Lepton asymmetry and primordial nucleosynthesis in the era of precision cosmology
Pasquale D. Serpico, Georg G. Raffelt
TL;DR
The paper addresses how a possible lepton asymmetry, parameterized by a common neutrino degeneracy parameter $\xi$ for all flavors, impacts primordial nucleosynthesis. It computes light-element abundances within the BBN framework with $\eta_B$ fixed by CMB data and $N_{\rm eff}$ set by neutrino physics, mapping the results to the helium yield $Y_p$. The helium yield is found to be the most sensitive probe of $\xi$, yielding bounds such as $-0.04 \lesssim \xi \lesssim 0.07$ or $\xi = 0.0245 \pm 0.0092$, with improved $\eta_B$ precision having little impact. A precise $Y_p$ measurement that distinguishes $\xi=0$ from $\xi\neq 0$ would test the cosmological standard assumption that sphaleron processes equilibrate the cosmic lepton and baryon asymmetries, providing a crucial probe of potential new physics.
Abstract
We calculate and display the primordial light-element abundances as a function of a neutrino degeneracy parameter ξcommon to all flavors. It is the only unknown parameter characterizing the thermal medium at the primordial nucleosynthesis epoch. The observed primordial helium abundance Y_p is the most sensitive cosmic ``leptometer.'' Adopting the conservative Y_p error analysis of Olive and Skillman implies -0.04 \alt ξ\alt 0.07 whereas the errors stated by Izotov and Thuan imply ξ=0.0245+-0.0092 (1 sigma). Improved determinations of the baryon abundance have no significant impact on this situation. A determination of Y_p that reliably distinguishes between a vanishing or nonvanishing ξis a crucial test of the cosmological standard assumption that sphaleron effects equilibrate the cosmic lepton and baryon asymmetries.