Is the dark matter particle its own antiparticle?

We propose a test based on direct detection data that allows us to determine if the dark matter particle is different from its antiparticle. The test requires the precise measurement of the dark matter spin-independent direct detection cross sections off three different nuclei and consists of interpreting such signals in terms of self-conjugate ($\text{particle}=\text{antiparticle}$) dark matter to see if such interpretation is consistent. If it is not, the dark matter must be different from its antiparticle. We illustrate this procedure for two sets of target nuclei, $\{\mathrm{Xe},\mathrm{Ar},\mathrm{Si}\}$ and $\{\mathrm{Xe},\mathrm{Ar},\mathrm{Ge}\}$, identifying the regions of the parameter space where it is particularly feasible. Our results indicate that future signals in direct detection experiments, if sufficiently accurate, might be used to establish that the dark matter particle is not its own antiparticle—a major step towards the determination of the fundamental nature of the dark matter.

Figure 1

An illustration of the test we are proposing: after combining direct detection data from three different nuclei (e.g. Xe, Ar, and Si) the Dirac nature of the DM could be singled out.

Figure 2

The regions in the plane (${\lambda }_p^M$, ${\lambda }_n^M$) consistent with a Majorana interpretation of the signals expected in three different targets: Xe (red), Ar (blue) and Si (green). Since these regions do not overlap, a Majorana DM particle can be excluded. The width of the bands is the result of a 40% uncertainty assumed in the determination of the cross sections with the nuclei. The assumed signal is due to a Dirac DM particle of 100 GeV mass and couplings $\{{\lambda }_p^D,{\lambda }_p^{\overline{D}},{\lambda }_n^D,{\lambda }_n^{\overline{D}}\}=\{6.7,2.0,-5.6,-1.0\}\times {10}^{-9}{\mathrm{GeV}}^{-2}$, which, if one assumes isospin conservation (as is usually done when presenting experimental limits), corresponds to dark matter-nucleon cross sections of [22] $\{0.9,0.3,1\}\times {10}^{-10}\mathrm{pb}$, respectively, for $\{\mathrm{Xe},\mathrm{Ar},\mathrm{Si}\}$.

Figure 3

Scatter plots of ${\lambda }_n^D/{\lambda }_p^D$ versus ${\lambda }_n^{\overline{D}}/{\lambda }_p^{\overline{D}}$ for our sample of points for Xe, Ar, Si and Xe, Ar, Ge target sets. The color code indicates the maximum allowed uncertainty (M.U.) in the measurement of the cross sections that enables to distinguish Dirac from Majorana DM: dark cyan for points with $\mathrm{M}.\mathrm{U}.<1\%$, red for points with $1\%<\mathrm{M}.\mathrm{U}.<10\%$, and black for points featuring $\mathrm{M}.\mathrm{U}.>10\%$.