Mean Free Path for Decay in Flight

The mean free path $\lambda$ is calculated for each step using

$$\begin{eqnarray*} \lambda = \gamma \beta c \tau \end{eqnarray*}$$

where $\tau$ is the lifetime of the particle and

$$\begin{eqnarray*} \gamma = \frac{1}{\sqrt{1 - \beta^2}}. \end{eqnarray*}$$

$\beta$ and $\gamma$ are calculated using the momentum at the beginning of the step. The decay time in the rest frame of the particle (proper time) is then sampled and converted to a decay length using $\beta$.