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

We calculate the effects of a light, very weakly-coupled boson X arising from a spontaneously broken U(1)(B-L) symmetry on Delta N-eff as measured by the CMB and Y-p from BBN. Our focus is the mass range 1 eV less than or similar to m(X) less than or similar to 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 Delta N-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 Delta N-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 Delta N-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 Delta N-eff < 0.