Spinning Gluons in the Proton

Particle physics experiments conducted at the CERN, DESY, JLab, RHIC, and SLAC laboratories have revealed that only about 30% of the proton’s spin is carried by the spin of its quark constituents [1]. This discovery has inspired a 30-year global program of dedicated experiments and theoretical activity to understand the internal spin structure of the proton. But there are several questions. Why is the quark contribution to the proton’s spin so small? How much of the proton’s remaining “spin budget” is contributed by gluons, the particles that mediate the strong force between quarks (Fig. 1)? And how much is contributed by orbital angular momentum? Yi-Bo Yang from the University of Kentucky, Lexington, and colleagues now present the first theoretical calculation of the gluon contribution to the proton’s spin that uses state-of-the-art computer simulations of quark-gluon dynamics on a spacetime lattice [2]. Their new result suggests that gluon spin constitutes a substantial fraction of the proton’s spin.