Synopsis: Gluons Chip in for Proton Spin

A new analysis of high-energy data shows that gluons may provide some of the proton’s missing spin.
Synopsis figure
Brookhaven National Laboratory

The proton has a spin that comes from its constituent quarks and gluons. Experiments in the 1980s found—unexpectedly—that the contribution from the intrinsic spins of the quarks was small. This so-called “proton spin crisis” remains unresolved, but a new comprehensive analysis of proton scattering data, reported in Physical Review Letters, finds the first clear evidence that the gluon spin polarization is not zero, suggesting that gluons may have a significant role in the spin of the proton.

The proton is a spin 1/2 particle made up of three quarks held together by gluons that carry the strong force. The quarks have spin 1/2, so physicists originally assumed that two of the quarks were in opposite alignment (cancelling their spin), leaving one unpaired quark to give the proton spin. However, measurements of muon-proton collisions found only a quarter of the proton’s spin comes from quark spins. The rest has to come from gluon spins and/or the orbital motion of quarks and gluons inside the proton.

To determine the contribution from gluons, which have spin 1, physicists measure the probability that a gluon with a particular momentum is aligned, or polarized, with respect to the proton spin. Earlier observations of proton-proton collisions at the Relativistic Heavy Ion Collider (RHIC) found the gluon polarization was close to zero for moderate momenta values. However, using more recent RHIC results, Daniel de Florian of the University of Buenos Aires, Argentina, and his colleagues find a nonzero gluon polarization. More data is still needed at low momentum, but the current best fit suggests that as much as half of the proton’s spin comes from gluon spins. — Michael Schirber


Announcements

More Announcements »

Subject Areas

Particles and Fields

Previous Synopsis

Next Synopsis

Metamaterials

Sound Switch

Read More »

Related Articles

Viewpoint: Extending an Alternative to Feynman Diagrams
Particles and Fields

Viewpoint: Extending an Alternative to Feynman Diagrams

A simplifying technique for calculating scattering amplitudes—the basis for predictions in particle physics experiments—has been extended to cover a class of effective quantum field theories. Read More »

Synopsis: A Little Empty Inside
Cosmology

Synopsis: A Little Empty Inside

A new model has allowed researchers to test a theory for why the centers of dark matter halos are less dense than expected. Read More »

Synopsis: Still Waiting For Electron Decay
Particles and Fields

Synopsis: Still Waiting For Electron Decay

Scientists have placed new limits on how often electrons decay into neutrinos and photons, a reaction that—if it occurred—would violate the law of charge conservation. Read More »

More Articles