Revisiting big bang nucleosynthesis with a new particle species : effect of co-annihilation with neutrons. (arXiv:2207.10499v2 [hep-ph] UPDATED)
<a href="http://arxiv.org/find/hep-ph/1/au:+Ghosh_D/0/1/0/all/0/1">Deep Ghosh</a>

In big bang nucleosynthesis (BBN), the light matter abundance is dictated by
the neutron-to-proton ($n/p$) ratio which is controlled by the standard weak
processes in the early universe. Here, we study the effect of an extra particle
species ($chi$) which co-annihilates with neutron, thereby potentially
changing the ($n/p$) ratio in addition to the former processes. We find a novel
interplay between the co-annihilation and the weak interaction in deciding the
($n/p$) ratio and the yield of $chi$. At the initial stage of BBN for the
large co-annihilation strength ($G_D$) in comparison to the weak coupling
($G_F$), more neutrons are removed from the thermal bath modifying the ($n/p$)
ratio from its standard evolution. We find that the standard BBN prediction is
restored for $G_D/G_F lesssim 10^{-1}$, while the mass of $chi$ being much
smaller than the neutron mass. When the mass of $chi$ is comparable to the
neutron mass, we can allow large $G_D/G_F$ values, as the thermal abundance of
$chi$ becomes Boltzmann-suppressed. Therefore, the ($n/p$) ratio is restored
to its standard value via dominant weak processes in later epochs. We also
discuss the viability of this new particle to be a dark matter candidate.

In big bang nucleosynthesis (BBN), the light matter abundance is dictated by
the neutron-to-proton ($n/p$) ratio which is controlled by the standard weak
processes in the early universe. Here, we study the effect of an extra particle
species ($chi$) which co-annihilates with neutron, thereby potentially
changing the ($n/p$) ratio in addition to the former processes. We find a novel
interplay between the co-annihilation and the weak interaction in deciding the
($n/p$) ratio and the yield of $chi$. At the initial stage of BBN for the
large co-annihilation strength ($G_D$) in comparison to the weak coupling
($G_F$), more neutrons are removed from the thermal bath modifying the ($n/p$)
ratio from its standard evolution. We find that the standard BBN prediction is
restored for $G_D/G_F lesssim 10^{-1}$, while the mass of $chi$ being much
smaller than the neutron mass. When the mass of $chi$ is comparable to the
neutron mass, we can allow large $G_D/G_F$ values, as the thermal abundance of
$chi$ becomes Boltzmann-suppressed. Therefore, the ($n/p$) ratio is restored
to its standard value via dominant weak processes in later epochs. We also
discuss the viability of this new particle to be a dark matter candidate.

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