Bounds on neutrino-scalar nonstandard interactions from big bang nucleosynthesis

Abstract

Coherent forward scattering processes by neutrino-scalar nonstandard interactions (SNSI) induce an effective neutrino mass. In the early Universe, a large neutrino effective mass restricts the production of neutrinos. The SNSI effect is modulated by two effective couplings; these account for the coupling between neutrinos and electrons or positrons, ${G}_{\mathrm{eff}}$, and the neutrino self-interaction ${G}_{\mathrm{S}}$. These parameters are directly related to the effective number of relativistic species, and nonzero values imply a smaller than expected ${N}_{\mathrm{eff}}$. We employ big bang nucleosynthesis to constraint the SNSI effect. We find that ${G}_{\mathrm{eff}}<1.2\text{ }\text{ }{\mathrm{MeV}}^{\ensuremath{-}2}$ and ${G}_{\mathrm{S}}<2.0\ifmmode\times\else\texttimes\fi{}{10}^{7}\text{ }\text{ }{\mathrm{MeV}}^{\ensuremath{-}2}$ at 68% C.L. For a scalar mass in the range ${10}^{\ensuremath{-}15}\text{ }\text{ }\mathrm{eV}\ensuremath{\lesssim}{m}_{\ensuremath{\phi}}\ensuremath{\lesssim}{10}^{\ensuremath{-}5}\text{ }\text{ }\mathrm{eV}$, our neutrino-scalar coupling constraint is more restrictive than any previous result.