Two Sides of the Same Coin: Sterile Neutrinos and Dark Radiation, Status and Perspectives

Abstract

The presence of light sterile neutrinos is one of the unanswered questions of particle physics. The cosmological counterpart is represented by dark radiation, i.e., any form of radiation present in the early Universe besides photons and standard (active) neutrinos. This short review provides a comprehensive overview of the two problems and of their connection. We review the status of neutrino oscillation anomalies, commenting on the most recent oscillation data and their mutual tensions, and we discuss the constraints from other terrestrial probes. We show the shortcomings of translating light sterile neutrinos in cosmology as additional thermalised relativistic species, produced by neutrino oscillations, and we detail alternative solutions, specifically focusing on neutrino nonstandard interactions, and on their link to the Hubble constant problem. The impact of a new force leading to dark radiation–dark matter interactions is also discussed in the realm of new physics in the dark sector.

Late-time interacting cosmologies and the Hubble constant tension

“…In the following, however, we will focus primarily on the eV-scale sterile neutrino. Sterile neutrinos at the eV scale have been introduced to give a possible neutrinooscillation explanation to the so-called Short-Baseline (SBL) anomalies [39,40,41,42], which originally included the excess appearance of electron antineutrinos measured by LSND [43], later confirmed also by MiniBooNE [44,45], and the anomalous disappearance of electron neutrinos at Gallium experiments [46,47] and of electron antineutrinos from nuclear reactors [48]. The original anomalies have been studied at different experimental probes and at the theoretical level for more than ten years.…”

Updated neutrino mass constraints from galaxy clustering and CMB lensing-galaxy cross-correlation measurements

“…Prospects for distinguishing the normal and inverted orderings based on physical effects associated to the individual mass eigenstates (as opposed to overall parameter space volume effects) do not appear promising in near-future cosmological data (see [237]). 13…”

Cosmology-friendly time-varying neutrino masses via the sterile neutrino portal

“…The red shaded regions indicate our KATRIN limits. The constraint from the ∆N eff requirement [84] is shown as the gray region in the top-left panel, while there are no such constraints for the other three cases. The black dashed lines show the region favored by gallium and reactor data [125], while the brown shaded region indicate the solar neutrino constraint [126].…”

“…The details for this observation are given in Appendix A. The sterile neutrino can be as light as O(eV), which is relevant for short baseline anomalies [79][80][81][82][83][84]. A general issue with the light sterile neutrino is the possibility of its thermalization in the early Universe, leading to an increase in ∆N eff [85][86][87][88].…”

“…Several regions are excluded from solar neutrino data (in brown), Big Bang nucleosynthesis (in blue), and KATRIN (in pink). The parameter space excluded based on ∆N eff < 0.25[84,102] from Planck TT, TE, EE + lowE + lensing + BAO is shown in gray, while for the other three cases the resultant ∆N eff is vanishingly small.…”