## Today on arxiv

07/12/2010

As usual I read the daily coming from arxiv for some new papers to talk you about. This morning I have found some interesting ones I would like to say something on. Firstly, I would like to point out to you the paper by Marco Ruggieri and Raoul Gatto (see here). These authors discuss the behavior of QCD in presence of a strong magnetic field. The main tool they consider is the Nambu-Jona-Lasinio model. As you may know, I showed that this is the low-energy limit of QCD (see here and here) but there is also a paper by Kondo (see here) giving the same conclusion even if an expression for the Nambu-Jona-Lasinio constant is not obtained. Gatto and Ruggieri arrive at the important conclusion that a strong magnetic field changes in some way the phase diagram of QCD. I think that this conclusion is strongly supported by the consistency of the model they use. By my side, I think that this area of research is very promising to test my derivation of low-energy QCD.

An important paper as well is the one posted by BESIII Collaboration (see here). This paper gives the most precise measure of the $\eta'\rightarrow\eta\pi\pi$ decay obtained so far due to their larger statistics. They arrive at the important conclusion that for this decay interactions of the decay products is important. This conclusion is really important as implies a production of intermediate resonances as $\sigma$ and a0(980) as already discussed in my preceding post. The reason why this is so important is that this gives a strong support to the view of the $\sigma$ resonance as a glueball and to our current understanding of QCD given above.

Indeed, today there is again a paper of Juan Sanz-Cillero discussing this matter (see here). A more extended discussion has been given in my post here.

## The nature of eta’

30/11/2010

$\eta'$ is a very peculiar particle. It mixes with $\eta$ that has a lower mass. Recently, in their report on KLOE-2 physics, this group reported here that $\eta'$ has a significant glue component besides quarks. This means that understanding its most important decay $\eta'\rightarrow\eta\pi^+\pi^-$ is not a trivial matter. As my readers may know, I have done a computation in my contribution to proceedings to QCD 10 conference where the decay process is seen to happen through an intermediate step with the $\sigma$ resonance followed by the decay of this into two pions. The agreement we get is so good to give a correct estimation of the decay constant of the $\eta$. This implies that the $\sigma$ is a true glue state. Of course our computation is rough enough to exclude mixing with other hadronic states that should exist.

Today, on arxiv, an interesting paper appeared authored by Rafel Escribano, Pere Masjuan, Juan José Sanz-Cillero (see here). These authors give an initial overview of the experimental status of the decay we have considered above. Then, using both the technique of Chiral Perturbation Theory (ChPT) and that of Resonant Chiral Perturbation Theory (RChPT), they try to fit experimental data. I have the luck to hear a talk of Juan José in Montpellier last year about this same matter and I was aware of his struggle to reach an agreement between a successful technique, as ChPT is, and experimental data for this particular process. The leading order of the theory is well below the experimental value and so, already in that first talk, Juan José showed the need for higher order corrections. But he proved that this cannot be enough and said at that time that some other states should be accounted for to reach a satisfactory agreement. This paper goes in this direction showing that if one accounts for the presence of the $\sigma$ and a0(980), the latter being dominant, the agreement is reached. These authors were also able to show a consistent relation between ChPT and RChPT that are in some way complimentary.

This paper is relevant as gives a strong support to the idea that I put forward about $\eta'$ decay. But these authors go further implying a higher level of understanding accounting for the presence of other hadronic states in a technical affordable way. I expect further improvement by them and it will be interesting to see how these could be obtained.