Shielding of a direct detection experiment and implications for the DAMA annual modulation signal

Abstract

Previous work has argued that, in the framework of plasma dark matter models, the DAMA annual modulation signal can be consistently explained with electron recoils. In the specific case of mirror dark matter, that explanation requires an effective low velocity cutoff, v c 30, 000 km/s, for the halo mirror electron distribution at the detector. We show here that this cutoff can result from collisional shielding of the detector from the halo wind due to Earth-bound dark matter. We also show that shielding effects can reconcile the kinetic mixing parameter value inferred from direct detection experiments with the value favoured from small scale structure considerations, ǫ ≈ 2 × 10 −10 .

First direct detection constraint on mirror dark matter kinetic mixing using LUX 2013 data

“…Therefore, using v rot ≈ 220 kms −1 and assuming the halo is in hydrostatic equilibrium, local mirror electron temperature ∼0.3 keV is expected. In such plasma dark matter models, it is important to consider capture of the dark matter by the Earth [17].…”

“…Here we follow the formalism presented in Refs. [15,17,18], first validating the calculations for NaI (as given in [17]) then performing the calculations for Xe.…”

“…Here E R is electron recoil energy, v velocity of the incoming mirror electron, m e electron mass, ε the kinetic mixing parameter, and α the fine structure constant. The scattering rate, calculated by multiplying with the integral of the velocity distribution of the incoming mirror dark matter and Taylor expanding around the yearly average, is given by [17] dR…”

“…The A v cos ωðt − t 0 Þ term describes annual modulation resulting from the change of velocity of the Earth with respect to the dark matter halo. Here ω ¼ 2π=year, t 0 ¼ 153 days (June 2) and modulation amplitude A v ¼ 0.7 [17]. The A θ ðθ −θÞ term describes diurnal and annual modulation due to the rotation of the Earth and the variation of the Earth's spin axis relative to the incoming dark matter wind.…”

“…Below this velocity collisions are important until mirror electron energy is reduced to ∼25 eV, after which energy loss to the captured mirror helium is no longer important. From energy loss considerations, the cutoff velocity may be estimated as [17]…”