Non-thermal processes in standard big bang nucleosynthesis: III. Reactions with slow nuclei and the overall effect

Abstract

The present paper completes a series of our works on non-thermal nuclear processes in big bang nucleosynthesis (BBN) started in JCAP05(2008)010 (Part I) and 05(2009)001 (Part II). The processes are triggered by non-Maxwellian particles naturally born in the main BBN reactions. Half of these reactions generate fast particles k^+ (= n,p,t,3He,alpha). The other half, being radiative capture processes, produce slow nuclei k^- (= d,t,3He,7Li,7Be) which can undergo (k^-,n) reactions with neutrons having large cross sections. The particle production rate R_k, thermalization time tau_k, and effective number density n_k are determined. It is shown that the values of n_k at the Universe temperatures T > 65 keV can exceed the number densities of Maxwellian 7Li and 7Be ions. To clarify the overall non-Maxwellian effect on BBN, both types of the non-Maxwellian particles are taken into account in the reaction network. Particular attention is paid to two-step sequential processes like p(n,gamma)d^-(n,gamma)t, d(p,gamma)3He^-(n,p)t, t(alpha,gamma)7Li^-(n,gamma)8Li, 3He(alpha,gamma)7Be^-(n,p)7Li, d(t,alpha)n^+(A,n)a_1a_2, and d(3He,alpha)p^+(A,p)a_1a_2 with (A,a_1,a_2) = 7Li,t,alpha) and (7Be,3He,alpha. It is obtained that the non-Maxwellian particles can selectively affect the element abundances, e.g., improve the prediction on 7Li/H by ~1.5% and at the same time leave unchanged the 4He abundance. The main conclusion however is that these particles are unable to significantly change the standard picture of BBN in general, and provide a pathway toward a solution of the cosmological lithium problem in particular.