## The many faces of QCD

After a long silence, due to technical impediments as many of you know, I turn back to you from Ghent (Belgium). I am participating at the conference “The many faces of QCD”. You can find the program here. The place is really beautiful as the town that I had the chance to look out yesterday evening. Organizers programmed a visit downtown tomorrow and I hope to see this nice town also at the sun light. The reason why this conference is so relevant is that it gathers almost all the people working on this matter of Green functions of Yang-Mills theory and QCD whose works I cited widely in my blog and in my papers. Now, I have the chance to meet them and speak to them. I am writing after the second day ended. The atmosphere is really exciting and discussion is always alive and it happens quite often that speakers are interrupted during their presentations. The situation this field is living is simply unique in the scientific community. They are at the very start of a possible scientific revolution as they are finally obtaining results of non-perturbative physics in a crucial field as that of QCD.

Disclaimer: The talks I will comment on are about results very near my research area. Talks I will not cite are important and interesting as well and the fact that I will not comment about them does not imply merit for good or bad. Anyhow, I will appreciate any comment by any participant to the conference aiming to discuss his/her work.

I would like to cite some names here but I fear to forget somebody surely worthwhile to be named. From my point of view, there have been a couple of talks that caught my attention more strongly than others, concerning computations on the lattice. This happened with the talk of Tereza Mendes yesterday and the one of Orlando Oliveira today. Tereza just studied the gluon propagator at higher temperatures obtaining again striking and unexpected results.  There is this plateau in the gluon propagator appearing again and again when lattice volume is increased. It would have been interesting to have also a look to the ghost and the running coupling. Orlando, by his side, showed for the first time an attempt to fit with the function $G(p)=\sum_n\frac{Z_n}{p^2+m^2_n}$ that you can recognize as the one I proposed since my first analysis to explain the infrared behavior of Yang-Mills theory. But Orlando went further and found the next to leading order correction to the mass appearing in a Yukawa-like propagator.  The idea is to see if the original hypothesis of Cornwall can agree with the current lattice computations. So, he shows that for the sum of propagators one can get even better agreement in the fitting increasing the number of masses (at least 4)  and for the Cornwall propagator you will need a mass corrected as $M^2+\alpha p^2$. Shocking as may seem, I computed this term this summer and you can find it in this paper of mine. Indeed, this is a guess I put forward after a referee asked to me an understanding of the next-to-leading corrections to my propagator and, as you can read from my paper, I guessed it would have produced a Cornwall-like propagator. Indeed, this is just a first infrared correction that can arise by expanding the logarithm in the Cornwall’s formula.

The question of the gluon condensate, that I treated in my blog extensively thanks to the help of Stephan Narison, has been presented today by Olivier Péne through a lattice computation. Olivier works in the group of Philippe Boucaud and contributed to the emerging of the now called decoupling solution for the gluon propagator. The importance of this work relies on the fact that a precise determination of the gluon condensate from lattice is fundamental for our understanding of low-energy behavior of QCD. For this analysis is important to have a precise determination of the constant $\Lambda_{QCD}$. Boucaud’s group produced an approach to this aim. Similarly, Andre Sternbeck showed how this important constant could be obtained by a proper definition of the running coupling and he showed a very fine agreement with the result of Boucaud’s group.

Finally, I would like to remember the talk of Valentine Zakharov. I talked extensively about Valentine in my previous blog’s entries. His discoveries in this area of physics are really fundamental and so it is important to have a particular attention to his talks. Substantially, he mapped scalar fields and Yang-Mills fields to get an understanding of confinement! As I am a strong supporter of this view, as my readers may know from my preceding posts, I was quite excited to see such a an idea puts forward by Valentine.

As conference’s program unfolds I will take you updated with an eyes toward the aspects that are relevant to my work. Meantime, I hope to have given to you the taste of the excitement this area of research conveys to us that pursue it.

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### 8 Responses to The many faces of QCD

1. Alejandro Rivero says:

About the QCD scale, you can tell a joke in the dinner table tonight:

sqrt(mass_neutron/3)= 17.697 (Mev^1/2)
(sqrt(electron)+sqrt(muon)+sqrt(tau))/3 = 17.716 (Mev^1/2)

A quotient of 0,9989…, or 99.7% if you do the quotient in MeVs instead of the square root. I am very amazed because I have been looking at this quantity (the one of leptons) for five years and never noticed the coincidence. I actually wonder if the people of QCD speaks of it sometimes, after a couple of beers -or a bit more-. To take it seriously, one would need some model with SU(3)_family and SU(3)_colour interlocked someway.

2. tbs says:

Hi Marco,
Great to see your new entry! Please do report on the conference more. It really is quite useful to get the first-hand information about such conferences while they are still on, to people like me who couldn’t attend them for one reason or another. Sorry for being too demanding, but I would really appreciate your discussing about the other talks too! It would be really great if you could spare some time for that.
Cheers,
tbs

• mfrasca says:

Dear tbs,

Thank you a lot for your comment. Currently the program of the conference does not contain yet all the talks. As soon as the organizers will update it I will do another post.

Cheers,

Marco

• tbs says:

Thanks Marco!
Will be waiting for your posts, as always. I have a general, friendly, complaint – why don’t you update your blog more frequently these days? There are many papers appearing related to IR QCD both from the continuum and lattice (recently, the lattice10 proceedings have also started appearing on arxiv) communities. There is no other blog that discusses such issues so frequently. I am sure you would have other commitments, but it would really be great if you could spare some more time on posting new blog entries. Remember, there are many readers (like me) who don’t usually comment on the blogosphere for various reasons but do read, enjoy and wait for the blogs!
Cheers,
tbs

• mfrasca says:

Thank you very much for reading my blog. The problem with the updating of the blog is that, currently, I cannot access it from my office as before. My mangement installed Websense, the same software that China and Iran use for their aims. This cut out all blogosphere. This means in turn that I have very few time to add posts. Of course, organizers of the conference asked for a free wi-fi connection and widely available to all participants. This made for me very easy to make some comments.

Meantime, you can go to

http://sites.google.com/site/facingqcd/pictures

and see some nice pictures of the conference taken by Nele. She is a nice post-doc at Ghent University and gave an important contribution to the organization making this conference a fine meeting point for our community. In that pictures there is me too even if I think you will be in trouble to find me out.

Cheers,

Marco

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