= Charged-current neutrino-electron threshold
{title2=$E_{\nu,\min}=(m_\ell^2-m_e^2)/(2m_e)$}
For a massless incident <neutrino> and a stationary target <electron>, the invariant squared center-of-mass <energy> is $s=m_e^2+2m_eE_\nu$ in <natural units>. Production of a charged <lepton> of mass $m_\ell$ and a massless neutrino requires $s\ge m_\ell^2$, giving the displayed threshold. For the electron flavour the threshold is zero; for heavier charged leptons it is enhanced by the small stationary-target mass.
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