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I have a question about this decay: [tex]\pi^- \rightarrow e^- \overline{\nu_e}[/tex]

there is a factor:

[tex]1 - \frac{v_e}{c}[/tex]

what are these values, [tex]v_e,c[/tex], and why this quantity is equal to:

[tex]\frac{m^2_e}{m^2_\pi + m^2_e}[/tex]?

Thank you

Ciccio