Constraining Light Thermal Inelastic Dark Matter with NA64

TL;DR

This paper uses the NA64 experiment to set new constraints on inelastic dark matter models with a vector portal, exploring parameter space relevant for relic density and muon g-2 anomaly explanations.

Contribution

It introduces a recast analysis of NA64 data to constrain inelastic dark matter models with a vector mediator, expanding the experimental bounds in the sub-GeV mass range.

Findings

NA64 places new limits on inelastic dark matter models.

The analysis extends the exclusion region in parameter space.

Future data could further probe semi-visible dark sector particles.

Abstract

A vector portal between the Standard Model and the dark sector is a predictive and compelling framework for thermal dark matter. Through co-annihilations, models of inelastic dark matter (iDM) and inelastic Dirac dark matter (i2DM) can reproduce the observed relic density in the MeV to GeV mass range without violating cosmological limits. In these scenarios, the vector mediator behaves like a semi-visible particle, evading traditional bounds on visible or invisible resonances, and uncovering new parameter space to explain the muon $(g-2)$ anomaly. By means of a more inclusive signal definition at the NA64 experiment, we place new constraints on iDM and i2DM using a missing energy technique. With a recast-based analysis, we contextualize the NA64 exclusion limits in parameter space and estimate the reach of the newly collected and expected future NA64 data. Our results motivate the development of an optimized search program for semi-visible particles, in which fixed-target experiments like NA64 provide a powerful probe in the sub-GeV mass range.