Hydrogenated carbon structures as directional sub-GeV dark matter detectors

TL;DR

This work tackles the challenge of detecting sub-GeV dark matter by proposing hydrogenated carbon structures—graphene, graphite, and carbon nanotube forests—as ultra-low-threshold DM–nucleon targets. It develops a benchmark heavy-mediator, spin-independent DM–proton contact interaction and computes the ejection rate of protons from bound hydrogen in the lattice, using DFT to estimate the naked-proton ejection probability and standard halo-model kinematics. The results indicate that 2D graphene can achieve sensitivity far surpassing current bounds, with a minimal detectable DM mass around 1 MeV, while 3D CNT forests offer larger target mass and strong directional modulation, subject to proton-transport effects. The detector concept is inexpensive, scalable, and operational at room temperature, with practical validation paths and background considerations, opening a new avenue for MeV-scale dark matter searches.

Abstract

We propose hydrogenated carbon structures as targets with a remarkable sensitivity to dark matter-nucleon interactions, in the mass range between the 1 MeV and 100 MeV. The ejection of a proton following the interaction with a dark matter particle is a quasi-elastic process, with an extremely small energy threshold, and a clear experimental signature. The proposed detectors are simple, technologically ready, and inexpensive. Yet, they can be considerably more sensitive than current experiments. They also allow strong directionality, to be used towards efficient background rejection.

Figures (3)

• Figure 1: Left and central panels: Schematic representation of the hydrogenated carbon structures at 100% loading, and a pictorial dark matter interaction on a hydrogen nucleus, resulting in the ejection of a proton. Dark and light spheres represent respectively carbon and hydrogen atoms, with the latter being covalently bound to their carbon site. Right panel: Detector concept. After its ejection, the proton is accelerated by an electric field, and its trajectory is focused by shaping electrodes, until it is detected by an SSD.
• Figure 2: Projected reach at 90% C.L., assuming no background. The target mass corresponding to graphene of area $100 \text{ cm}^2$ and $1 \text{ m}^2$ is, respectively, $M_{\rm H} = 0.66$ and $66 \text{ }\mu\text{g}$. For CNTs of area $100 \text{ cm}^2$ and $1 \text{ m}^2$ it is, instead, $M_{\rm H} = 0.84$ and $84$ mg. The gray shaded areas correspond to currently excluded region, as obtained using the Migdal effect, whose leading bounds are set by SENSEI SENSEI:2023zdf and PandaX-4T PandaX-4T_2023. The reference cross section is defined as $\bar{\sigma}_p \equiv g_\chi^2 \mkern2mu g_p^2 \mkern2mu \mu_{\chi p}^2/(\pi \mkern1mu m_\phi^4)$.
• Figure 3: Probability for a proton to leave from the top side of the array of CNTs as a function of the dark matter mass, computed for a dark matter wind parallel ( red curve) and anti-parallel ( blue curve) to the orientation of the CNTs.