Abstract
The search for dark matter weakly interacting massive particles with noble liquids has probed masses down and below a GeV/c2. The ultimate limit is represented by the experimental threshold on the energy transfer to the nuclear recoil. Currently, the experimental sensitivity has reached a threshold equivalent to a few ionization electrons. In these conditions, the contribution of a Bremsstrahlung photon or a so-called Migdal electron due to the sudden acceleration of a nucleus after a collision might be sizable. In the present work, we use a Bayesian approach to study how these effects can be exploited in experiments based on liquid argon detectors. In particular, taking inspiration from the DarkSide-50 public spectra, we develop a simulated experiment to show how the Migdal electron and the Bremsstrahlung photon allow to push the experimental sensitivity down to masses of 0.1 GeV/c2, extending the search region for dark matter particles of previous results. For these masses we estimate the effect of the Earth shielding that, for strongly interacting dark matter, makes any detector blind. Finally, we show how the sensitivity scales for higher exposure.
Highlights
ME, n = 1, 2 - 10-35 cm2 Brem - 10-33 cm2ME, n = 1, 2, 3 - 10-35 cm2 DarkSide-50 data Ne-for an exposure E = 6786 kg d [29].and a DM mass of 0.5 GeV/c2, we find ER,max ∼ 0.09 keV, while δmax ∼ 1.8 keV
Figure 5. 90% C.I. upper bounds on the σSI exploiting the Migdal electron and photon Bremsstrahlung signals for the tea-lab simulated experiment loosely inspired by DarkSide-50
The tealab bounds are computed for a threshold Ne− = 4
Summary
We start this section describing our notation and our assumptions for the elastic DM-nucleus scattering rates, and continue presenting the computation of the differential rates for the Migdal effect and the photon Bremsstrahlung process. The elastic DM-nucleus differential rate with respect to the nuclear recoil energy ER, per unit detector mass, is dRN R dER. Here NT is the number of target nuclei per unit detector mass, ER is the recoil energy given by an incoming dark matter particle with velocity v > vmin = (mN ER)/(2μ2N ),. The rate depends on our assumptions on the local dark matter density, ρχ, and the dark matter velocity distribution f (v). The nuclear form factor F (ER) ∼ 1 for small momentum transfers, while A is the atomic mass, leading to the coherent enhancement of the cross section
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