Non-thermal processes in standard big bang nucleosynthesis: I. In-flight nuclear reactionsinduced by energetic protons

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The standard model of big bang nucleosynthesis (BBN) relies on a nuclear reaction networkoperating with thermal reactivities for Maxwellian plasma. In the primordial plasma,however, a number of non-thermal processes triggered by energetic particles ofvarious origins can take place. In the present work we examine in-flight nuclearreactions induced in the plasma by MeV protons generated in D(d, p)T and3He(d, p)4He fusions. We particularly focus on several low threshold endoergic processes. These arereactions omitted in the standard network—proton-induced break-ups of loosely bound D,7Li,7Be nuclei—andthe 3H(p, n)3He charge-exchange reaction important for the interconversion ofA = 3 nuclei in the early universe. It is found that the break-up processesin the plasma take the form of Maxwellian processes at temperaturesT>70 keV, while in the lower temperature range they proceed as non-thermal reactions. It is shown thatat T<70 keV the in-flight reaction channels can enhance the break-up reactivitiesby several orders of magnitude. The levels of these reactivities howeverremain insufficiently high to affect BBN kinetics and change the standardprediction of light element abundances. The abundances are found to be:Yp = 0.2457,D/H = 2.542 × 10−5,3He/H = 1.004 × 10−5,7Li/H = 4.444 × 10−10. Future steps in the study of non-thermal processes in the primordial plasma are brieflydiscussed.