S-factor

In nuclear physics, the astrophysical S-factor is a rescaling of a nuclear reaction's total cross section to account for the Coulomb repulsion between the charged reactants. It determines the rates of nuclear fusion reactions that occur in the cores of stars.
Definition.
The quantity is defined as
Here, is the dimensionless Sommerfeld parameter,
where
Motivation.
The Coulomb barrier causes the cross section to have a strong exponential dependence on the center-of-mass energy . The S-factor remedies this by factoring out the Coulomb component of the cross section.