Gluons contribute to the spin of protons

Physicists have identified 4 sources for the existence of proton's spin: -) quark spin, -) gluon spin, -) the motion of quarks, and -) the motion of gluons. 30 years ago experiments showed that the spin of quarks, the elementary components of protons, explained only 30% of proton spin. Scientists from the Kentucky University have recently computed the share of gluons to the proton spin. It appears that 50% of the proton spin is due to the gluon spin. These calculations are complex as they mingle quantum physics with special relativity and they have recently benefited from theoretical advances that have made them computable. (A.C.)