Abstract
We present a first calculation of the rate for plasmon production in semiconductors from nuclei recoiling against dark matter. The process is analogous to bremsstrahlung of transverse photon modes, but with a longitudinal plasmon mode emitted instead. For dark matter in the 10 MeV - 1 GeV mass range, we find that the plasmon bremsstrahlung rate is 4-5 orders of magnitude smaller than that for elastic scattering, but 4-5 orders of magnitude larger than the transverse bremsstrahlung rate. Because the plasmon can decay into electronic excitations and has characteristic energy given by the plasma frequency $\omega_p$, with $\omega_p \approx 16$ eV in Si crystals, plasmon production provides a distinctive signature and new method to detect nuclear recoils from sub-GeV dark matter.
Highlights
There have been significant efforts recently to directly detect dark matter (DM) in the low-mass regime [1]
With μNχ the nucleus-DM reduced mass. This is identical to the threshold velocity for inelastic DM scattering with mass splitting δ 1⁄4 ωp. (We have neglected the weak dispersion in the plasmon mode to simplify the velocity integral.) In order to estimate the effects of the k-dependent dispersion and wave function renormalization, the rate is computed from Eq (8) using the results of Ref. [34] for ωLðkÞ, ZLðkÞ
As for direct detection, bosonic dark matter can be absorbed into plasmon modes [8,50,71]
Summary
There have been significant efforts recently to directly detect dark matter (DM) in the low-mass (sub-GeV) regime [1]. The recoiling ion is a current source and can lose energy into both transverse photon and longitudinal plasmon modes With these approximations, we find that the rate for plasmon production through the process in Eq (1) is typically 4–5 orders of magnitude smaller than the elastic nuclear recoil rate, and cannot explain the excesses studied in Ref. [21] involved a plasmon produced in association with many phonons, and is not captured by our approach.) bremsstrahlung emission of plasmons by a recoiling nucleus is a novel signature of dark matter scattering in semiconductor targets, and we find that the corresponding rate is around 5 orders of magnitude larger than that for bremsstrahlung emission of transverse modes.
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