We explore a two component dark matter model with a fermion and a scalar. In this scenario the Standard Model (SM) is extended by a fermion, a scalar and an additional pseudo scalar. The fermionic component is assumed to have a global U(1) DM and interacts with the pseudo scalar via Yukawa interaction while a Z 2 symmetry is imposed on the other component -the scalar. These ensure the stability of both dark matter components. Although the Lagrangian of the present model is CP conserving, the CP symmetry breaks spontaneously when the pseudo scalar acquires a vacuum expectation value (VEV). The scalar component of the dark matter in the present model also develops a VEV on spontaneous breaking of the Z 2 symmetry. Thus the various interactions of the dark sector and the SM sector occur through the mixing of the SM like Higgs boson, the pseudo scalar Higgs like boson and the singlet scalar boson. We show that the observed gamma ray excess from the Galactic Centre as well as the 3.55 keV X-ray line from Perseus, Andromeda etc. can be simultaneously explained in the present two component dark matter model and the dark matter self interaction is found to be an order of magnitude smaller than the upper limit estimated from the observational results.

With the consideration of a fast expanding Universe in effect due to an additional scalar field, we present a study of leptogenesis in non-standard cosmology. The Hubble expansion rate is modified by the new added scalar field ϕ, which can change the abundance of lepton asymmetry resulted by the leptogenesis mechanism. We report a significant deviation from the standard unflavored leptogenesis scenario can be achieved in presence of the scalar field ϕ that dominates the energy budget of the early Universe. We present our results for leptogenesis from type-I seesaw with heavy right-handed Majorana neutrinos. The results are based on Boltzmann equations and effects of the scalar field are similar for other kinds of leptogenesis framework. *

Although theoretically well-motivated as a minimal consistent dark matter (DM) model, the inert doublet model (IDM) fell short in explaining the existence of DM in the intermediate mass region (100-500 GeV). We try to address this problem by introducing an additional vector dark matter. We show that the relic density of inert dark matter candidate can be enhanced significantly with new interactions in the intermediate mass region 100-500 GeV in the multicomponent dark matter model when compared with the usual single component inert doublet model. We also show that we can get a reasonable excess in the / E T distribution if we do not apply a very hard / E T cut on it as is customary in any dark matter search at the LHC.

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We extend the so-called singlet doublet dark matter model, where the dark matter is an admixture of a Standard Model singlet and a pair of electroweak doublet fermions, by a singlet scalar field. The new portal coupling of it with the dark sector not only contributes to the dark matter phenomenology (involving relic density and direct detection limits), but also becomes important for generation of dark matter mass through its vacuum expectation value. While the presence of dark sector fermions affects the stability of the electroweak vacuum adversely, we find this additional singlet is capable of making the electroweak vacuum absolutely stable upto the Planck scale. A combined study of dark matter phenomenology and Higgs vacuum stability issue reflects that the scalar sector mixing angle can be significantly constrained in this scenario. 1 amitdbanik@iitg.ac.in 2 abhijit.saha@iitg.ac.in 3 asil@iitg.ac.in 1 arXiv:1806.08080v3 [hep-ph]