Low-temperature enhancement of semi-annihilation and the AMS-02 positron anomaly

Abstract

Semi-annihilation is a generic feature of particle dark matter that is most easily probed by cosmic ray experiments. We explore models where the semi-annihilation cross section is enhanced at late times and low temperatures by the presence of an s-channel resonance near threshold. The relic density is then sensitive to the evolution of the dark matter temperature, and we compute expressions for the associated Boltzmann equation valid in general semi-annihilating models. At late times, a self-heating effect warms the dark matter, allowing number-changing processes to remain effective long after kinetic decoupling of the dark and visible sectors. This allows the semi-annihilation signal today to be enhanced by up to five orders of magnitude over the thermal relic cross section. As a case study, we apply this to a dark matter explanation of the positron excess seen by AMS-02. We see that unlike annihilating dark matter, our model has no difficulty fitting the data while also giving the correct relic density. However, constraints from the CMB and γ-rays from the galactic centre do restrict the preferred regions of parameter space.

Improved bounds on ℤ3 singlet dark matter

“…Similar behaviour of DM also occurs in a more complicated DM model based on D 3 symmetry [54]. Indirect detection of Z 3 DM was considered in [55,56]. Further developments in connection with radiative neutrino mass were studied in [57][58][59].…”

“…The collision term C[f ] contains contributions from all possible interactions including elastic scattering, annihilation and semi-annihilation. The explicit expressions for the collision term and its moments can be found in [76] and for the contribution of semi-annihilation in the appendix A of [56]. 3 This system of cBE has been then used to find the improved relic density for the points situated on the boundary of the parameter space set by the absolute stability bound, i.e.…”

Dark matter freeze-in from semi-production

“…Semi-annihilations are present in a number of realistic models (see e.g. [10][11][12][13]) and not only are they subject to weakened direct detection limits, but also lead to peculiar phenomenology concerning the indirect detection [14] and the core-cusp cosmological problem [15][16][17].…”

“…This has been done recently in the context of DM freeze-out in [31,32], where it has been also shown that solving the system of equations for coupled 0-th and 2-nd moment (cBE) of (4.1) instead often gives from fairly to very good estimate of the resulting relic density. 4 At the same time the structure of the semi-annihilation and semi-production rates is technically more challenging due to the necessity of performing thermal averages on pairs of particles with different temperatures [14,15,33]. Having all this in mind we therefore leave full numerical solution of (4.1) to future work, while here we focus on the system of cBE.…”

Gravitational wave signals of pseudo-Goldstone dark matter in the Z3 complex singlet model

“…[24]. Indirect detection of Z 3 DM was considered in [25,26]. The cubic coupling can contribute to 3 → 2 scattering for Z 3 strongly interacting (SIMP) DM [27][28][29][30].…”