# Luminous Signals of Inelastic Dark Matter in Large Detectors

## TL;DR

This paper explores how large underground detectors can detect inelastic dark matter through modulating photon signals resulting from excited state decays, offering new detection strategies that outperform traditional methods for certain mass splittings.

## Contribution

It introduces the concept of using large scintillator detectors to identify inelastic dark matter via sidereal modulation of photon signals, highlighting their potential advantages over existing detection techniques.

## Key findings

- Borexino and JUNO can probe inelastic dark matter for mass splittings above 180-240 keV.
- CYGNUS's sensitivity is comparable or better than xenon detectors for certain mass splittings.
- Inelastic signals can surpass elastic scattering in dark matter searches with larger detector volumes.

## Abstract

We study luminous dark matter signals in models with inelastic scattering. Dark matter $\chi_1$ that scatters inelastically off elements in the Earth is kicked into an excited state $\chi_2$ that can subsequently decay into a monoenergetic photon inside a detector. The photon signal exhibits large sidereal-daily modulation due to the daily rotation of the Earth and anisotropies in the problem: the dark matter wind comes from the direction of Cygnus due to the Sun's motion relative to the galaxy, and the rock overburden is anisotropic, as is the dark matter scattering angle. This allows outstanding separation of signal from backgrounds. We investigate the sensitivity of two classes of large underground detectors to this modulating photon line signal: large liquid scintillator neutrino experiments, including Borexino and JUNO, and the proposed large gaseous scintillator directional detection experiment CYGNUS. Borexino's (JUNO's) sensitivity exceeds the bounds from xenon experiments on inelastic nuclear recoil for mass splittings $\delta \gtrsim 240 (180)$ keV, and is the only probe of inelastic dark matter for ${350 \text{ keV} \lesssim \delta \lesssim 600 \text{ keV}}$. CYGNUS's sensitivity is at least comparable to xenon experiments with $\sim 10 \; {\rm m}^3$ volume detector for $\delta \lesssim 150$ keV, and could be substantially better with larger volumes and improved background rejection. Such improvements lead to the unusual situation that the inelastic signal becomes the superior way to search for dark matter even if the elastic and inelastic scattering cross sections are comparable.