I see that one popular method for forming bound states in Quantum Field Theory is the Covariant Projection Method where we project onto desired Spin-1 states by replacing the $\upsilon$ $v $ spinor product with the Dirac matrix:
Could you please help show it ? For example equation 2.1a here :
$\epsilon^\mu$ is the polarization vector of the meson and P , M are the 4-momentum and mass of the meson.
Thank you very much.