Figure 1: The spin of the proton is built up from the intrinsic spin and orbital angular momentum of its quark and gluon constituents.
Computer simulations indicate that about 50% of the proton’s spin comes from the spin of the gluons that bind its quark constituents.
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 . 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 . Their new result suggests that gluon spin constitutes a substantial fraction of the proton’s spin.
Protons behave like spinning tops. But unlike the classical spin of a top, the spin of the proton and of other elementary particles is an intrinsic quantum phenomenon. This spin is responsible for many fundamental properties and phenomena, including the proton’s magnetic moment and the phases of low-temperature matter.
The proton is described in quantum chromodynamics (or QCD, the theory of quarks and gluons) by an inner core of three confined valence quarks and a sea of virtual quark-antiquark pairs and gluons, all surrounded by a diffuse cloud of virtual pions (light-mass bound states of a quark-antiquark pair). The relativistic motion of all of these particles means that they each carry orbital momentum. The spin of the proton is built up from the intrinsic spin of the valence and sea quarks (each 1/2) and the gluons (spin 1) and their orbital angular momentum, where spin is measured in units of Planck’s constant divided by 2𝜋. The proton spin puzzle is the challenge to understand how these contributions combine to yield the total spin 1/2 of the proton.
Viewpoint: Spinning Gluons in the Proton
Steven D. Bass, Marian Smoluchowski Institute of Physics, Jagiellonian University, 30-348 Krakow, Poland
March 6, 2017• Physics 10, 23