Finally, evidence has emerged of gluons' contribution to the proton spin. The results were published recently in Physical Review Letters by de Florian et al. de Florian is from the Department of Physics, University of Buenos Aires, Argentina.
But could this lead to the resolution of the long-standing proton spin crisis? Apparently, the issue is still open.
It is well-known that the proton has spin equal to half. . However, where the proton spin comes from is still a puzzle.
It would not be puzzling if the proton were an elementary particle itself. But since the proton is itself made up of quarks and gluons (collectively called partons), all its properties, including spin, should come from the properties of the constituent particles.
The first guess was that the quarks, which have spin value of half, were totally contributing to the proton spin. However, even in the 1980s, it came to be known from experiments, for instance, the European Muon Collaboration (and later by Spin Muon Collaboration at CERN and a series of experiments at SLAC) that the quarks only contributed about 20 per cent of the proton’s spin.
This led to the speculation that the gluons were contributing the rest or a large fraction of the proton’s spin. Sourendu Gupta, from TIFR, Mumbai, told this Correspondent: “People built models of various kinds in the late 1980s. My collaborators — D. Indumathi and M.V.N. Murthy who are at IMSc, Chennai — and I took a completely different approach at that time. We asked ourselves what else would happen if the gluons carried the rest of the spin of the proton. Our 1989 paper was perhaps the first to ask this question, and, happily, some of the ideas we had have been incorporated into current day measurements.”
Recent analysis by the authors of the data presented by the STAR and PHENIX experiments at the Relativistic Heavy Ion Collider, at Brookhaven National Laboratory, shows that the gluons may be contributing about 40 per cent to the proton spin budget.
“This is an exciting result, and a tremendous effort by the theorists,” Abhay Deshpande, of Stony Brook University (SUNY) and RIKEN BNL Research Centre, who is part of the PHENIX experiment at RHIC said in an email to this Correspondent.
But this discovery may still come with a rider — the uncertainty associated with the theory and experiment itself. “While the central value of the gluon contribution is about 40 per cent of the proton spin budget, every experiment has an uncertainty and this will have to be coded in the results,” Prof. Deshpande said.
One way to settle the issue is to make measurements over a wider range in the gluon momentum fraction. “One needs the future Electron Ion Collider (EIC) to accomplish this,” Prof. Deshpande noted. “Future experiments should also measure the orbital motion of the quarks and gluons.
Measurements at the future EIC would be able to explicitly measure the proton spin contributions from the individual components — intrinsic quark and gluon spin with high precision and also their orbital angular momenta. This would be able to solve the proton spin puzzle, once and for all.
Knowing what contributes to the proton’s spin tests our understanding of the standard model of particle physics.
