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Register a new userW/Z Bremsstrahlung as the Dominant Annihilation Channel for Dark Matter, Revisited
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We revisit the calculation of electroweak bremsstrahlung contributions to dark matter annihilation. Dark matter annihilation to leptons is necessarily accompanied by electroweak radiative corrections, in which a $W$ or $Z$ boson is also radiated. Significantly, while many dark matter models feature a helicity suppressed annihilation rate to fermions, bremsstrahlung process can remove this helicity suppression such that the branching ratios Br($ell u W $), Br($ell^+ell^-Z$), and Br($bar u u Z$) dominate over Br($ell^+ell^-$) and Br($bar u u$). We find this is most significant in the limit where the dark matter mass is nearly degenerate with the mass of the boson which mediates the annihilation process. Electroweak bremsstrahlung has important phenomenological consequences both for the magnitude of the total dark matter annihilation cross section and for the character of the astrophysical signals for indirect detection. Given that the $W$ and $Z$ gauge bosons decay dominantly via hadronic channels, it is impossible to produce final state leptons without accompanying protons, antiprotons, and gamma rays.
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A conservative upper bound on the total dark matter (DM) annihilation rate can be obtained by constraining the appearance rate of the annihilation products which are hardest to detect. The production of neutrinos, via the process $chi chi to bar u u $, has thus been used to set a strong general bound on the dark matter annihilation rate. However, Standard Model radiative corrections to this process will inevitably produce photons which may be easier to detect. We present an explicit calculation of the branching ratios for the electroweak bremsstrahlung processes $chi chi to bar u u Z$ and $chi chi to bar u e W$. These modes inevitably lead to electromagnetic showers and further constraints on the DM annihilation cross-section. In addition to annihilation, our calculations are also applicable to the case of dark matter decay.
Interpretations of indirect searches for dark matter (DM) require theoretical predictions for the annihilation or decay rates of DM into stable particles of the standard model. These predictions include usually only final states accessible as lowest order tree-level processes, with electromagnetic bremsstrahlung and the loop-suppressed two gamma-ray line as exceptions. We show that this restriction may lead to severely biased results for DM tailored to produce only leptons in final states and with mass in the TeV range. For such models, unavoidable electroweak bremsstrahlung of Z and W-bosons has a significant influence both on the branching ratio and the spectral shape of the final state particles. We work out the consequences for two situations: Firstly, the idealized case where DM annihilates at tree level with 100% branching ratio into neutrinos. For a given cross section, this leads eventually to minimal yields of photons, electrons, positrons and antiprotons. Secondly, the case where the only allowed two-body final states are electrons. The latter case is typical of models aimed at fitting cosmic ray e^- and e^+ data. We find that the multimessenger signatures of such models can be significantly modified with respect to results presented in the literature.
Gamma-rays induced by annihilation or decay of dark matter can be its smoking gun signature. In particular, gamma-rays generated by internal bremsstrahlung of Majorana and real scalar dark matter is promising since it can be a leading emission of sharp gamma-rays. However in the case of Majorana dark matter, its cross section for internal bremsstrahlung cannot be large enough to be observed by future gamma-ray experiments if the observed relic density is assumed to be thermally produced. In this paper, we introduce some degenerate particles with Majorana dark matter, and show they lead enhancement of the cross section. As a result, increase of about one order of magnitude for the cross section is possible without conflict with the observed relic density, and it would be tested by the future gamma-ray experiments such as GAMMA-400 and Cherenkov Telescope Array (CTA). In addition, the constraints of perturbativity, positron observation by the AMS experiment and direct search for dark matter are discussed.
We consider an extension of the standard model in which a singlet fermionic particle, to serve as cold dark matter, and a singlet Higgs are added. We perform a reanalysis on the free parameters. In particular, demanding a correct relic abundance of dark matter, we derive and plot the coupling of the singlet fermion with the singlet Higgs, $g_s$, versus the dark matter mass. We analytically compute the pair annihilation cross section of singlet fermionic dark matter into two photons. The thermally averaged of this cross section is calculated for wide range of energies and plotted versus dark matter mass using $g_s$ consistent with the relic abundance condition. We also compare our results with the Fermi-Lat observations.
We propose a model of asymmetric dark matter (DM) where the dark sector is an identical copy of both forces and matter of the standard model (SM) as in the mirror universe models discussed in literature. In addition to being connected by gravity, the SM and DM sectors are also connected at high temperature by a common set of heavy right-handed Majorana neutrinos via their Yukawa couplings to leptons and Higgs bosons. The lightest nucleon in the dark (mirror) sector is a candidate for dark matter. The out of equilibrium decay of right-handed neutrino produces equal lepton asymmetry in both sectors via resonant leptogenesis which then get converted to baryonic and dark baryonic matter. The dark baryon asymmetry due to higher dark nucleon masses leads to higher dark matter density compared to the familiar baryon density that is observed. The standard model neutrinos in this case acquire masses from the inverse seesaw mechanism. A kinetic mixing between the U(1) gauge fields of the two sectors is introduced to guarantee the success of Big-Bang Nucleosynthesis.
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