Abstract
It is well known that the annihilation of Majorana dark matter into fermions is helicity suppressed. Here, we point out that the underlying mechanism is a subtle combination of two distinct effects, and we present a comprehensive analysis of how the suppression can be partially or fully lifted by the internal bremsstrahlung of an additional boson in the final state. As a concrete illustration, we compute analytically the full amplitudes and annihilation rates of supersymmetric neutralinos to final states that contain any combination of two standard model fermions, plus one electroweak gauge boson or one of the five physical Higgs bosons that appear in the minimal supersymmetric standard model. We classify the various ways in which these three-body rates can be large compared to the two-body rates, identifying cases that have not been pointed out before. In our analysis, we put special emphasis on how to avoid the double counting of identical kinematic situations that appear for two-body and three-body final states, in particular on how to correctly treat differential rates and the spectrum of the resulting stable particles that is relevant for indirect dark matter searches. We find that both the total annihilation rates and the yields can be significantly enhanced when taking into account the corrections computed here, in particular for models with somewhat small annihilation rates at tree-level which otherwise would not be testable with indirect dark matter searches. Even more importantly, however, we find that the resulting annihilation spectra of positrons, neutrinos, gamma-rays and antiprotons differ in general substantially from the model-independent spectra that are commonly adopted, for these final states, when constraining particle dark matter with indirect detection experiments.
Highlights
The prime hypothesis for the cosmologically observed dark matter (DM) [1] is a new type of elementary particle [2]
We compute analytically the full amplitudes and annihilation rates of supersymmetric neutralinos to final states that contain any combination of two standard model fermions, plus one electroweak gauge boson or one of the five physical Higgs bosons that appear in the minimal supersymmetric standard model
Among theoretically well-motivated candidates, weakly interacting massive particles (WIMPs) play a prominent role. This is because such WIMPs very often appear in theories that attempt to cure the fine-tuning problems in the Higgs sector of the standard model of particle physics (SM), and because thermal relics with weak masses and cross sections at the electroweak scale are typically produced with the correct abundance to account for the DM density today [3, 4]
Summary
The prime hypothesis for the cosmologically observed dark matter (DM) [1] is a new type of elementary particle [2]. The resulting suppression by a factor of m2f /m2χ can be lifted by allowing for an additional vector [11] or scalar [20] boson in the final state, implying that for DM masses at the electroweak scale the radiative ‘corrections’ can be several orders of magnitude larger than the result from lowest order in perturbation theory.1 We revisit these arguments and point out that the effect commonly referred to as helicity suppression is the culmination of two distinct suppression mechanisms, in the sense that they can be lifted independently. In a more technical appendix, we describe the details of our analytical calculations to obtain the three-body matrix elements for fully general neutralino annihilation in the MSSM (appendix A), the numerical implementation of these results in DarkSUSY (appendix B), and how to correctly treat spin correlations of decaying resonances (appendix C)
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