Observational constraints on secret neutrino interactions from big bang nucleosynthesis

We investigate possible interactions between neutrinos and massive scalar bosons via g_φννφ (or massive vector bosons via gVνγ^μνV_μ) and explore the allowed parameter space of the coupling constant g_φ (or gV) and the scalar (or vector) boson mass m_φ (or mV) by requiring that these secret neutrino interactions (SNIs) should not spoil the success of big bang nucleosynthesis (BBN). Incorporating the SNIs into the evolution of the early Universe in the BBN era, we numerically solve the Boltzmann equations and compare the predictions for the abundances of light elements with observations. It turns out that the constraint on g_φ and m_φ in the scalar-boson case is rather weak, due to a small number of degrees of freedom (d.o.f.). However, in the vector-boson case, the most stringent bound on the coupling gV6×10^-10 at 95% confidence level is obtained for mV1 MeV, while the bound becomes much weaker gV8×10^-6 for smaller masses mV10^-4 MeV. Moreover, we discuss in some detail how the SNIs affect the cosmological evolution and the abundances of the lightest elements.