## Gluons are not all the story: An update

30/05/2009

In a recent post of mine (see here) I have pointed out to you a beautiful paper by Dan Pirjol and Carlos Schat. This paper is now appeared on Physical Review Letters (see here) but you can find also the preprint on arxiv (see here). I think its content is really important as it gives a serious clue toward our understanding of low-energy QCD. Dan agreed to publish here a contribution about his work and I am glad to post it.

We find that, within experimental errors on the hadron masses, the
so-called gluon-exchange model (OGE) is disfavored by data. This is probably not very  surprising, since at low energies the real degrees of freedom of QCD should include, in addition to the  gluons, also pions (the Goldstone bosons of the spontaneously broken  chiral symmetry). The OGE model does not include the pion exchange effects; an alternative to the OGE model  which includes their effects is the so-called GBE (Goldstone boson exchange) model.

There has been a long-standing debate about the most appropriate model
of quark forces in the framework of the constituent quark model, in  particular about their spin-flavor dependence. The main candidates are
the OGE and GBE models (see e.g. the second paper in Ref.[3] for a
discussion of this controversy). Our letter attempts to resolve this controversy using only minimal assumptions about the orbital dependence
of the hadronic wave functions. More precisely, we assume only isospin
symmetry, but no other assumption is made about the form of the wave
functions. The novel mathematical tool which makes our analysis possible is the application of the permutation group $S_3$, which allows one to study the implications of the most general spin-flavor structure of the quark forces.