On underestimation of the inelastic interactions in the direct dark matter search

Abstract

In the paper expressions are obtained for the event rates expected in experiments aimed at direct detection of dark matter (DM) particles. These expressions allow one to estimate the rates taking into account simultaneously elastic (coherent) and inelastic (incoherent) channels of DM particle interaction with nuclei. The nonzero nuclear excitation energies are used in the calculation of the inelastic scattering contributions. A strong correlation between the excitation energy and the recoil energy of the excited nucleus limits the possibility of the inelastic channel detection with a number of nuclei. Together with the standard model of the DM distribution in the Galaxy some models are considered, which allow higher speeds of the DM particle. As the nuclear recoil energy, TA, increases, the dominance of the elastic interaction channel is smoothly replaced by the dominance of the inelastic one. Therefore, if a detector is set up to detect only elastic scattering events, it starts to lose capability of seeing anything. The only way to notice the interaction remains the gamma radiation from the deexcitation of the nucleus. In the case of spin-independent DM interaction, as TA increases, the inelastic contribution quickly dominates. If the DM particle interacts only spin-dependently, the detectors focused on registration of the elastic spin-dependent DM signal will see nothing, since the signal goes through the inelastic channel. It looks like the desired DM interaction could have a noticeable intensity, but the DM detector is unable to detect it. Therefore, a setup aimed at the direct DM detection should register two signals. The first is the nuclear recoil energy and the second is the gamma-quanta with a certain energy from the target nucleus deexitation. The experiment will provide the complete information about the DM interaction.