Protons are seemingly elementary particles and as such one might assume that science knows all there is to know about them. But, together with the origin of its positive charge, physicists have been at a loss to add up the proton’s “spin”. Until now.
The spin of a sub-atomic particle is one of its characteristic properties along with its charge. It is a quantum property, although it can be pictured simply as a kind of rotation. As is often the case with quantum concepts, however, the analogy only stretches so far in that a proton has a spin 1/2, which means it has to “rotate” through 720 degrees, rather than 360 degrees, to get back to its initial state; like tracing one’s fingertip along a “Moebius strip”.
Protons consist of two “up” and one “down” quark linked by gluon chains. Each quark has a spin 1/2, two ups add up to 1 and then the down subtracts a half leaving the proton with a net spin 1/2. However, researchers at the European Muon Collaboration demonstrated in the 1980s that the proton’s spin is not produced by its quarks, In fact, they contribute only a quarter of the value of this quantum property.
“This result was so surprising that it was called the spin-crisis,” explains Yasuyuki Akiba, a PHENIX team member. Particle physicists were therefore confronted with a fundamental question: What else contributes to the spin of the proton?
Scientists suspected that the deficit might be paid for by the gluons that hold the quarks together.
Now, by analyzing data from a year-long experiment carried out at the Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC) in 2006, the PHENIX collaboration at BNL in Upton, USA, together with scientists from the RIKEN BNL Research Center and the RIKEN Nishina Center for Accelerator-Based Science have shown that the gluons are not the main source of the proton’s spin either.
Some models predict that the missing spin comes mainly from gluons, while others suggest that the contribution from the orbital angular momentum of quarks within the proton may also be significant. The analysis suggests that the gluon contribution is about 40%. With 25% from the quarks, that leaves 35% still to be accounted for, which may be due to angular momentum or some other factor.
“Although there is still a significant uncertainty in this result, our data show that models predicting large gluon spin can now be firmly excluded,” Akiba says.