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, $\mathcal{Z}$, 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 $1\times10^{-7}$
counts/pixel/day have simultaneously reported a DM signal, we find that the
smallest cross section value compatible with $\mathcal{Z}=3$ ($\mathcal{Z}=5$)
is about $8\times10^{-42}$ cm$^2$ ($1\times10^{-41}$ cm$^2$) for contact
interactions, and $4\times10^{-41}$ cm$^2$ ($7\times10^{-41}$ cm$^2$) 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.
