Lower part of QCD spectrum

Today in arxiv appeared the contribution of Heinrich Leutwyler at QCD 08 (see here). According to Britannica online Leutwyler is one of the inventors of the concept of color (see here) together with Harald Fritzsch and Murray Gell-Mann. I have the luck to listen to his talk at Montpellier for QCD 08 and I was aware of his work due to a very beatiful paper he published recently with Irinel Caprini and Gilberto Colangelo (see here and here) where they get the mass and the width of the $\sigma$ resonance and her sibling f0(980). This paper is absolutely relevant for an understanding of the nature of this resonance that is is the vault key to connect theoretical analysis and experimental data for a Yang-Mills theory. A different approach by Yndurain, Garcia-Martin and Pelaez (see here and here) gave slightly different values for the $\sigma$.

This Leutwyler’s paper gives a deep insight of our current understanding of the lower part of the QCD spectrum. What really striked me is the deep relation between $\sigma$ and f0(980) as these resonances seem to emerge from such analysis always together. This is in deep agreement both with my analysis (see here) and that of Narison, Mennessier and Ochs (see here). We note as the QCD mass gap is appreciably lower (pion mass) than that of a pure Yang-Mills when the nature of the $\sigma$ is elucitated being a glueball. We discussed this in a post. This means that a strict connection can exist between the phenomenological analysis of Leutwyler and our current understanding of a Yang-Mills theory in the infrared. What we miss now is a substantial support from lattice QCD (see McNeile’s paper).

Corrigenda: I have to correct the fact that Leutwyler is a proponent of color degree of freedom. This is not true. He is one of the proponents of QCD using color as charge. This is the same information correctly conveyed by Britannica online.