## Sigma resonance again

José Pelaéz and Guillermo Rìos published today a paper on arxiv (see here). The argument is an understanding of the nature of $\sigma$ and $\kappa$ resonances. The technique they use is Chiral Perturbation Theory (ChPT) but the idea is to see the behavior of the amplitudes at increasing number of colors. They get again a confirmation that the very nature of $\sigma$ is not a typical $\bar qq$ state. Rather, a subdominant $\bar qq$ component is seen at larger energies with larger values of the number of colors. This conclusion  agrees with our theorem proved here.

The current situation forces the authors to prudence. They do not draw any conclusion about the real nature of $\sigma$ and $\kappa$ but their results still appear impressive. These authors have a long file of very good works about the quest for an understanding of the lower part of QCD spectrum and they have given the mass and the width of $\sigma$  with really increased precision. They belong to a group headed by Paco Yndurain. You can find a tribute to Paco by Stephan Narison here.

From my view you can see this as another confirmation to the idea that $\sigma$ is a glueball and the lowest state of a pure Yang-Mills theory. This evidence is becoming overwhelming but other interpretations are not ruled out yet. The fact that $\kappa$ or else f0(980) are glueballs would give further strong support to this as I expect a glueball state at this value of energy.

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