Hadronic tensor ($W^{\mu \nu }$)

The inclusive product of hadronic-current matrix elements, summed over final states with a Lorentz-invariant phase-space measure. Electromagnetic current conservation makes both indices transverse to the exchanged four-momentum.

The two scalar coefficients in the unpolarized electromagnetic hadronic tensor. Extra polarization or parity-violating currents can require further coefficients.

In the massless-target convention, the Callan-Gross relation sets this coefficient to zero at leading order in the parton model.