Prospects for determining the particle/antiparticle nature of WIMP dark matter with direct detection experiments

It was recently pointed out that direct detection signals from at least three different targets may be used to determine whether the Dark Matter (DM) particle is different from its antiparticle. In this work, we examine in detail the feasibility of this test under different conditions, motivated by proposals for future detectors. Specifically, we perform likelihood fits to mock data under the hypotheses that the DM particle is identical to or different from its antiparticle, and determine the significance with which the former can be rejected in favor of the latter. In our analysis, we consider 3 different values of the DM mass (50 GeV, 300 GeV, 1 TeV) and 4 different experimental ensembles, each consisting of at least 3 different targets — Xe and Ar plus one among the following: Si, Ge, CaWO₄, or Ge/CaWO₄. For each of these experimental ensembles and each DM mass, the expected discrimination significance is calculated as a function of the DM-nucleon couplings. In the best case scenario, the discrimination significance can exceed O(3σ) for three of the four ensembles considered, reaching O(5σ) at special values of the DM-nucleon couplings. For the ensemble including Si, O(5σ) significance can be achieved for a range of DM masses and over a much wider range of DM-nucleon couplings, highlighting the need for a variety of experimental targets in order to determine the DM properties. These results show that future direct detection signals could be used to exclude, at a statistically significant level, a Majorana or a real DM particle, giving a critical clue about the identity of the Dark Matter.