Abstract
A plasma-kinetic model to properly describe the behavior of neutrons in the primordial plasma during the epoch of big bang nucleosynthesis (BBN) is formulated. For the first time, this model is applied to calculate the realistic energy distribution function of these neutrons and examine their main characteristics. The fact that the realistic neutron distribution departs from a conventional Maxwellian function is obtained; its high-energy tail is essentially enhanced by nonthermal neutrons produced in the $\mathrm{T}(d,n{)}^{4}\mathrm{He}$ and $\mathrm{D}(d,n{)}^{3}\mathrm{He}$ reactions. The fraction of these neutrons ${\ensuremath{\eta}}_{n}^{\ensuremath{'}}$ in the total neutron component is at a level of ${10}^{\ensuremath{-}2}%--{10}^{\ensuremath{-}3}%$, while their effective temperature ${T}_{n}^{\ensuremath{'}}$ reaches several MeV and exceeds the plasma temperature in the BBN epoch by about a factor of ${10}^{2}$. The nonthermal neutron influence on individual reactions is examined on the example of the threshold $\mathrm{D}(n,2n)p$, $^{7}\mathrm{Li}(n,nt)^{4}\mathrm{He}$, and $^{7}\mathrm{Be}(n,n^{3}\mathrm{He})^{4}\mathrm{He}$ processes. We show that at plasma temperatures ${T}_{9}\ensuremath{\lesssim}1.2$ the nonthermal neutrons strongly maintain these reactions, increasing their rates by orders of magnitude as compared with the respective Maxwellian estimates. Note that the obtained phenomenon has a general nature. It may manifest in other nuclear systems and becomes a natural supplement of nonthermal effects triggered by dark matter decay, which has been extensively studied elsewhere. An important question remains---to what extent such fast particles may affect chain reaction kinetics in the plasma and change the predictions of standard BBN.
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