Consider the decay process τ- → π- + Vτ in the rest frame of the τ-. The masses of the τ-, π- and Vτ are Mτ, Mπ and zero respectively.
The velocity of π- is
1
\(\frac{\left(M_{\mathrm{r}}^{2}-M_{\pi}^{2}\right) c}{M_{\tau}^{2}+M_{\pi}^{2}}\)
2
\(\frac{\left(M_{\tau}^{2}+M_{\pi}^{2}\right) c}{M_{\tau}^{2}-M_{\pi}^{2}}\)
3
\(\frac{M_{\pi} c}{M_{\tau}}\)
4
\(\frac{M_{\tau} c}{M_{\pi}}\)