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The effects of introducing a small amount of non-thermal distribution (NTD)of elements in big bang nucleosynthesis (BBN) are studied by allowing afraction of the NTD to be time-dependent so that it contributes only during acertain period of the BBN evolution. The fraction is modeled as aGaussian-shaped function of $\log(T)$, where $T$ is the temperature of thecosmos, and thus the function is specified by three parameters; the centraltemporal position, the width and the magnitude. The change in the averagenuclear reaction rates due to the presence of the NTD is assumed to beproportional to the Maxwellian reaction rates but with temperature $T_{\rm NTD}\equiv \zeta T$, $\zeta$ being another parameter of our model. By scanning awide four-dimensional parametric space at about half a million points, we havefound about 130 points with $\chi^2< 1$, at which the predicted primordialabundances of light elements are consistent with the observations. Themagnitude parameter $\varepsilon_0$ of these points turns out to be scatteredover a very wide range from $\varepsilon_0 \sim 10^{-19}$ to $\sim 10^{-1}$,and the $\zeta$-parameter is found to be strongly correlated with the magnitudeparameter $\varepsilon_0$. The temperature region with $0.3\times 10^9 \mbox{K}\lesssim T \lesssim 0.4\times 10^9 \mbox{K}$ or the temporal region $t\simeq10^3$ s seems to play a central role in lowering $\chi^2$.

Effects of transient non-thermal particles on the big bang nucleosynthesis