Parton momentum fraction
= Parton momentum fraction
{title2=$x$}
In the collinear approximation, a <parton> carries a fraction $0<x<1$ of its parent hadron's longitudinal <momentum>. A <parton distribution function> $f_i(x,Q^2)$ gives its distribution at the specified factorization scale. For production of one on-shell particle with no transverse recoil, $x_+x_-s=M^2$ and $x_\pm=(M/\sqrt s)e^{\pm y}$, where $y$ is its <rapidity>.