Massless collinear parton approximation
= Massless collinear parton approximation
{title2=$k=\xi P,\quad P^2=k^2=0$}
The leading high-energy approximation neglects <quark> and target <masses> and transverse constituent <momentum>. A lightlike reference <momentum> is used for the <hadron>; this does not describe a massless particle at rest.