Big Bang Nucleosynthesis as a probe of non-standard neutrino interactions and non-unitary three-neutrino mixing

TL;DR

This study probes two Beyond the Standard Model neutrino frameworks—non-standard neutrino interactions (NSI) and non-unitary three-neutrino mixing—by examining their impact on Big Bang Nucleosynthesis (BBN) and the effective number of relativistic species, $N_{ m eff}$. Using modified public codes NUDEC_BSM and PRyMordial, the authors quantify how NSI and non-unitarity alter the early-universe thermodynamic background and weak-rate normalizations, revealing that BBN can provide competitive constraints with terrestrial experiments, especially for the non-diagonal CC-NSI parameters $\varepsilon^{udV}_{e\alpha}$ and the NU parameter $\alpha_{22}$. The analysis shows a notable synergy between neutrino decoupling and BBN, tightening constraints when both observables are combined, though results depend sensitively on the adopted nuclear reaction rates (NACRE II vs PRIMAT). The work highlights the need for improved nuclear-rate measurements to robustly interpret BBN in the presence of NP and demonstrates the growing power of precision cosmology to test neutrino-sector BSM physics.

Abstract

In this work we investigate the impact of two phenomenological Beyond the Standard Model (BSM) scenarios concerning the role of neutrinos in the early universe: non-standard neutrino interactions (NSI) and non-unitary three-neutrino mixing. We evaluate the impact of these frameworks on two key cosmological observables: the effective number of relativistic neutrino species (\Neff), related to neutrino decoupling, and the abundances of light elements produced at Big Bang Nucleosynthesis (BBN). For the first time, neutrino CC-NSI with quarks and non-unitary three-neutrino mixing are studied in the context of BBN, and the constraints on such interactions are found to be remarkably restrictive. In particular, the BBN limits are competitive with the ones derived from terrestrial experiments for the non-diagonal CC-NSI parameter $\varepsilon^{udV}_{e α}$, with $α\neq e$ and for the non-unitarity parameter $α_{22}$. In the case of non-unitarity, the combination between neutrino decoupling and BBN imposes stringent constraints that can either mildly favour the existence of New Physics (NP), or reinforce the SM, depending on the choice of the experimental nuclear rates involved in the BBN calculation. These results stress the already noted need for further nuclear rates measurements in order to obtain more robust BBN theoretical predictions.

Figures (6)

• Figure 1: Helium-4 (top panel) and deuterium (bottom panel) abundances as a function of neutrino NC-NSI with electrons parameters. BBN is mainly insensitive to $\mathcal{O}(1)$$\varepsilon^X_{\alpha \beta}$.
• Figure 2: Nuclear abundances as a function of neutrino CC-NSI with quarks, mainly modifying neutron-to-proton conversion. Solid lines correspond to $\varepsilon^{udV}_{ee}$ and dashed lines to $\varepsilon^{udV}_{e\alpha}$. Blue and red lines are obtained with NACRE II rates, while green and yellow lines with PRIMAT rates.
• Figure 3: $N_{\rm eff}$ as a function of NU parameters, when the exact calculation is made with FortEPiaNO (solid lines) and when we use the approximate mapping in eq. \ref{['eq:NUtoNSI']} (dashed lines). Note the slight discrepancies ($\sigma (N_{\rm eff}) \lesssim 0.006$) for $\alpha_{11} > 0.5$.
• Figure 4: Nuclear abundances as a function of NU diagonal parameters: $\alpha_{11}$ (blue lines), $\alpha_{22}$ (yellow lines) and $\alpha_{33}$ (red lines). Results are obtained with NACRE II rates (solid lines) and PRIMAT rates (dashed lines).
• Figure 5: Allowed $1\sigma$ (red), $2\sigma$ (turquoise) and $3\sigma$ (blue) regions on the $\alpha_{22}$-$\alpha_{11}$ plane. Upper panels: $N_{\rm eff}$, helium-4 and deuterium abundances constraints. Lower panels: $\Delta G_F$ minimum values and BBN constraints alone ($Y_p$ and $D/H$) and in combination with $N_{\rm eff}$. It has been profiled over $\alpha_{21}$ values, satisfying the unitarity condition, eq. \ref{['eq:unitarity condition']}. In the case of $N_{\rm eff}$ we consider the future measurements, while for BBN we consider the current ones. The crosses mark the best-fit for each case; further details can be found in the main text.