Revisiting ${}^7$Be Weak and Radiative Transition Rates in Big Bang Nucleosynthesis: Implications for the Primordial Lithium Problem

Abstract

The primordial 7Li abundance predicted by standard Big Bang nucleosynthesis (BBN) exceeds observations in old, metal-poor stars by a factor of 3-4. Since most primordial 7Li is produced as 7Be and subsequently converted by electron capture (EC), additional 7Be destruction channels may affect its final abundance. We investigate EC and antineutrino capture (AC), positron decay from the 7Be nuclear excited state, and proton capture (PC), including 7Be(p,gamma)8B, stimulated emission (SE), plasma screening, and a three-body Auger-like channel transferring the capture energy to a continuum electron. Weak rates are calculated from first principles using perturbation theory with explicitly evaluated hadronic and leptonic currents, while thermally averaged nuclear rates are obtained from the relevant cross sections over 10 < kT < 100 keV. The EC rate rapidly decreases as the Universe expands and cools, while AC enhances weak destruction mainly at early times. SE and screening increase the 7Be(p,gamma)8B rate by only 1-3% at kT about 87 keV. The Auger-like cross section is about 4 x 10^-3 of the radiative channel at kT = 100 keV and falls to about 10^-10 at 10 keV. Our first-principles weak rates differ substantially from previous log(ft)-based estimates, yielding a 7Be half-life of about two days under BBN conditions, nearly one order of magnitude different from phenomenological predictions. Nevertheless, EC, PC, and beta+ decay provide only percent-level corrections to the dominant 7Be(n,p)7Li channel and cannot resolve the cosmological lithium problem. These results motivate a first-principles reassessment of the full BBN nuclear network before invoking physics beyond the Standard Model.

Comments: 24 pages, 7 figures