Fuelling the search for light dark matter-electron scattering with spherical proportional counters

Abstract

Dark matter (DM) detectors employing a Spherical Proportional Counter (SPC) have demonstrated a single-electron detection threshold and are projected to have small background rates. We explore the sensitivity to DM-electron scattering with SPC detectors in the context of DarkSphere, a proposal for a 300 cm diameter fully-electroformed SPC. SPCs can run with different gases, so we investigate the sensitivity for five targets: helium, neon, xenon, methane, and isobutane. We use tools from quantum chemistry to model the atomic and molecular systems, and calculate the expected DM induced event rates. We find that DarkSphere has the potential to improve current exclusion limits on DM masses above 4 MeV by up to five orders of magnitude. Neon is the best all-round gas target but using gas mixtures, where methane and isobutane constitute 10% of the gas, can improve the sensitivity, especially when combined with helium. Our study highlights the currently untapped potential of SPCs to search for DM-electron scattering in the MeV-to-GeV DM mass range.

DarkSPHERE: Exploring light dark matter with a spherical proportional counter electroformed underground at the Boulby Underground Laboratory

“…We follow the simplified treatment described in Ref. [42] to model the detector response to the electron that has been ionised through the Migdal effect. The solid lines in Figure 11 show the leading constraints on the SI DMnucleon cross-section derived using the Migdal effect.…”

“…The single ionisation electron threshold of the spherical proportional counter, which is possible because of the small detector capacitance and high gain operation, allows for the possibility of searching for DM-electron interactions [42]. 1 The kinematics of DM-electron scattering in atoms and molecules mean that this search channel can probe DM candidates in the mass range from approximately 5 MeV to 1 GeV [147].…”

“…We use the dimensionless ionisation form factors for helium and isobutance provided in Ref. [42] to calculate the signal rate. For the detector response, we follow the simplified treatment described in Ref.…”

“…For the detector response, we follow the simplified treatment described in Ref. [42] and use a 1 (28) eV threshold on the electron kinetic energy as a proxy for the single (n e − ≥ 1) and double (n e − ≥ 2) electron threshold. The F DM = (αm e /q) 2 scenario is particularly sensitive to the threshold so the sensitivity in this case decreases rapidly as the threshold is increased.…”

“…The DarkSPHERE design presented in this article is principally optimised to search for the light DMnucleon interaction. However, the characteristics of the DarkSPHERE detector provide for a multi-physics platform: the spherical proportional counter's singleelectron threshold enables excellent sensitivity to the DM-electron interaction in the 10 MeV to GeV mass range [42]; and the large diameter of the spherical proportional counter enables sensitivity to ultra-heavy DM with masses close to the Planck mass [43]. Furthermore, in addition to DM searches, the energy resolution and light-readout capabilities of a large spherical proportional counter filled with 136 Xe gas lends itself to a neutrinoless double β-decay search, which is being explored by the Rare Decays with Radial Detector (R2D2) R&D effort [44,45], and as a potential tool for supernova neutrino searches [46,47].…”