= Physical momentum support of a deep-inelastic hadronic tensor
{title2=$p_X=P+q,\quad p_X^2=M^2+Q^2(1/x-1)$}
For an absorbed spacelike photon, use $q^2=-Q^2<0$, $\nu=P\cdot q>0$, $x=Q^2/(2\nu)$ and the physical momentum delta function $\delta^{(4)}(P+q-p_X)$. The displayed invariant final mass is compatible with <deep inelastic scattering> for $0<x<1$. Using $P-q$ with the same positive-$x$ definition instead gives $M^2-Q^2(1+1/x)$, which becomes negative in the <Bjorken scaling> limit and has no physical hadronic spectral support. Reversing the photon convention requires reversing the associated scalar-product and scaling-variable conventions too.
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