Cosmological implications of gauged U(1) B-L on ΔN eff in the CMB and BBN

Abstract

Abstract We calculate the effects of a light, very weakly-coupled boson X arising from a spontaneously broken U(1) B-L symmetry on ΔN eff as measured by the CMB and  Yp from BBN. Our focus is the mass range 1 eV ≲ mX ≲ 100 MeV; masses lighter than about an eV have strong constraints from fifth-force law constraints, while masses heavier than about 100 MeV are constrained by other probes, including terrestrial experiments. We do not assume N eff began in thermal equilibrium with the SM; instead, we allow N eff to freeze-in from its very weak interactions with the SM. We find U(1) B-L is more strongly constrained by ΔN eff than previously considered. The bounds arise from the energy density in electrons and neutrinos slowly siphoned off into N eff bosons, which become nonrelativistic, redshift as matter, and then decay, dumping their slightly larger energy density back into the SM bath causing ΔN eff > 0. While some of the parameter space has complementary constraints from stellar cooling, supernova emission, and terrestrial experiments, we find future CMB observatories including Simons Observatory and CMB-S4 can access regions of mass and coupling space not probed by any other method. In gauging U(1) B-L , we assume the [U(1) B-L ]3  anomaly is canceled by right-handed neutrinos, and so our ΔN eff calculations have been carried out in two scenarios: neutrinos have Dirac masses, or, right-handed neutrinos acquire Majorana masses. In the latter scenario, we comment on the additional implications of thermalized right-handed neutrinos decaying during BBN. We also briefly consider the possibility that X decays into dark sector states. If these states behave as radiation, we find weaker constraints, whereas if they are massive, there are stronger constraints, though now from ΔN eff < 0.