Erratum: Coherent photon scattering background in sub- GeV/c2 direct dark matter searches [Phys. Rev. D 95 , 021301(R) (2017)]

Abstract

Proposed dark matter detectors with eV-scale sensitivities will detect a large background of atomic (nuclear) recoils from coherent photon scattering of MeV-scale photons. This background climbs steeply below ∼10 eV, far exceeding the declining rate of low-energy Compton recoils. The upcoming generation of dark matter detectors will not be limited by this background, but further development of eV-scale and sub-eV detectors will require strategies, including the use of low nuclear mass target materials, to maximize dark matter sensitivity while minimizing the coherent photon scattering background.PACS numbers: 13.60. Fz, 95.35.+d Interest in sub-GeV/c 2 mass thermal relic dark matter models has inspired ideas for direct detection experiments with eV-scale and sub-eV thresholds [1]. For such light dark matter, the recoil energy differential scattering rate is restricted to energies below the detection thresholds of direct detection experiments motivated by weakscale and supersymmetric models [2,3].Penetrating MeV-scale photons are a background for all of these experiments with different mechanisms at high and low recoil energies. Incoherent (Compton) scattering from electrons is suppressed when insufficient energy is deposited to excite a bound electron [4], or when the energy range of interest is narrow compared to typical MeV scale energy depositions. This dominant mechanism for photon backgrounds in most existing direct detection experiments has been considered negligible for future eV-scale and sub-eV experiments [1]. In contrast, the coherent scattering of neutral particles, such as coherent neutrino scattering [5] or coherent dark matter scattering, produces an enhanced spectrum of low-energy recoils. Coherent photon scattering across an atom produces a low-energy background spectrum that may overwhelm low threshold dark matter detectors.A photon with energy E γ = c p γ ≈ 1 MeV scattering at angle 0 ≤ θ ≤ π from an atom with mass M ≈ 10 GeV/c 2 , will transfer a small momentum q and recoil energy E r .The differential coherent scattering cross section is also strongly suppressed when coherence is lost for q >h/a B = 3.7 keV/c, where a B is the Bohr radius. This small energy deposition can be safely ignored for most applications, but not for upcoming dark matter searches. The angle differential cross section for coherent photon scattering and its effects on photon transport have * fbfree@fnal.gov

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

“…2, we learn that numerous low energy depositions that can mimic DM signals are expected, even in a well-shielded environment. The low-energy background event rates are consistent with those in [51]. For example, in silicon (germanium) we expect N bk = 23 (93) total background events in the energy range 1−100 meV.…”

“…We have confirmed that the same is true for all materials included in our analysis. Thus, the dominant process in partially coherent photon-ion scattering with energy transfers 100 meV is well described by the photonelectron Rayleigh scattering cross section (in agreement with [51]),…”

“…One promising such direction is the use of semiconductor detectors, which can be sensitive to light DM in the 10 keV − 100 MeV mass range through electron and phonon signals (see, e.g., [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50]). Characterizing, quantifying, and mitigating the backgrounds in these experiments is a crucial task to ensure their success [51][52][53].…”

“…High-energy (∼ 100 keV − few MeV) photons are produced in DM detectors from the decay of radioactive impurities in the detector or shielding material. These photons have a large scattering cross-section at small momentum transfers [51] and can scatter in the target material to produce a single-or multi-phonon excitation, mimicking a low-mass DM signal. Such low-energy (∼ 1 − 100 meV) excitations could be within the reach of upcoming technologies such as transition edge sensors and form the basis of R&D efforts for detecting sub-GeV DM.…”

First results from a search for coherent elastic neutrino-nucleus scattering (CE$ν$NS) at a reactor site

“…Special attention was paid to the possibility that coherent photon scattering from any energetic gammas able to reach the PPC might produce low-energy NRs able to compete with the CEνNS signal. This process has been proposed as a possible background affecting searches for low-mass dark matter candidates [44,45]. Again, we find that its contribution to the ROI is more than three orders of magnitude lower than the present background level, which is dominated by elastic scattering from epithermal neutrons.…”