Calculation of momentum distribution function of a non-thermal fermionic dark matter

Abstract

The most widely studied scenario in dark matter phenomenology is the thermal WIMP scenario. Inspite of numerous efforts to detect WIMP, till now we have no direct evidence for it. A possible explanation for this non-observation of dark matter could be because of its very feeble interaction strength and hence, failing to thermalise with the rest of the cosmic soup. In other words, the dark matter might be of non-thermal origin where the relic density is obtained by the so-called freeze-in mechanism. Furthermore, if this non-thermal dark matter is itself produced substantially from the decay of another non-thermal mother particle, then their distribution functions may differ in both size and shape from the usual equilibrium distribution function. In this work, we have studied such a non-thermal (fermionic) dark matter scenario in the light of a new type of U(1) B−L model. The U(1) B−L model is interesting, since, besides being anomaly free, it can give rise to neutrino mass by Type II see-saw mechanism. Moreover, as we will show, it can accommodate a non-thermal fermionic dark matter as well. Starting from the collision terms, we have calculated the momentum distribution function for the dark matter by solving a coupled system of Boltzmann equations. We then used it to calculate the final relic abundance, as well as other relevant physical quantities. We have also compared our result with that obtained from solving the usual Boltzmann (or rate) equations directly in terms of comoving number density, Y . Our findings suggest that the latter approximation is valid only in cases where the system under study is close to equilibrium, and hence should be used with caution.

The dawn of FIMP Dark Matter: A review of models and constraints

“…[277,352,353] and [17,18], where the authors considered decays and annihilations of the inflaton field or a generic scalar s as a production mechanism for sterile neutrinos, respectively. More recent studies have considered similar scenarios where the scalar s either decays into sterile neutrinos while in equilibrium [271,280,283,285,306,340,[354][355][356][357][358][359][360][361][362][363], or where it first undergoes a freeze-in itself [271, 279, 281-285, 330, 354, 358, 361, 362]. Recently, sterile neutrinos undergoing or related to freeze-in have also been studied in supersymmetric models [272,278,361,364].…”

Cold keV dark matter from decays and scatterings

“…The constraints are in fact becoming so strong that even many out-ofequilibrium mechanisms are already excluded, forcing the DM to be increasingly weakly coupled, e.g. by successive freeze-in mechanisms [32][33][34].…”

Type III seesaw for neutrino masses in U(1)B−L model with multi-component dark matter

“…These four chiral fermions constitute two Dirac fermion mass eigenstates, the lighter of which becomes the DM candidate having either thermal [100] or non-thermal origins [101]. The light neutrino mass in this model had its origin from a variant of type II seesaw mechanism and hence remained disconnected to the anomaly cancellation conditions.…”