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.


12 Responses to PHENIX says gluons are not all the story

  1. Daniel de França MTd2 says:

    Hi Marco!

    This is what I posted at Tommaso’s blog:

    “Thank you Alejandro! But on clearer terms I am not looking for any kind of effective theory from higher dimensions, instead I am trying to see if it is possible an aproximative corresponcence GR in meso – galactic scale and QCD on subnuclear scale such that it’d yield a kind of MOND theory in the GR side. The reason I am looking for QCD it is that the strong force carrier on the strong coupling can be aproximated by phonon-like particles of spin 0, or at least, this is what I understood from Marco Frasca. Thus, maybe I should find that dark matter is in fact gravity potential stored in meso galactic sized solitons with 0 spin.

    What do you think?”

    So, Marco, what do you think?


  2. mfrasca says:

    Dear Daniel,

    What you can find in literature is the attempt to explain hadrons through GR equations. I have seen this kind of approach but not the other way. The reason relies on the smallness of gravitational coupling. What I get in QCD arises from the fact that the non-linear terms are quite strong in Yang-Mills equations changing in this way the dynamics. For GR one works most of time in a weak field approximation.

    A favorable situation for GR is met near singularities. In this case one has a behavior with the non-linear terms driving the dynamics. But the problem here is that Einstein equations are not so friendly as Yang-Mills equations are as quantum dynamics is obtained with a continuous spectrum. So, no bound states and no mass gap. Things would change if time would be Wick-rotated but in this case you are no more in the physical case.



  3. Daniel de França MTd2 says:

    “The reason relies on the smallness of gravitational coupling. What I get in QCD arises from the fact that the non-linear terms are quite strong in Yang-Mills equations changing in this way the dynamics. For GR one works most of time in a weak field approximation.”

    But that’s but I want, the weak field aproximation working like the strong one in QCD. Shouldn’t GR become non linear on meso-galactic scale? I really cannot understand why people think that the way Newtonian limit is taken is correct on that scale. I realy cannot see why supposed small efects like framing drag couldn’t summ up on large scales and create complex stabel solutions, like gravitational solitons. I really would like to see anything that would show that this would not happen.

  4. mfrasca says:

    Dear Daniel,

    Let me clarify a point. Solitons as solution of Einstein equations do exist. This has been proved by my professor Vladimir Belinski ( ) and this is one of the results that made him renowned ( ). The other is BKL solution that now I will not discuss.

    Of course, if you look for something like I did for Yang-Mills you are in trouble with general relativity as I have already discussed elsewhere. The problem is that the spectrum is continuous here and I think that, for quantum gravity, we have to do something to Einstein equations to make them at least treatable. A lot of people is doing good work here and I will avoid to discuss it.



  5. Daniel de França MTd2 says:

    I am aware of these works on quantum gravity. But what I am looking for is simply a MOND like aproximation of GR that could show that GR could trap energy without or with very little mass, I am not trying anything quantum mechanical. I repeat, just an aproximation, that is, an aproximative model that matches qualitatively.

    But thank you very much for that book 🙂 Maybe there is a loophole somewhere, like if one has a small mass instead of nothing.

  6. mfrasca says:


    Beliski’s work is purely classical. No quantum whatsoever. He has found classical soliton solutions of Einstein equations.



  7. Daniel de França MTd2 says:

    Lol! 😀 I AM SORRY! I read that they “DO NOT” exist. This is what happens when I go to be at 3AM to wake up at 7AM! :O I’ve been doing this a lot because I am also trying to understand other stuff. And I am at work posting this things! My boss doesn’t know! 😀

    So, yes sure, I am just thinking in classical solutions, as I said.

  8. Daniel de França MTd2 says:

    Hi Marco!

    Speaking of Physics Letters B, remember that paper that used a non-abelian langragian, like QCD, to describe gravity on galactic scale? That one that started making thinking about the things such as the one I posted on post?

    It will also be published on Physics Letters B! 🙂 Just like the article you mentioned in the update :).
    The final version appeared yesterday on, it was highly changed, but the idea is the same:



  9. […] of the current situation mostly from the experimental side. As reader from this blog may know (see here), glue contribution to spin is about zero and the proton spin appears mostly due to valence quarks […]

  10. […] with the complex vacuum structure of QCD.” [Visto en The Gauge Connection]. La entrada “PHENIX says gluons are not all the story,” también será de vuestro […]

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