Hierarchical Bayesian Thermonuclear Rate for the $^7$Be(n,p)$^7$Li Big Bang Nucleosynthesis Reaction. (arXiv:1912.06210v1 [astro-ph.CO])
<a href="http://arxiv.org/find/astro-ph/1/au:+Souza_R/0/1/0/all/0/1">Rafael S. de Souza</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kiat_T/0/1/0/all/0/1">Tan Hong Kiat</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Coc_A/0/1/0/all/0/1">Alain Coc</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Iliadis_C/0/1/0/all/0/1">Christian Iliadis</a>

Big bang nucleosynthesis provides the earliest probe of standard model
physics, at a time when the universe was between 100 seconds and 1000 seconds
old. It determines the abundances of the lightest nuclides, which give rise to
the subsequent history of the visible matter in the Universe. The present work
derives new $^7$Be(n,p)$^7$Li thermonuclear reaction rates based on all
available experimental information. This reaction sensitively impacts the
primordial abundances of $^{7}$Be and $^7$Li during big bang nucleosynthesis.
For the nuclear model, we adopt an incoherent sum of single-level, two-channel
R-matrix approximation expressions, which are implemented into a hierarchical
Bayesian model, to analyze the remaining six data sets we deem most reliable.
The nuclear structure of $^8$Be near the neutron threshold has also been
evaluated to estimate appropriate prior densities for our analysis. In the
fitting of the data, we consistently model all known sources of uncertainty and
also take the variation of the neutron and proton channel radii into account,
hence providing less biased estimates of the $^7$Be(n,p)$^7$Li thermonuclear
rates. From the resulting posteriors, we extract R-matrix parameters and derive
excitation energies, partial and total widths. Our fit is sensitive to the
contributions of the first three levels above the neutron threshold. Values of
excitation energies and total widths for these states are in overall agreement
with previous results, although our results have significantly smaller
uncertainties. Our $^7$Be(n,p)$^7$Li thermonuclear rates have uncertainties
between 1.5% and 2.0% at temperatures of $leq$1 GK. We compare our rates to
previously published results and find that the $^7$Be(n,p)$^7$Li rates most
commonly used in big bang simulations have too optimistic uncertainties.

Big bang nucleosynthesis provides the earliest probe of standard model
physics, at a time when the universe was between 100 seconds and 1000 seconds
old. It determines the abundances of the lightest nuclides, which give rise to
the subsequent history of the visible matter in the Universe. The present work
derives new $^7$Be(n,p)$^7$Li thermonuclear reaction rates based on all
available experimental information. This reaction sensitively impacts the
primordial abundances of $^{7}$Be and $^7$Li during big bang nucleosynthesis.
For the nuclear model, we adopt an incoherent sum of single-level, two-channel
R-matrix approximation expressions, which are implemented into a hierarchical
Bayesian model, to analyze the remaining six data sets we deem most reliable.
The nuclear structure of $^8$Be near the neutron threshold has also been
evaluated to estimate appropriate prior densities for our analysis. In the
fitting of the data, we consistently model all known sources of uncertainty and
also take the variation of the neutron and proton channel radii into account,
hence providing less biased estimates of the $^7$Be(n,p)$^7$Li thermonuclear
rates. From the resulting posteriors, we extract R-matrix parameters and derive
excitation energies, partial and total widths. Our fit is sensitive to the
contributions of the first three levels above the neutron threshold. Values of
excitation energies and total widths for these states are in overall agreement
with previous results, although our results have significantly smaller
uncertainties. Our $^7$Be(n,p)$^7$Li thermonuclear rates have uncertainties
between 1.5% and 2.0% at temperatures of $leq$1 GK. We compare our rates to
previously published results and find that the $^7$Be(n,p)$^7$Li rates most
commonly used in big bang simulations have too optimistic uncertainties.

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