Bounds on Transverse Momentum Dependent Distribution Functions

Here, P and S denote the parent hadron momentum and spin, and the relevant component of the quark momentum is x = p/P, the light-cone momentum fraction. The components a = a · n∓ stem from vectors n+ and n−, satisfying n2+ = n 2 − = 0 and n+ ·n− = 1, which are fixed by the momentum that introduces the large scaleQ, together with a (soft) hadron momentum. When only the leading part in orders of 1/Q is considered, just the Φγ part of the correlator suffices. This part is usually parametrized in terms of the following quark distribution functions [5]