Do quarks grant confinement?


In 2010 I went to Ghent in Belgium for a very nice Conference on QCD. My contribution was accepted and I had the chance to describe my view about this matter. The result was this contribution to the proceedings. The content of this paper was really revolutionary at that time as my view about Yang-Mills theory, mass gap and the role of quarks was almost completely out of track with respect to the rest of the community. So, I am deeply grateful to the Organizers for this opportunity. The main ideas I put forward were

  • Yang-Mills theory has an infrared trivial fixed point. The theory is trivial exactly as the scalar field theory is.
  • Due to this, gluon propagator is well-represented by a sum of weighted Yukawa propagators.
  • The theory acquires a mass gap that is just the ground state of a tower of states with the spectrum of a harmonic oscillator.
  • The reason why Yang-Mills theory is trivial and QCD is not in the infrared limit is the presence of quarks. Their existence moves the theory from being trivial to asymptotic safety.

These results that I have got published on respectable journals become the reason for rejection of most of my successive papers from several referees notwithstanding there were no serious reasons motivating it. But this is routine in our activity. Indeed, what annoyed me a lot was a refeee’s report claiming that my work was incorrect because the last of my statement was incorrect: Quark existence is not a correct motivation to claim asymptotic safety, and so confinement, for QCD. Another offending point was the strong support my approach was giving to the idea of a decoupling solution as was emerging from lattice computations on extended volumes. There was a widespread idea that the gluon propagator should go to zero in a pure Yang-Mills theory to grant confinement and, if not so, an infrared non-trivial fixed point must exist.

Recently, my last point has been vindicated by a group that was instrumental in the modelling of the history of this corner of research in physics. I have seen a couple of papers on arxiv, this and this, strongly supporting my view. They are Markus Höpfer, Christian Fischer and Reinhard Alkofer. These authors work in the conformal window, this means that, for them, lightest quarks are massless and chiral symmetry is exact. Indeed, in their study quarks not even get mass dynamically. But the question they answer is somewhat different: Acquired the fact that the theory is infrared trivial (they do not state this explicitly as this is not yet recognized even if this is a “duck” indeed), how does the trivial infrared fixed point move increasing the number of quarks? The answer is in the following wonderful graph with N_f the number of quarks (flavours):

QCD Running CouplingFrom this picture it is evident that there exists a critical number of quarks for which the theory becomes asymptotically safe and confining. So, quarks are critical to grant confinement and Yang-Mills theory can happily be trivial. The authors took great care about all the involved approximations as they solved Dyson-Schwinger equations as usual, this is always been their main tool, with a proper truncation. From the picture it is seen that if the number of flavours is below a threshold the theory is generally trivial, so also for the number of quarks being zero. Otherwise, a non-trivial infrared fixed point is reached granting confinement. Then, the gluon propagator is seen to move from a Yukawa form to a scaling form.

This result is really exciting and moves us a significant step forward toward the understanding of confinement. By my side, I am happy that another one of my ideas gets such a substantial confirmation.

Marco Frasca (2010). Mapping theorem and Green functions in Yang-Mills theory PoS FacesQCD:039,2010 arXiv: 1011.3643v3

Markus Hopfer, Christian S. Fischer, & Reinhard Alkofer (2014). Running coupling in the conformal window of large-Nf QCD arXiv arXiv: 1405.7031v1

Markus Hopfer, Christian S. Fischer, & Reinhard Alkofer (2014). Infrared behaviour of propagators and running coupling in the conformal
window of QCD arXiv arXiv: 1405.7340v1

Dust is finally settling…


The situation about Yang-Mills theory is finally settling down. I do not mean that mathematicians’ community has finally decided the winner of the Millenium prize but rather that people working on the study of two-point functions on a pure Yang-Mills theory have finally a complete scenario for it. These studies have seen very hot debates and breakthrough moments with the use of important computing resources at different facilities. I have tried to sum up this very beautiful piece of history of physical science here. Just today a paper by Attilio Cucchieri, David Dudal and Nele Vandersickel is appeared on arXiv making clear a fundamental aspect of this scenario. Attilio is a principal figure in the Brazilian group that carried out fundamental results in this area of research and was instrumental in the breakthrough at Regensburg 2007. David and Nele were essential into the realization of Ghent conference on 2010 and their work, as we will see in a moment, displays interesting results that could be important for a theoretical understanding of Yang-Mills theory.

The question of the Green functions for Yang-Mills theory can be recounted in two very different views about their behavior at very low energies. Understanding the behavior of these functions in this energy limit could play an essential role to understand confinement, one of the key problems of physics today. Of course, propagators depend on the gauge choice and so, when we talk of them here we just mean in the Landau gauge.  But they also code some information that does not depend on the gauge at all as the mass spectrum. So, If one wants to know if the gluon becomes massive and how big is that mass then, she should turn her attention to these functions. But also, if I want to do QCD at very low energies I need these functions to be able to do computations, something that theoretical physicists are not able to perform precisely yet missing this piece of information.

In the ’90, the work performed by several people seemed to convince everyone that the gluon propagator should go to zero lowering momenta and the ghost propagator should run to infinity faster than the case of a free particle. Difficulties with computational resources impeded to achieve the right volume dimensions to draw clearcut conclusions about, working on the lattice. But another solution was emerging, with a lot of difficulties and while a paradigm seemed to be already imposed, proving that the gluon propagator should reach a finite non-null limit at zero momenta and the ghost propagator was behaving like a free particle. A massive gluon propagator was already proposed in the ’80 by John Cornwall and this idea was finally gaining interest. After Regensburg 2007, this latter solution finally come into play as lattice results on huge volumes were showing unequivocally that the massive solution was the right one. The previous solution was then called “scaling solution” while the massive one was dubbed “decoupling solution”.

A striking result obtained by Axel Maas (see here) showed that, in two dimensions, the propagators agree with the scaling solution. This is quite different from the three and four dimensional case where the massive solution is seen instead. This problem was a main concern for people working on the lattice as a theoretical understanding was clearly in need here. Attilio asked to me if I could come out with an explanation with my approach. I have found a possible answer here but this was not the answer Attilio was looking for. With this paper he has found the answer by himself.

The idea is the following. In order to understand the behavior of the propagators in different dimensions one has to solve the set of coupled Dyson-Schwinger equations for the ghost and gluon propagators as one depends on the other.  In this paper they concentrate just on the equation for the ghost propagator and try to understand, in agreement with the no-pole idea of Gribov that the ghost propagator must have no poles, when its solution is consistent. This is a generalization of an idea due to Boucaud, Gómez, Leroy, Yaouanc, Micheli, Pène and Rodríguez-Quintero (see here): Consider the equation of the ghost propagator and compute it fixing a form for the gluon propagator, then see when the solution is physically consistent. In their work, Boucaud et al. fix the gluon propagator to be Yukawa-like, a typical massive propagator for a free particle. Here I was already happy because this is fully consistent with my scenario (see here): I have a propagator being the sum of Yukawa-like propagators typical of a trivial infrared fixed point where the theory becomes free. Attilio, David and Nele apply this technique to a propagator devised by Silvio Paolo Sorella, David Dudal, John Gracey, Nele Vandersickel and Henry Verschelde that funded the so-called “Refined Gribov-Zwanziger” scenario (see here). The propagator they get can be simply rewritten as the sum of three Yukawa propagators and so, it is fully consistent with my results. Attilio, David and Nele use it to analyze the behavior of the ghost propagator and to understand its behavior at different dimensions, using Gribov no-pole condition. Their results are indeed striking. They recover a critical coupling at which the scaling solution works in 2 and 3 dimensions: Only when the coupling has this particular value the scaling solution can apply but this is not the real case. Also, as Attilio, David and Nele remeber us, this critical point is unstable as recently showed by Axel Weber (see here). This agrees with the preceding finding by Boucaud et al. but extends the conclusions to different dimensions. In two dimensions a strange thing happen: There is a logarithmic singularity at one-loop for the ghost propagator that can only be removed taking the gluon propagator going to zero and to make the Gribov no-pole condition hold. This is indeed a beautiful physical explanation and gives an idea on what is going on by changing dimensions to these propagators. I would like to emphasize that also the refined Gribov-Zwanziger scenario agrees perfectly well with my idea of a trivial infrared fixed point that is also confirmed by lattice data, having the gluon propagator the sum of Yukawa propagators. I think we can merge our results at some stage fixing the parameters.

Given all this clear view that is finally emerged, maybe it is time to turn to phenomenology. There is a lot of people, for example there at CERN, waiting for fully working models of low-energy QCD. All the people I cited here and a lot more I would like to name have given the answer.

Attilio Cucchieri, David Dudal, & Nele Vandersickel (2012). The No-Pole Condition in Landau gauge: Properties of the Gribov Ghost
Form-Factor and a Constraint on the 2d Gluon Propagator arXiv arXiv: 1202.1912v1

Axel Maas (2007). Two- and three-point Green’s functions in two-dimensional Landau-gauge Yang-Mills theory Phys.Rev.D75:116004,2007 arXiv: 0704.0722v2

Boucaud, P., Gómez, M., Leroy, J., Le Yaouanc, A., Micheli, J., Pène, O., & Rodríguez-Quintero, J. (2010). Low-momentum ghost dressing function and the gluon mass Physical Review D, 82 (5) DOI: 10.1103/PhysRevD.82.054007

Marco Frasca (2007). Infrared Gluon and Ghost Propagators Phys.Lett.B670:73-77,2008 arXiv: 0709.2042v6

David Dudal, John Gracey, Silvio Paolo Sorella, Nele Vandersickel, & Henri Verschelde (2008). A refinement of the Gribov-Zwanziger approach in the Landau gauge: infrared propagators in harmony with the lattice results Phys.Rev.D78:065047,2008 arXiv: 0806.4348v2

Axel Weber (2011). Epsilon expansion for infrared Yang-Mills theory in Landau gauge arXiv arXiv: 1112.1157v1

Yang-Mills mass gap scenario: Further confirmations


Alexander (Sasha) Migdal was a former professor at Princeton University. But since 1996, he is acting as a CEO of a small company. You can read his story from that link. Instead, Marco Bochicchio was a former colleague student of mine at University of Rome “La Sapienza”. He was a couple of years ahead of me. Now, he is a researcher at Istituto Nazionale di Fisica Nucleare, the same of OPERA and a lot of other striking contributions to physics. With Marco we shared a course on statistical mechanics held by Francesco Guerra at the department of mathematics of our university. Today, Marco posted a paper of him on arXiv (see here). I am following these works by Marco with a lot of interest because they contain results that I am convinced are correct, in the sense that are describing the right scenario for Yang-Mills theory. Marco, in this latter work, is referring to preceding publications from Sasha Migdal about the same matter that go back till ’70s! You can find a recollection of these ideas in a recent paper by Sasha (see here). So, what are these authors saying? Using somewhat different approaches than mine (that you can find well depicted here), they all agree that a Yang-Mills theory has a propagator going like


being Z_n some numbers and m_n is given by the zeros of some Bessel functions. This last result seems quite different from mine that I get explicitly m_n=(n+1/2)m_0 but this is not so because, in the asymptotic regime, J_k(x)\propto \cos(x-k\pi/2-\pi/4)/\sqrt{x} and zeros for the cosine go like (n+1/2)\pi and then, my spectrum is easily recovered in the right limit. The right limit is properly identified by Sasha Migdal from Padè approximants for the propagator that start from the deep Euclidean region \Lambda\rightarrow\infty, being \Lambda an arbitrary energy scale entering into the spectrum. So, the agreement between the scenario proposed by these authors and mine is practically perfect, notwithstanding different mathematical approaches are used.

The beauty of these conclusions is that such a scenario for a Yang-Mills theory is completely unexpected but it is what is needed to grant confinement. So, the conclusion about the questions of mass gap and confinement is approaching. As usual, we hope that the community will face these matters as soon as possible making them an important part of our fundamental knowledge.

Marco Bochicchio (2011). Glueballs propagators in large-N YM arXiv arXiv: 1111.6073v1

Alexander Migdal (2011). Meromorphization of Large N QFT arXiv arXiv: 1109.1623v2

Marco Frasca (2010). Mapping theorem and Green functions in Yang-Mills theory PoS FacesQCD:039,2010 arXiv: 1011.3643v3

The Gribov obsession


I have treated the question of Yang-Mills propagators in-depth in my blog being one of my main concerns. There is an important part of the scientific community aimed to understand how these functions behave both at lower energies and overall on the whole energy range. The motivation to write down these few lines today arises from a number of interesting comments that an anonymous reader yielded to this post. If you already read it you know the main history about this matter otherwise you are urged to do so. The competitors in this arena are a pair of different solutions to the question of the propagators: The scaling solution and the decoupling solution. In the former case one expects the gluon propagator to go to zero as momenta lower and the ghost propagator should run to infinity faster than the free case. Similarly, one should have the running coupling to reach a finite value in the same limit. In the other case, the gluon propagator reaches a finite non-zero value toward zero momenta, the ghost propagator behaves as that of a free massless particle and the running coupling seems not to reach any finite value but rather bends significantly toward zero signaling a trivial infrared fixed point for Yang-Mills theory. In this post I would like to analyze the question of the genesis of the scaling solution. It arises from the Gribov obsession.

So, what is the Gribov obsession? Let us consider the case of electromagnetism. This does not give full reason to all this matter but just a hint about what is going on. The question bothering people is gauge fixing. To do computations in quantum field theory you need the gauge properly fixed and this is done in different ways. In the Lorenz gauge for example you will be able to do explicitly covariant computations but states have not all positive norm. But if you fix your gauge in the usual way, there is a residual as you can always add a solution of the wave equation for the gauge function and the physics does not change. This residual freedom is just harmless and, indeed, quantum electrodynamics is one of the most successful theories in the history of physics.

In non-Abelian gauge theories, Lorenz gauge is also called Landau gauge and the situation is well richer for residual gauge freedom that gauge fixing does not appear to be enough to grant consistent computations. This question was put forward firstly by Gribov and one has to cope with Gribov copies. Gribov copies should be renamed Gribov obsession as I did. If you want a fine description of the problem you can read this paper by Alfred Actor, appendix H or also the beautiful paper by Silvio Sorella and Rodrigo Sobreiro (see here). Now, we all know that when people is doing perturbation theory in QCD and uncover asymptotic freedom, there is no reason to worry about Gribov copies. They are simply harmless. So, the question is how much are important in the low energy (infrared) case.

This question transformed the original Gribov obsession in the obsession of many. Gribov himself proposed a solution limiting solutions to the so called first Gribov horizon as Gribov pointed out that the set of gauge orbits can be subdivided in regions with the first one having the Fadeed-Popov determinant with all positive eigenvalues and the next ones with eigenvalues becoming zero and then going to negative. In this way he was able to get a confining propagator that unfortunately is not causal. The question is then if limiting in this way the solutions of Yang-Mills theory gives again meaningful physical results. We should consider that this was a conjecture by Gribov and, while surely Gribov copies exist, it could be that imposing such a constraint is simply wrong as could be imposing any other constraint at all. One can also assert with the same right that Gribov copies can be ignored and starting to do physics from this. Now, the point is that the scaling solution arises from the Gribov obsession.

Of course, in my papers I showed (see here and refs therein), through perturbation theory, that in the deep infrared we can completely forget about Gribov copies. This is due to the appearance of an infrared trivial fixed point that makes the theory free in this limit reducing the case to the same of the ultraviolet limit. Starting perturbation theory from this point makes all the matter simply harmless. This scenario has been shown correct by lattice computations that recover the infrared fixed point and so are surely sound. The decoupling solution, now found by many researchers, is there to testify the goodness of the work researchers working with lattices and computers have done so far.

Finally, let me repeat my bet:

I bet 10 euros, or two rounds of beer at the next conference after the result is made manifestly known, that Gribov copies are not important in Yang-Mills theory at very low energies.

Nobody interested?

Actor, A. (1979). Classical solutions of SU(2) Yang—Mills theories Reviews of Modern Physics, 51 (3), 461-525 DOI: 10.1103/RevModPhys.51.461

R. F. Sobreiro, & S. P. Sorella (2005). Introduction to the Gribov Ambiguities In Euclidean Yang-Mills Theories arxiv arXiv: hep-th/0504095v1

Marco Frasca (2010). Mapping theorem and Green functions in Yang-Mills theory arxiv arXiv: 1011.3643v1

The many faces of QCD (2)


Back at home, conference ended. A lot of good impressions both from the physics side and other aspects as the city and the company. On Friday I held my talk. All went fine and I was goodly inspired so to express my ideas at best. You can find all the talks here. The pictures are here. Now it should be easier to identify me.

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.

On Tuesday afternoon started a session about phases in QCD. This field is very active and is a field where some breakthroughs are expected to be seen in the near future. I have had a lot of fun to know Eduardo Fraga that was here with two of his students: Leticia Palhares and Ana Mizher. I invite you to read their talks as this people are doing a real fine work. On the same afternoon I listened to the talk of Pedro Bicudo. Pedro, besides being a nice company for fun, is also a very good physicist performing relevant work in the area of lattice QCD. He is a pioneer in the use of CUDA, parallel computing using graphic processors, and I intend to use his code, produced with his student Nuno Cardoso, on my machine to start doing lattice QCD at very low cost. On his talk you can see a photo of one of my graphic cards. He used lattice computations to understand the phase diagram of QCD. Quite interesting has been the talk of Jan Pawlowski about the phase diagram of two flavor QCD. He belongs to a group of people that produced the so called scaling solution and it is a great moment to see them to recognize the very existence of the decoupling solution, the only one presently seen on lattice computations.

On Wednesday the morning session continued on the same line of the preceding day. I would like to cite the work of Marco Ruggieri because, besides being a fine drinking companion (see below), he faces an interesting problem:  How does the ground state of QCD change in presence of a strong magnetic field? Particularly interesting is to see how the phase diagram gets modified. On the same line were the successive talks of Ana Mizher and Maxim Chernodub. Chernodub presented a claim that in this case vacuum is that of an electromagnetic superconductor due to \rho meson condensation. In this area of research the main approach is to use some phenomenological model. Ana Mizher used a linear sigma model while Marco preferred the Nambu-Jona-Lasinio model. The reason for this is that the low-energy behavior of QCD is not under control and the use of well-supported effective models is the smarter approach we have at our disposal. Of course, this explains why the work of our community is so important: If we are able to model the propagator of the gluon in the infrared, all the parameters of the Nambu-Jona-Lasinio model are properly fixed and we have the true infrared limit of QCD. So, the stake is very high here.

In the afternoon there were some talks that touched very near the question of infrared propagators. Silvio Sorella is an Italian theoretical physicist living in Brazil. He is doing a very good work in this quest for an understanding of the low-energy behavior of QCD. This work is done in collaboration with several other physicists. The idea is to modify the Gribov-Zwanziger scenario, that by itself will produce the scaling solution currently not seen on the lattice, to include the presence of a gluon condensate. This has the effect to produce massive propagators that agree well with lattice computations. In this talk Silvio showed how this approach can give the masses of the lowest states of the glueball spectrum. This has been an important step forward showing how this approach can be used to give experimental forecasts. Daniel Zwanziger then presented a view of the confinement scenario. The conclusion was very frustrating: So far nobody can go to the Clay Institute to claim the prize. More time is needed. Daniel has been the one who proposed the scenario of infrared Yang-Mills theory that produced the scaling solution. The idea is to take into account the problem of Gribov copies and to impose that all the computations must be limited to the first Gribov horizon. If you do this the gluon propagator goes to zero lowering momenta and you get positivity maximally violated obtaining a confining theory. So, this scenario has been called Gribov-Zwanzinger. From lattice computations we learned that the gluon propagator reaches a non zero finite value lowering momenta and this motivated Silvio and others to see if one could maintain the original idea of Gribov horizon and agreement with lattice computations of the Gribov-Zwanzinger scenario. Matthieu Thissier presented a talk with an original view. The idea is to consider QCD with a small perturbation expansion at one loop and a mass term added by hand. He computed the gluon propagator and compared with lattice data till the infrared obtaining a very good agreement. Arlene Aguilar criticized strongly this approach as he worked with a coupling larger than one (a huge one said Arlene) even if he was doing small perturbation theory. I talked about this with Matthieu. My view is that the main thing to learn from this kind of  computations is that if you take a Yukawa-like propagator with a mass going at least as m^2+cq^2 (do you remember Orlando Oliveira talk?) the agreement with lattice data is surely fairly good and so, even if you have done something that is mathematically questionable, surely we apprehend an important fact! The afternoon session was concluded by the talk of Daniele Binosi. With Daniele we spent a nice night in Ghent. He is a student of Joannis Papavassiliou and, together with Arlene Aguilar, this group is doing fine work on numerically solving Dyson-Schwinger equations to get the full propagator of Yang-Mills theory. They get a very good agreement with lattice data and support the view that, on the full range of energies, the Cornwall propagator for the gluon with a logarithmic running mass reaching a constant in the infrared is the right description of the theory. Daniele presented a beautiful computation based on Batalin-Vilkoviski framework that supported the conclusions of his group. It should be said that he presented a different definition of the running coupling that grants a non-trivial fixed point at infrared. This is  a delicate matter as, already a proper definition of the running coupling for the infrared is not a trivial question. Daniele’s definition is quite different from that given by Andre Sternbeck in his talk as the latter has just the trivial fixed point as is emerging from the lattice computations.

On Thursday the first speaker was Attilio Cucchieri. Attilio and his wife, Tereza Mendes, are doing a fine work on lattice computations that reached a breakthrough at Lattice 2007 when they showed, with a volume of (27fm)^4, that the gluon propagator in the Landau gauge reaches a finite non-zero value lowering momenta. This was a breakthrough, confirmed at the same conference by two others groups (Orlando Oliveira by one side and I. Bogolubsky, E.M. Ilgenfritz, M. Muller-Preussker and A. Sternbeck by the other side), as for long time it was believed that the only true solution was the scaling one and the gluon propagator should have gone  to zero lowering momenta. This became a paradigm so that papers have got rejected on the basis that they were claiming a different scenario. Attilio this time was on a very conservative side presenting an interesting technical problem. Tereza’s talk was more impressive showing that, with higher temperatures and increasing volumes, in the Landau gauge the plateau is still there. With Tereza and Attilio we spent some nice time in a pub discussing together with Marco Ruggeri about the history of their community, how they went to change everything about this matter and their fighting for this. I hope one day this people will write down this history because there is a lot to learn from it. In the afternoon session there was a talk by Reinhard Alkofer. Alkofer has been instrumental in transforming the scaling solution into a paradigm for a lot of years in the community. Unfortunately lattice computations talked against it and, as Bob Dylan one time said, times are changing. He helped the community with discovering a lot of smart students that have given an important contribution to it. In his talk he insisted with his view with a proposal for the functional form for the propagator (this was missing until now for the scaling solution) and a computation of the mass of the \eta'. \eta' is a very strange particle. From {\rm DA}\Phi{\rm NE} (KLOE-2) we know that this is not just a composite state of quarks but it contains a large part made of glue: It is like to have to cope with an excited hydrogen atom and so, also its decay is to be understood (you can read my paper here). So, maybe a more involved discussion is needed before to have an idea of how to get the mass of this particle. After Alkofer’s talk followed the talks of Aguilar and Papavassiliou. I would like to emphasize the relevance of the work of this group. Aguilar showed how they get an effective quark mass from Schwinger-Dyson equations when there is no enhancement in the ghost propagator. Papavassiliou proposed to extend the background field method to Schwinger-Dyson equations. I invite you to check the agreement they get for the Cornwall propagator of the gluon with lattice data in Arlene’s talk and how this can give the form m^2+cq^2  at lower momenta. My view is that, combining my recent results on strongly coupled expansions for Yang-Mills and scalar field theories and the results of this group, a meaningful scenario is emerging giving a complete comprehension of what is going on for Yang-Mills theory at lower energies. Joannis gave us an appointment for the next year in Trento. I will do everything I can to be there! Finally, the session was completed with Axel Mass’ talk. Axel has been a student of Alkofer and worked with Attilio and Tereza. He put forward a lattice computation of Yang-Mills propagators in two dimensions that, for me, should have completely settled the question but produced a lot of debate instead. He gave in his talk another bright idea: To study on the lattice a scalar theory interacting with gluons. I think that this is a very smart way to understand the mechanism underlying mass generation in these theories. From the works discussed so far it should appear clear that Schwinger mechanism (also at classical level (see my talk)!) is at work here.  The talk of Axel manifestly shows this. It would be interesting if he could redo the computations taking a massless scalar field to unveil completely the dynamical generation of masses.

On Friday the morning session started with an interesting talk by Hans Dierckx trying to understand cardiac behavior using string theory. A talk by Oliver Rosten followed. Oliver produced a PhD thesis on the exact renormalization group of about 500 pages (see here). His talk was very beautiful and informative and in some way gave a support to mine. Indeed, he showed, discussing on the renormalization group, how a strong coupling expansion could emerge. In some way we are complimentary. I will not discuss my talk here but you are free to ask questions. The conference was concluded by a talk of Peter van Baal. Peter has a terrible story about him and I will not discuss it here. I can only wish to him the best of the possible lucks.

Finally, I would like to thank the organizers for the beautiful conference they gave me the chance to join. The place was very nice (thanks Nele!) and city has an incredible beauty. I think these few lines do not do justice to them and all the participants for what they have given. See you again folks!

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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