Solution
= Solution
Two-body energy-momentum conservation in the pion rest frame gives $E_e=(m_\pi^2+m_e^2)c^2/(2m_\pi)$ and $p_e=(m_\pi^2-m_e^2)c/(2m_\pi)$. Thus $v_e=c(m_\pi^2-m_e^2)/(m_\pi^2+m_e^2)$.
= Solution
Two-body energy-momentum conservation in the pion rest frame gives $E_e=(m_\pi^2+m_e^2)c^2/(2m_\pi)$ and $p_e=(m_\pi^2-m_e^2)c/(2m_\pi)$. Thus $v_e=c(m_\pi^2-m_e^2)/(m_\pi^2+m_e^2)$.