QCD 08 – The Report

The place was beatiful, the weather excellent and the company very good. I can summarize in this sentence my experience in Montpellier. This town is at the south of France facing Mediterranean sea. The Conference was absolutely interesting and mostly fitted the bill for what are my current interests in QCD. The only regret I have is to not have had the chance to stay till the end of the event. Indeed, I departed on thursday and so this report could be only partial. The organizer of this Conference is Stephan Narison and accomplished the task at best.

I apologize since now for any omissions that does not diminish at all the relevance of the work of all people at Conference.

The program started with QCD at colliders. I should say that there has been a lot of “would be” here and there as the LHC has not started yet. But what hit more here has been how good proved to be QCD at energies large enough and how asymptotic freedom and DL-Altarelli-Parisi equation describe correctly the physics we observe. Some talks were somewhat near my interests for different reasons. E.g. I was hit by several italian researchers working at foreigner institutions and performing very well at their job. I can think at the talks of Maria Ubiali or Silvia Niccolai, the former at University of Edinburgh and the latter at Orsay. While is widely known that scientific endeavour is a completely international activity, I am also aware of the difficulties our Universities here in Italy meet to hold the best young researchers. I have also find of interest the talk of Matteo Giordano. He is a doctorate student at University of Pisa and his tutor is Enrico Meggiolaro. They considered to study hadron-hadron scattering with a lattice computation taking (16)^4 points and \beta=6.0. Their conclusions seem to agree fairly well with AdS/CFT computations.

Two really striking talks were given by C. Schill and J. Pretz of the COMPASS collaboration. These people managed to get the contribution to the spin of the nucleon due to the gluons. They expected a large contribution due to the small one of the quarks. What they measured is almost zero! From my preceding posts one could understand why the result is so. In the infrared limit the true particles of the theory are not gluons but some constituent states carrying no spin and so no contribution on this side is expected. What remains is almot all orbital contribution.

I would like also emphasize the relevant work of CLAS Collaboration at Jefferson Lab and Hera that now stopped activity.

For my theoretical activity I was mostly engaged since the second day, tuesday. In the morning there were the talks of Harald Fritzsch, Mikhail Shifman and Adriano Di Giacomo. Fritzsch talked about the dependence of the time of the QCD scale as emerging on unified framework of fundamental interactions as also proposed by himself. Shifman showed how lower dimensional models could be the key to understand QCD. Di Giacomo proposed an interpretation of confinement as breaking of simmetry. In the afternoon several talks were presented about tau decay and helps that this process can give to our understanding of QCD.

Wednesday session was the most important for me as I had to present my talk. This session was aimed at theoretical methods to cope with QCD in non-perturbative regime. Paolo di Vecchia presented the string theory view. An overview has been given of AdS/CFT methods. As already said elsewhere this approach requires in input the value of the ground state that is generally taken from lattice computations. For your comments, if any, my talk is here. Following to my talk, Craig McNeile gave an overview of our comprehension of glueballs on lattice and why we do not see sigma resonance there. Some more effort is required.

Wedsnesday afternoon was really interesting. Firstly a host of experimental results was given. I noticed the talks by Catalina Curceanu and Stefano Fiore both working at the DAPHNE facility in Frascati at INFN. The former was about Kaonic atoms and the latter was about phi decays and their relevance into our understanding of light scalar mesons. I have the luck to know both the speakers. I met Catalina two years ago in Piombino (Italy) at DICE 2006 Conference and there she gave a nice talk about an experiment to verify Pauli exclusion principle. We have a dinner together with Stephan Adler on my left and Steve Carlip on her right and some other nice guys. The talk about Kaonic atoms was really fascinating due also to her speaker ability. With Stefano I have spent a nice evening in Montpellier together with Matteo Giordano and three other nice fellows. We took a beer at “Le Petit Ness” asking what Ness stood for. I do not still have an answer and no Scottish clues were given. Stefano’s talk proved how far we can go in Italy to do very good science having means and chances. DAPHNE is a very good opportunity to improve our understanding of QCD spectrum.

Several interesting talks were given that afternoon. Achasov showed theoretical evidence that sigma resonance should be a tetraquark. Achasov is one of the pioneers in using chiral perturbation theory when the very existence of sigma met a lot of skepticism by the community. But Mennessier showed a strong evidence that there should be two poles, sigma and sigma’, the latter at about 1 GeV (f0(980)?). Mennessier started his talk in French until mumbling of the audience convinced him to turn to english. The talk of Kunihiro was interesting as well. With Teiji I have had some scientifc exchange as I cited his work and he cited mine. Teiji gave a major contribution to the use of renormalization group in perturbation theory. I used his envelope method extensively. I had the pleasure to greet him as this was the first time we met.

At the end of the session there were the talks of Peter Minkowski and Stephan Narison. I should say that at this point I was really excited. The reason is that I share their view of the light scalar meson spectrum as I have corroborated it with my theoretical findings. We agree that sigma resonance is the glueball and now we also see, as showed by Mennessier, that there is an excited state at about 1 GeV! Check my talk above and you will find all this. I have had also the honor to be cited by Stephan Narison in his talk. My view is that these people are on the right track. But, of course, Narison’s talk was the start of a very vivid discussion with Achasov that has the opposite view supporting quark molecular structure of these mesons. This has been my last day at the Conference. I had to leave without hearing by Pelaez and Leutwyler, a reason to regret for. I should say that Pelaez, working with Paco Yndurain (recently deceased and commemorated at the Conference), arrived at the same conclusions as ours about sigma. This resonance has a large gluonic content and I have presently a lot of reasons to be happy.

To conclude this report I should summarize the main points I have heard about. High energy behavior, asymptotic freedom and DLAP, is well verified also at NLO and NNLO. Spectrum at high energies has some missing states with respect to our theoretical understanding. The low energies spectrum is instead crowded and some understanding is starting to emerge. The identification of the first glueball is currently hotly debated but evidences are emerging that sigma resonance is the one. News from experiments are expected and, indeed, a lot of experimental acitivity is underway around the world. It is possible that in a few years we will be able to understand infrared QCD quite well.

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