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

Abstract

We present the conceptual design and the physics potential of DarkSPHERE, a proposed 3 m in diameter spherical proportional counter electroformed underground at the Boulby Underground Laboratory. This effort builds on the R&D performed and experience acquired by the NEWS-G Collaboration. DarkSPHERE is primarily designed to search for nuclear recoils from light dark matter in the 0.05-10 GeV mass range. Electroforming the spherical shell and the implementation of a shield based on pure water ensures a background level below 0.01 dru. These, combined with the proposed helium-isobutane gas mixture, will provide sensitivity to the spin-independent nucleon cross-section of 2 × 10 −41 (2 × 10 −43 ) cm 2 for a dark matter mass of 0.1 (1) GeV. The use of a hydrogen-rich gas mixture with a natural abundance of 13 C provides sensitivity to spin-dependent nucleon cross-sections more than two orders of magnitude below existing constraints for dark matter lighter than 1 GeV. The characteristics of the detector also make it suitable for searches of other dark matter signatures, including scattering of MeV-scale dark matter with electrons, and superheavy dark matter with masses around the Planck scale that leave extended ionisation tracks in the detector.