Fig. 5: Emergence of plasmon with β and mV.
a shows the differential cross section per atom, weighted by β2, for a dark matter particle with \({\bar{\sigma }}_{e}=1{0}^{-35}\,{{{{{\rm{cm}}}}}}^{2}\) interacting with a silicon via a light mediator. Different colors indicate the speed of the incoming dark matter particle. The plasmon peak appears in the differential cross-section as the speed of the dark-matter particle is increased, and dominates the cross section for β ≳ 0.01. b shows the differential cross section per atom for a boosted dark matter particle with β = 1 for various mediator masses, mV. The dark matter particles interacting via lighter mediators have a cross section that is dominated by the plasmon whenever mV ≲ kF. For mV → 0, the cross section agrees with that for millicharged-particle scattering shown in Fig. 3. These plots are made with the Lindhard approximation for the silicon dielectric function.
