Electroweak and Higgs boson internal bremsstrahlung. General considerations for Majorana dark matter annihilation and application to MSSM neutralinos

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.

GeV-scale thermal WIMPs: Not even slightly ruled out

“…Note that 2 → 3 bremsstrahlung processes can be the dominant DM annihilation mode in the scenario the 2 → 2 annihilation mode is suppressed [123][124][125][126][127][128][129][130][131][132][133][134][135][136][137][138]. Bremsstrahlung can lift helicity suppression for direct annihilation for Majorana DM to neutrinos, but the annihilation rate is generally still not sufficiently large to produce a thermal relic cross section.…”

DarkSUSY 6: an advanced tool to compute dark matter properties numerically

“…Another complication is that for these processes -unlike in the case of the U (1) and SU (3) corrections discussed above -intermediate particles can go on-shell, so a sophisticated subtraction scheme has been implemented both at the cross section and at the yield level to avoid double-counting when including con- Figure 4. Cosmic ray spectra for an MSSM benchmark model with a degenerate sfermion spectrum, featuring a lightest neutralino with mass m χ ∼ 3.4 TeV and the correct thermal relic abundance (model D2 in [111]). The left panel shows leptonic cosmic ray particles e ± , ν µ , and ν τ (green, blue and cyan line respectively), and the right panel the case of photons (red) and antiprotons (orange).…”

“…For illustration, we show in Fig. 4 the example of cosmic-ray spectra computed with DarkSUSY 6 , showing separately the impact of electromagnetic and electroweak IB (for a pMSSM-7 benchmark model taken from [111]).…”

Enhanced Neutrino Emissivities in Pseudoscalar-mediated Dark Matter Annihilation in Neutron Stars

“…Let us note that besides the quoted annihilation processes we consider, there may be additional pseudoscalar boson production s-wave aaa cc  (Abdullah et al 2014), initial/final state radiation and internal bremsstrahlung processes ff a cc ¯or a ff cc  ( Bell et al 2017). However, since the cross sections for these processes are proportional to g g f 2 4 c and g g f 4 2 c , respectively, they are subdominant in the case of a Dirac fermion DM candidate (Ibarra et al 2013;Bringmann et al 2017). Similarly, radiative a-production can arise from the SM particles' interaction inside the star, but this process is found to be only relevant in the case of very light mediators (eV) like axions or Majorons (Farzan 2003;Sedrakian 2016).…”