In this paper, it is reported that the neutron decay rate is:
$$W=\dfrac{m_e^5c^4}{2\pi^3 \hbar^7}|\mathcal{M}| f$$ where $\mathcal{M}$ is the matrix element and $f$ is a factor space statistical factor and $m_e$ is the electron mass. The paper details the calculation of the matrix element, but does not explain how to calculate $f$ and the constant factor in front of the expression.
Can anyone suggest how to derive them or point out a reference that does?
