The surprising influence of late charged current weak interactions on Big Bang Nucleosynthesis

TL;DR

This paper reveals that late-time charged current weak interactions significantly influence Big Bang Nucleosynthesis outcomes, especially helium-4 abundance, even below the temperature of traditional weak freeze-out, highlighting their importance in early universe physics.

Contribution

It demonstrates the persistent impact of low-temperature weak processes on BBN and emphasizes their sensitivity to lepton energy distributions and beyond-standard-model physics.

Findings

Late-time weak interactions affect helium-4 yields.

BBN codes already include these effects, but their significance is underappreciated.

BBN can probe subtle beyond-standard-model physics in early universe conditions.

Abstract

The weak interaction charged current processes ($\nu_e+n\leftrightarrow p+e^-$, $\bar\nu_e +p\leftrightarrow n+e^+$, $n\leftrightarrow p+e^-+\bar\nu_e$) interconvert neutrons and protons in the early universe and have significant influence on Big Bang Nucleosynthesis (BBN) light-element abundance yields, particulary that for $^{4}{\rm He}$. We demonstrate that the influence of these processes is still significant even when they operate well below temperatures $T\sim0.7\,{\rm MeV}$ usually invoked for "weak freeze-out," and in fact down nearly into the alpha-particle formation epoch ($T \approx 0.1\,{\rm MeV}$). This physics is correctly captured in commonly used BBN codes, though this late-time, low-temperature persistent effect of the isospin-changing weak processes, and the sensitivity of the associated rates to lepton energy distribution functions and blocking factors are not widely appreciated. We quantify this late-time influence by analyzing weak interaction rate dependence on the neutron lifetime, lepton energy distribution functions, entropy, the proton-neutron mass difference, and Hubble expansion rate. The effects we point out here render BBN a keen probe of any beyond-standard-model physics that alters lepton number/energy distributions, even subtly, in epochs of the early universe all the way down to near $T=100\,{\rm keV}$.