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

• Main Authors: E. Grohs, George M. Fuller
• Format: Article
• Language: English
• Published: Elsevier 2016-10-01
• Series: Nuclear Physics B
• Online Access: http://www.sciencedirect.com/science/article/pii/S0550321316302644

The weak interaction charged current processes (νe+n↔p+e−; ν¯e+p↔n+e+; n↔p+e−+ν¯e) interconvert neutrons and protons in the early universe and have significant influence on Big Bang Nucleosynthesis (BBN) light-element abundance yields, particularly that for 4He. We demonstrate that the influence of these processes is still significant even when they operate well below temperatures T∼0.7 MeV usually invoked for “weak freeze-out,” and in fact down nearly into the alpha-particle formation epoch (T≈0.1 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 keV.