Probing displaced (dark)photons from low reheating freeze-in at the LHC

TL;DR

This paper proposes a model extending the Standard Model with a dark photon and pseudo-scalar mediator, exploring their freeze-in production at low reheating temperatures and potential detection via displaced photons at the LHC.

Contribution

It introduces a novel dark matter scenario with a low reheating freeze-in mechanism and connects cosmological relic abundance with LHC search constraints.

Findings

Dark photon relic abundance can be achieved via low reheating freeze-in.

LHC searches for displaced photons constrain the Higgs portal coupling.

Excluded parameter space includes thermalized mediators compatible with observed relic abundance.

Abstract

We extend the Standard Model (SM) by introducing a $U(1)'$ gauge boson and a real pseudo-scalar field, both odd under a $\mathbb{Z}_2$ symmetry. The resulting low-energy spectrum consists of a stable vector as the dark matter candidate, and a pseudo-scalar mediator, which interacts with the SM via a Higgs portal coupling and a dimension-five portal connecting it to both the dark and visible photons. We explore the freeze-in of both particles at low reheating temperature, finding a rich yield evolution dynamics in the early Universe. This setup brings a consistent dark matter scenario in which the dark photon relic abundance is generated through freeze-in at low reheating temperatures. In addition to its cosmological viability, the model can be tested at the LHC: Higgs bosons can decay into dark photons and displaced visible photons via the long-lived mediator. These signatures allow us to constrain the Higgs portal coupling using recent searches for non-pointing photons and limits on invisible or undetected Higgs decays. We derive meaningful constraints on the dark matter parameter space, in particular excluding a thermalized mediator in the region compatible with the observed relic abundance.