Cosmology with self-interacting sterile neutrinos and dark matter: A pseudoscalar model

Abstract

Short baseline neutrino oscillation experiments have shown hints of the existence of additional sterile neutrinos in the eV mass range. Such sterile neutrinos are incompatible with cosmology because they suppress structure formation unless they can be prevented from thermalizing in the early Universe or removed by subsequent decay or annihilation. Here, we present a novel scenario in which both sterile neutrinos and dark matter are coupled to a new, light pseudoscalar. This can prevent thermalization of sterile neutrinos and make dark matter sufficiently self-interacting to have an impact on galactic dynamics and possibly resolve some of the known problems with the standard cold dark matter scenario. Even more importantly it leads to a strongly self-interacting plasma of sterile neutrinos and pseudoscalars at late times and provides an excellent fit to cosmic microwave background data. The usual cosmological neutrino mass problem is avoided by sterile neutrino annihilation to pseudoscalars. The preferred value of H 0 is substantially higher than in standard ΛCDM (lambda cold dark matter) and in much better agreement with local measurements.

Predictions for neutrinoless double-beta decay in the 3+1 sterile neutrino scenario

“…In cosmology, the effects of the larger mass m 4 can be disentangled from those of the smaller masses, because ν 4 becomes non-relativistic shortly after matter-radiation equality, much earlier than ν 1 , ν 2 , ν 3 . Moreover, it is possible that the contribution of m 4 to the energy density of the Universe is suppressed, for example by a large lepton asymmetry [84][85][86][87][88], or an enhanced background potential due to new interactions in the sterile sector [89][90][91][92][93][94][95], or a larger cosmic expansion rate at the time of sterile neutrino production [96], or MeV dark matter annihilation [97].…”

Sterile neutrinos with pseudoscalar self-interactions and cosmology

“…We stress that each value of g s corresponds to one value of N eff and one value of f int , but while N eff can be found as a function of g s [12], the calculation yielding f int is unfeasible to perform using current technology, and we simply leave f int as a free parameter when considering partial equilibration. When also letting N eff vary freely, the result is a two-dimensional parameter space, where the pseudoscalar models trace out a onedimensional path.…”

“…Recently, various models of neutrino self-interactions [6][7][8][9][10][11] have been proposed to reconcile sterile neutrinos in cosmology, by preventing additional neutrinos from being fully thermalized in the early Universe. Among the secret interaction models, the pseudoscalar model [12], where interactions are confined to the sterile sector and mediated by a light pseudoscalar, can naturally accommodate one eV-sterile neutrino in cosmology, solving the tension with BBN and CMB measurements.…”

Light sterile neutrinos

“…However, if the sterile neutrinos are generated by active-sterile oscillations in the early Universe [256][257][258][259][260][261], they are fully thermalized well before CMB decoupling, resulting in ∆N eff 1, which is disfavored by the bound (69) 19 . This problem led several authors to propose new mechanisms that can relieve the tension: a large lepton asymmetry [259,[300][301][302][303][304][305][306][307][308][309][310], new neutrino interactions [311][312][313][314][315][316][317][318][319][320][321], entropy production after neutrino decoupling [322], neutrino decay [323], very low reheating temperature [324,325], time varying dark energy components [298], a larger cosmic expansion rate at the time of sterile neutrino production [326], inflationary freedom [327]. The authors of Refs.…”