The transverse-momentum distribution of an up quark with longitudinal momentum fraction x = 0.1 in a transversely polarized proton moving in the z-direction, while polarized in the y-direction. The color code indicates the probability of finding the up quarks.

Alberto Accardi et al, Electron Ion Collider: The Next QCD Frontier - Understanding the glue that binds us all, arXiv:1212.1701