far-Forward Emulsion and Liquid Argon Detectors at the LHC

TL;DR

This paper explores the potential of far-forward detectors at the LHC, including emulsion and liquid argon types, to detect dark matter produced via dark photons, showing they could observe hundreds to thousands of events and probe key parameter space.

Contribution

It introduces and evaluates the use of emulsion and liquid argon detectors at the LHC for dark matter detection, demonstrating their sensitivity to dark photon decay products.

Findings

Detectors can detect hundreds to thousands of dark matter events.

Effective background separation techniques are devised for these detectors.

Results motivate constructing such detectors at the LHC, like at the proposed Forward Physics Facility.

Abstract

New light particles may be produced in large numbers in the far-forward region at the LHC and then decay to dark matter, which can be detected through its scattering in far-forward experiments. We consider the example of invisibly-decaying dark photons, which decay to dark matter through $A' \to \chi \chi$. The dark matter may then be detected through its scattering off electrons $\chi e^- \to \chi e^-$. We consider the discovery potential of detectors placed on the beam collision axis 480 m from the ATLAS interaction point, including an emulsion detector (FASER$\nu$2) and, for the first time, a Forward Liquid Argon Experiment (FLArE). For each of these detector technologies, we devise cuts that effectively separate the single $e^-$ signal from the leading neutrino- and muon-induced backgrounds. We find that 10- to 100-tonne detectors may detect hundreds to thousands of dark matter events in the HL-LHC era and will sensitively probe the thermal relic region of parameter space. These results motivate the construction of far-forward emulsion and liquid argon detectors at the LHC, as well as a suitable location to accommodate them, such as the proposed Forward Physics Facility.