Large contributions to dark matter annihilation from three-body final states

The annihilation rate of dark matter particles plays a crucial role in dark matter studies, for it determines their relic density and their indirect detection signal. In this paper, we show that this annihilation rate can receive large additional contributions from three-body final states consisting of a real and a virtual massive particle, such as WW* (Wf \bar f') and t \bar t* (tW\bar b). We consider two specific examples, from the singlet model and the MSSM, and find that, due to the new three-body final state contributions, the prediction for the relic density may decrease by more than a factor two, whereas the present dark matter annihilation rate gets enhanced by up to two orders of magnitude. Some of the implications of these results are briefly discussed.Comment: 6 page

Inverse decays and the relic density of the sterile sneutrino

We consider a weak scale supersymmetric seesaw model where the Higgsino is the next-to-lightest supersymmetric particle and the right-handed sneutrino is the dark matter candidate. It is shown that, in this model, inverse decays, which had been previously neglected, may suppress the sneutrino relic density by several orders of magnitude. After including such processes and numerically solving the appropriate Boltzmann equation, we study the dependence of the relic density on the mu parameter, the sneutrino mass, and the neutrino Yukawa coupling. We find that, even though much smaller than in earlier calculations, the sneutrino relic density is still larger than the observed dark matter density.Comment: 15 pages, 4 figure

Singlet-Doublet Dirac Dark Matter

We analyze a simple extension of the Standard Model where the dark matter particle is a Dirac fermion that is mixture of a singlet and an SU(2) doublet. The model contains only four free parameters: the singlet and the doublet masses and two new Yukawa couplings. Direct detection bounds in this model are very strong and require the dark matter particle to be singlet-like. As a result, its relic density has to be obtained via coannihilations with the doublet. We find that the dark matter mass should be below 750 GeV, that the singlet-doublet mass difference cannot exceed 9%, and that direct detection experiments offer the best chance to probe this scenario. Finally, we also show that this model can effectively arise in well-motivated extensions of the Standard Model.Comment: 14 page

Scalar dark matter in the B-L model

The B-L extension of the Standard Model requires the existence of right-handed neutrinos and naturally realizes the seesaw mechanism of neutrino mass generation. We study the possibility of explaining the dark matter in this model with an additional scalar field that is a singlet of the Standard Model but charged under $U(1)_{B-L}$. An advantage of this scenario is that the stability of the dark matter can be guaranteed by appropriately choosing its B-L charge, without the need of an extra ad hoc discrete symmetry. We investigate in detail the dark matter phenomenology of this model. We show that the observed dark matter density can be obtained via gauge or scalar interactions, and that semi-annihilations could play an important role in the latter case. The regions consistent with the dark matter density are determined in each instance and the prospects for detection in future experiments are analyzed. If dark matter annihilations are controlled by the B-L gauge interaction, the mass of the dark matter particle should lie below 5 TeV and its direct detection cross section can be easily probed by XENON1T; if instead they are controlled by scalar interactions, the dark matter mass can be much larger and the detection prospects are less certain. Finally, we show that this scenario can be readily extended to accommodate multiple dark matter particles.Comment: 24 page

Higgs decays in supersymmetric models with light neutralinos

In the Minimal Supersymmetric Standard Model, neutralinos lighter than 50 GeV are compatible with all accelerator, precision, and cosmological bounds. Such neutralinos might constitute a relevant decay channel for the Higgs boson, modifying its expected signatures at hadron colliders. We study the branching ratio h --> \chi\chi and determine the region in the supersymmetric parameter space where it is sizable. We have found that, in fact, the Higgs may dominantly decay into neutralino pairs. Besides, as a result of this new channel, the branching ratio into visible modes, such as h --> \gamma\gamma, gets suppressed.Comment: 9 pages, 5 figure