The Phonon Background from Gamma Rays in Sub-GeV Dark Matter Detectors

Abstract

High-energy photons with O(MeV) energies from radioactive contaminants can scatter in a solidstate target material and constitute an important low-energy background for sub-GeV dark matter direct-detection searches. This background is most noticeable for energy deposits in the 1−100 meV range due to the partially coherent scattering enhancement in the forward scattering direction. We comprehensively quantify the resulting single-and multi-phonon background in Si, Ge, GaAs, SiC, and Al2O3 target materials, which are representative of target materials of interest in low-mass dark matter searches. We use a realistic representation of the high-energy photon background, and contrast the expected background phonon spectrum with the expected dark matter signal phonon spectrum. An active veto is needed to suppress this background sufficiently in order to allow for the detection of a dark matter signal, even in well-shielded environments.

Dark matter direct detection from the single phonon to the nuclear recoil regime

“…with k the unit vector along the phonon propagation direction. Note that the r 0 2 dependence was neglected in the LA eigenvector in (27) and in [24]; here we have kept this additional phase so that the acoustic and optical eigenvectors are explicitly orthogonal across a unit cell. This additional phase factor will only be relevant in cases where there is a destructive interference in the leading coupling to acoustic phonons, which occurs for some final states [13].…”

“…A similar expansion in number of modes has been performed previously for the integrable toy model that is the harmonic oscillator [25]. Here we have generalized the approach to a harmonic crystal, analogous to the procedure followed in [26] and [27], in calculations of the Migdal effect and X-ray backgrounds, respectively. Fig.…”

Revisiting the Dark Matter Interpretation of Excess Rates in Semiconductors

“…is the weak charge of a nucleus with N neutrons and Z protons, and we have given the numerical values for silicon (ω max = 140 eV). For the photon cross section, we have assumed coherent Thomson scattering from X-ray or gamma-ray photons with E γ m N , which dominates over other photonuclear processes such as Delbrück scattering and resonant scattering [39]. For the neutrino cross section we have assumed coherent scattering with ω E ν and sufficiently low momentum transfer that the nuclear form factor is approximately unity, a reasonable approximation even for elastic scattering in the energy range relevant for the excesses.…”

Snowmass white paper: Light dark matter direct detection at the interface with condensed matter physics

“…On the particle physics side, effective field theory (EFT) provides the necessary framework and tools for both systematically exploring the DM theory space and matching onto target responses [2,64,65]. Accurate characterization of background for low-threshold detectors is also essential, and there has been rapid progress on this subject recently [66,67].…”