Observational Constraints on Secret Neutrino Interactions from Big Bang Nucleosynthesis

TL;DR

This paper constrains secret neutrino interactions with scalar or vector bosons during Big Bang nucleosynthesis by analyzing their effects on light element abundances, resulting in stringent bounds on the interaction strength, especially for vector bosons around 1 MeV.

Contribution

It provides new observational constraints on neutrino-secret interactions by incorporating them into early universe evolution and solving Boltzmann equations during BBN.

Findings

Weak constraints on scalar boson interactions due to low degrees of freedom.

Stringent bound on vector boson coupling: g_V < 6×10^{-10} at 95% CL for ~1 MeV mass.

Bounds weaken significantly for smaller vector boson masses below 10^{-4} MeV.

Abstract

We investigate possible interactions between neutrinos and massive scalar bosons via $g^{}_{\phi} \overline{\nu} \nu \phi$ (or massive vector bosons via $g^{}_V \overline{\nu} \gamma^\mu \nu V^{}_\mu$) and explore the allowed parameter space of the coupling constant $g^{}_{\phi}$ (or $g^{}_V$) and the scalar (or vector) boson mass $m^{}_\phi$ (or $m^{}_V$) 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 $g^{}_{\phi}$ and $m^{}_\phi$ in the scalar-boson case is rather weak, due to a small number of degrees of freedom. However, in the vector-boson case, the most stringent bound on the coupling $g^{}_V \lesssim 6\times 10^{-10}$ at $95~\%$ confidence level is obtained for $m^{}_V \simeq 1~{\rm MeV}$, while the bound becomes much weaker $g^{}_V \lesssim 8\times 10^{-6}$ for smaller masses $m^{}_V \lesssim 10^{-4}~{\rm MeV}$. Moreover, we discuss in some detail how the SNIs affect the cosmological evolution and the abundances of the lightest elements.