Non-thermal processes in standard big bang nucleosynthesis. Part 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 JCAP 05 (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, 3 He, α ). The other half, being radiative capture processes, produce slow nuclei k - (= d,t, 3 He, 7 Li, 7 Be) which can undergo ( k - , n ) reactions with neutrons having large cross sections. The particle production rate R k , thermalization time τ k , and effective number density n k are determined. It is shown that at the early stage of BBN the number density of slow deuterons (respectively, 3 He) can exceed the number densities of Maxwellian 7 Li and 7 Be (respectively, 7 Be) 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,γ ) d - ( n,γ ) t , d ( p,γ ) 3 He - ( n,p ) t , t ( α,γ ) 7 Li - ( n,γ ) 8 Li, 3 He( α,γ ) 7 Be - ( n,p ) 7 Li, d ( t,α ) n + ( A,n ) a 1 a 2 , and d ( 3 He, α ) p + ( A,p ) a 1 a 2 with ( A , a 1 , a 2 ) = ( 7 Li, t,α ) and ( 7 Be, 3 He, α ). It is obtained that the non-Maxwellian particles can selectively affect the element abundances, e.g., improve the prediction on 7 Li/H by ∼ 1.5% and at the same time leave unchanged the 4 He 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.