Abstract
We show that a general semi-annihilation scenario, in which a pair of dark matter (DM) particles annihilate to an anti-DM, and an unstable state that can mix with or decay to standard model states, can lead to particle anti-particle asymmetry in the DM sector. The present DM abundance, including the CP-violation in the DM sector and the resulting present asymmetry are determined entirely by a single semi-annihilation process at next-to-leading order. For large CP-violation in this process, we find that a nearly complete asymmetry can be obtained in the DM sector, with the observed DM density being dominated by the (anti-)DM particle. The presence of additional pair-annihilation processes can modify the ratio of DM and anti-DM number densities further, if the pair-annihilation is active subsequent to the decoupling of the semi-annihilation. For such a scenario, the required CP-violation for generating the same present asymmetry is generically much smaller, as compared to the scenario with only semi-annihilation present. We show that a minimal model with a complex scalar DM with cubic self-interactions can give rise to both semi- and pair-annihilations, with the required CP-violation generated at one-loop level. We also find that the upper bound on the DM mass from S-matrix unitarity in the purely asymmetric semi-annihilation scenario, with maximal CP-violation, is around 15 GeV, which is much stronger than in the WIMP and previously considered asymmetric DM cases, due to the required large non-zero chemical potential for such asymmetric DM.
Highlights
Φ weakly interacting massive particle (WIMP) scenario, while the latter is the so-called semiannihilation process [22]
We show that a general semi-annihilation scenario, in which a pair of dark matter (DM) particles annihilate to an anti-DM, and an unstable state that can mix with or decay to standard model states, can lead to particle anti-particle asymmetry in the DM sector
For large CP-violation in this process, we find that a nearly complete asymmetry can be obtained in the DM sector, with the observed DM density being dominated by theDM particle
Summary
In order to determine the DM relic abundance in a model-independent setup, we consider the thermally averaged cross-section for the semi-annihilation process ( σv S) as a free parameter. For a fixed DM mass, if we in turn keep increasing the CP violation , the reaction rate σv S needs to be correspondingly higher, for the same reason as described above This is shown in figure 3 (right), where for two fixed values of mχ (5 GeV and 10 TeV), we show the contours in the σv S − plane (red and pink solid lines respectively) in which the central value of the DM relic density Ωh2 = 0.12 is reproduced. We see that being entirely within the limits of maximal possible semi-annihilation rate and the maximal possible value of CP-violation, we can obtain a completely asymmetric DM scenario, with no requirement of subsequent pair-annihilations to remove the symmetric component This is one of the primary important observations of this paper
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