PHENIX says gluons are not all the story


PHENIX is a collaboration working with data extracted from RHIC (Relativistic Heavy Ion Collider) located at Brookhaven Labs. phenixed In an experiment with proton-antiproton colliding beams and looking at the ejected \pi^0 they were able to extract the contribution of the gluons to the proton spin. They did this using Next-to-Leading-Order perturbation theory fixing the theory scale at 4GeV^2. Their paper is here and will appear shortly in Physical Review Letters. Their result is

\Delta G^{[0.02,0.3]}_{\rm GRSV}=0.2\pm0.1{\rm (stat)}\pm0.1{\rm (sys)} ^{+0.0}_{-0.4}{\rm (shape)}\pm0.1{\rm (scale)}

that is consistent with zero. This is an independent confirmation of the results of the COMPASS Collaboration that we discussed here. These results let us know that in a proton no contribution to the spin comes from glue, rather this is mostly orbital angular momentum. So, why is this conclusion so relevant? From our point of view we know that, in the low energy limit, glue carries no spin. Rather, true excitations of the Yang-Mills field are some kind of colorless states that makes the spectrum and having the lower state with a massive glueball that can also be seen in labs. We know that this state is the \sigma resonance. This is the scenario that is emerging from experiments and that whatever theory one can think about should explain.

Update: COMPASS Collaboration confirms small polarization of the gluons inside the nucleon (see here, to appear in Physics Letters B). The current world situation is given in their figure that I put here with their caption (for the refs check their paper).


These results, emerging from several different collaborations, are saying to us a relevant information. Glue seems to carry no spin in the low-energy limit. I think that any sound approach to manage QCD in this case should address this result. The main conclusion to be drawn is that glue excitations seen in this case are different from those seen in the high-energy limit. This is a strong confirmation of our point of view presented here and in published papers. It is a mounting evidence that appears to outline a clear scenario of strong interactions at lower energies.

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