# Direct Detection of Spin-(In)dependent Nuclear Scattering of Sub-GeV Dark Matter Using Molecular Excitations

## TL;DR

This paper introduces a new method for detecting sub-GeV dark matter through molecular excitations in gases, producing infrared photons detectable by sensitive photodetectors, enabling exploration of previously inaccessible dark matter parameter space.

## Contribution

It proposes a novel detection technique using molecular excitations and photon emission, sensitive to both spin-dependent and independent interactions, with practical implementation using existing technology.

## Key findings

- Detects dark matter via vibrational and rotational excitations in molecules.
- Produces multi-infrared-photon signals observable with ultrasensitive detectors.
- Enables exploration of new dark matter parameter space with near-term technology.

## Abstract

We propose a novel direct detection concept to search for dark matter with 100~keV to 100~MeV masses. Such dark matter can scatter off molecules in a gas and transfer an $\mathcal{O}(1)$ fraction of its kinetic energy to excite a vibrational and rotational state. The excited ro-vibrational mode relaxes rapidly and produces a spectacular multi-infrared-photon signal, which can be observed with ultrasensitive photodetectors. We discuss in detail a gas target consisting of carbon monoxide molecules, which enable efficient photon emission even at a relatively low temperature and high vapor pressure. The emitted photons have an energy in the range 180~meV to 265~meV. By mixing together carbon monoxide molecules of different isotopes, including those with an odd number of neutrons, we obtain sensitivity to both spin-independent interactions and spin-dependent interactions with the neutron. We also consider hydrogen fluoride, hydrogen bromide, and scandium hydride molecules, which each provide sensitivity to spin-dependent interactions with the proton. The proposed detection concept can be realized with near-term technology and allows for the exploration of orders of magnitude of new dark matter parameter space.