Projected sensitivity to sub-GeV dark matter of next-generation semiconductor detectors

We compute the projected sensitivity to dark matter (DM) particles in the sub-GeV mass range of future direct detection experiments using germanium and silicon semiconductor targets. We perform this calculation within the dark photon model for DM-electron interactions using the likelihood ratio as a test statistic, Monte Carlo simulations, and background models that we extract from recent experimental data. We present our results in terms of DM-electron scattering cross section values required to reject the background only hypothesis in favour of the background plus DM signal hypothesis with a statistical significance, Z, corresponding to 3 or 5 standard deviations. We also test the stability of our conclusions under changes in the astrophysical parameters governing the local space and velocity distribution of DM in the Milky Way. In the best-case scenario, when a high-voltage germanium detector with an exposure of 50 kg-year and a CCD silicon detector with an exposure of 1 kg-year and a dark current rate of 1×10^-7 counts/pixel/day have simultaneously reported a DM signal, we find that the smallest cross section value compatible with Z=3 (Z=5) is about 4×10^-42 cm² (6×10^-42 cm²) for contact interactions, and 2.5×10^-41 cm² (4×10^-41 cm²) for long-range interactions. Our sensitivity study extends and refine previous works in terms of background models, statistical methods, and treatment of the underlying astrophysical uncertainties.