Longitudinal polarization vector
= Longitudinal polarization vector
{title2=$\epsilon_L^\mu=(|\mathbf k|,E\widehat{\mathbf k})/m$}
For an on-shell massive vector with $k^\mu=(E,|\mathbf k|\widehat{\mathbf k})$ in <Minkowski metric> signature $+---$, the displayed vector satisfies $k\cdot\epsilon_L=0$ and $\epsilon_L^2=-1$. It specifies the longitudinal one of the three physical polarization states. It is not a replacement for a final polarization sum when other states are unobserved.