Proton is made up of quarks, antiquarks and gluons. We also know that proton has spin 1/2. How does the spin of the proton gets generated from different contributions from quarks and gluons ? What is the role of their orbital angular momentum ? These are intriguing questions in hadron physics. The answer seems to be not so straightforward, as the number of qaurks and gluons in the proton is not constant, it depends on the momentum or energy scale of the probe with which we investigate.

So the contributions from intrinsic spin of quarks and gluons as well as their orbital angular momentum are also dependent on this momentum scale, and on the underlying quark-gluon interaction, which is governed by QCD (quantum chromodynamics). The monentum and spatial distritutions of the quarks and gluons in the proton are also correlated with the spin. In fact, the spatial distribution of quarks in a proton, which is polarized transversely with respect to the direction of its motion, is found to be distorted in the transverse plane.