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Register a new userEffect of Degenerate Particles on Internal Bremsstrahlung of Majorana Dark Matter
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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.
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There has been interest recently on particle physics models that may give rise to sharp gamma ray spectral features from dark matter annihilation. Because dark matter is supposed to be electrically neutral, it is challenging to build weakly interacting massive particle models that may accommodate both a large cross section into gamma rays at, say, the Galactic center, and the right dark matter abundance. In this work, we consider the gamma ray signatures of a class of scalar dark matter models that interact with Standard Model dominantly through heavy vector-like fermions (the vector-like portal). We focus on a real scalar singlet S annihilating into lepton-antilepton pairs. Because this two-body final-state annihilation channel is d-wave suppressed in the chiral limit, we show that virtual internal bremsstrahlung emission of a gamma ray gives a large correction, both today and at the time of freeze-out. For the sake of comparison, we confront this scenario to the familiar case of a Majorana singlet annihilating into light lepton-antilepton pairs, and show that the virtual internal bremsstrahlung signal may be enhanced by a factor of (up to) two orders of magnitude. We discuss the scope and possible generalizations of the model.
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.
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.
We revisit thermal Majorana dark matter from the viewpoint of minimal effective field theory. In this framework, analytic results for dark matter annihilation into standard model particles are derived. The dark matter parameter space subject to the latest LUX, PandaX-II and Xenon-1T limits is presented in a model-independent way. Applications to singlet-doublet and MSSM are presented.
The expression for the spin susceptibility $chi$ of degenerate quark matter is derived with corrections upto $ {cal O}(g^4ln g^2)$. It is shown that at low density, $chi^{-1}$ changes sign and turns negative indicating a ferromagnetic phase transition. To this order, we also calculate sound velocity $c_1$ and incompressibility $K$ with arbitrary spin polarization. The estimated values of $c_1$ and $K$ show that the equation of state of the polarized matter is stiffer than the unpolarized one. Finally we determine the finite temperature corrections to the exchange energy and derive corresponding results for the spin susceptibility.
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