A box seat at OPERA

26/09/2011

ResearchBlogging.org

While at Bari Conference (see here), the news was spreading that OPERA Collaboration, a long baseline experiment using muon neutrino beams launched by CERN by CNGS Project, detected a possible Lorentz violating effect. Initially, it started as a rumor in the comment area at Jester’s blog (see here). Then, Tommaso Dorigo provided a full account on his blog well before the Collaboration come out with its results (see here and here) so, he was kindly advised to remove his ill-timed report by his management at CERN. This post, as you can see, is now back and gives, as usual for Tommaso, a very good description of facts. Similarly, you can find good posts about at vixra and Jester’s blog. Meantime, OPERA Collaboration published its paper on arxiv (see here) and, on Friday, gave a seminar at CERN that was broadcast through all the web. Today, we know that they performed very well at the measurement and, after a struggle with the data lasting about three years, they forcefully published the result waiting for all the community to scrutinize it. Indeed, at first it is very difficult to find some drawback in this work and, being all well-trained physicists, it appears quite difficult to expect this. So, the next and more important step is just to have this result replicated or not by independent labs.

It is interesting to note that this experiment was originally conceived to observe neutrino oscillations. What they should observe are tau neutrinos arising from the muon neutrinos coming from CERN. But this has proved really difficult and, after about 16000 events, they were able to get a possible serendipitous discovery. The spokesperson of the Collaboration is Antonio Ereditato and you can find an interview here.

My first impressions about this result were a couple of important points that surely should have been emphasized: It could be the first evidence of a Lorentz-violating effect and string theory could be put in some difficulty if this result should be confirmed. Theories introducing Lorentz-violating terms have been known for years. These are generally connected to possible formulations of quantum gravity and someone claimed them incorrect just because string mainstream needed a perfect Lorentz symmetry. Besides, in the sixties of the last century, tachyons were introduced by Gerald Feinberg (see here). These are particle with an immaginary mass and so, they could never be seen at rest. But their quantum field theory has an instability in the ground state that would change their nature from superluminal to subluminal breaking symmetry. You can realize this immediately if you have in mind a Higgs field. Neutrinos are Fermions but this does not change too much such a conclusion as for these particles a formulation of spin-statistics theorem could be a mess. But even if we accept their existence, a paper today on arxiv by Giovanni Amelino-Camelia, Giulia Gubitosi, Niccoló Loret, Flavio Mercati, Giacomo Rosati and Paolo Lipari (see here) rules them out as a possible explanation for the OPERA effect. This paper and the other by Giacomo Cacciapaglia, Aldo Deandrea, Luca Panizzi (see here) show that a proper analysis should be accomplished using a modified dispersion relation between energy and momenta. This is perfectly in line with the recently proposal for a modified special relativity that has Amelino-Camelia as one of the proponents. Anyhow, as emphasized by these authors, an in-depth scrutiny of the OPERA experiment is in need as the fits seem to point toward a somewhat exotic dispersion relation even if a kind of fit can be found. On the other side, Cacciapaglia&al. seem to find a fit with non-integer exponent putting OPERA result somewhat out of the theoretical proposals of these last years.

From a string theory standpoint, it appears a rather strange situation even if it is possible to propose modified formulations accounting for the Lorentz-violation and the reason relies on the fact that, essentially, one starts from a fully-fledged quantum field theory preserving all the cherished symmetries. We just point out that what appears today in view is a world with no strings and supersymmetry not even in sight but this is a rapidly changing scenario having LHC at full steam.

Finally, this appears the first significant move toward new physics and a great one indeed arising from an important collaboration. With LHC at full power and other labs now tuned, the future appears quite exciting.

The OPERA Collaboraton: T. Adam, N. Agafonova, A. Aleksandrov, O. Altinok, P. Alvarez Sanchez, S. Aoki, A. Ariga, T. Ariga, D. Autiero, A. Badertscher, A. Ben Dhahbi, A. Bertolin, C. Bozza, T. Brugiére, F. Brunet, G. Brunetti, S. Buontempo, F. Cavanna, A. Cazes, L. Chaussard, M. Chernyavskiy, V. Chiarella, A. Chukanov, G. Colosimo, M. Crespi, N. D’Ambrosios, Y. Déclais, P. del Amo Sanchez, G. De Lellis, M. De Serio, F. Di Capua, F. Cavanna, A. Di Crescenzo, D. Di Ferdinando, N. Di Marco, S. Dmitrievsky, M. Dracos, D. Duchesneau, S. Dusini, J. Ebert, I. Eftimiopolous, O. Egorov, A. Ereditato, L. S. Esposito, J. Favier, T. Ferber, R. A. Fini, T. Fukuda, A. Garfagnini, G. Giacomelli, C. Girerd, M. Giorgini, M. Giovannozzi, J. Goldberga, C. Göllnitz, L. Goncharova, Y. Gornushkin, G. Grella, F. Griantia, E. Gschewentner, C. Guerin, A. M. Guler, C. Gustavino, K. Hamada, T. Hara, M. Hierholzer, A. Hollnagel, M. Ieva, H. Ishida, K. Ishiguro, K. Jakovcic, C. Jollet, M. Jones, F. Juget, M. Kamiscioglu, J. Kawada, S. H. Kim, M. Kimura, N. Kitagawa, B. Klicek, J. Knuesel, K. Kodama, M. Komatsu, U. Kose, I. Kreslo, C. Lazzaro, J. Lenkeit, A. Ljubicic, A. Longhin, A. Malgin, G. Mandrioli, J. Marteau, T. Matsuo, N. Mauri, A. Mazzoni, E. Medinaceli, F. Meisel, A. Meregaglia, P. Migliozzi, S. Mikado, D. Missiaen, K. Morishima, U. Moser, M. T. Muciaccia, N. Naganawa, T. Naka, M. Nakamura, T. Nakano, Y. Nakatsuka, D. Naumov, V. Nikitina, S. Ogawa, N. Okateva, A. Olchevsky, O. Palamara, A. Paoloni, B. D. Park, I. G. Park, A. Pastore, L. Patrizii, E. Pennacchio, H. Pessard, C. Pistillo, N. Polukhina, M. Pozzato, K. Pretzl, F. Pupilli, R. Rescigno, T. Roganova, H. Rokujo, G. Rosa, I. Rostovtseva, A. Rubbia, A. Russo, O. Sato, Y. Sato, A. Schembri, J. Schuler, L. Scotto Lavina, J. Serrano, A. Sheshukov, H. Shibuya, G. Shoziyoev, S. Simone, M. Sioli, C. Sirignano, G. Sirri, J. S. Song, M. Spinetti, N. Starkov, M. Stellacci, M. Stipcevic, T. Strauss, P. Strolin, S. Takahashi, M. Tenti, F. Terranova, I. Tezuka, V. Tioukov, P. Tolun, T. Tran, S. Tufanli, P. Vilain, M. Vladimirov, L. Votano, J. -L. Vuilleumier, G. Wilquet, B. Wonsak, J. Wurtz, C. S. Yoon, J. Yoshida, Y. Zaitsev, S. Zemskova, & A. Zghiche (2011). Measurement of the neutrino velocity with the OPERA detector in the CNGS
beam arXiv arXiv: 1109.4897v1

Feinberg, G. (1967). Possibility of Faster-Than-Light Particles Physical Review, 159 (5), 1089-1105 DOI: 10.1103/PhysRev.159.1089

Giovanni Amelino-Camelia, Giulia Gubitosi, Niccoló Loret, Flavio Mercati, Giacomo Rosati, & Paolo Lipari (2011). OPERA-reassessing data on the energy dependence of the speed of neutrinos arXiv arXiv: 1109.5172v1

Giacomo Cacciapaglia, Aldo Deandrea, & Luca Panizzi (2011). Superluminal neutrinos in long baseline experiments and SN1987a arXiv arXiv: 1109.4980v1


The XV Workshop on Statistical Mechanics and nonperturbative Field Theory

25/09/2011

ResearchBlogging.org

This week I was in Bari as the physics department of that university organized a major event: SM&FT 2011. This is a biennial conference having the aims to discuss recent achievements in fields as statistical mechanics and quantum field theory that have a lot of commonalities. The organizers are well-known physicists and so it was a pleasure for me to see my contribution accepted. Leonardo Cosmai wrote to me confirming my partecipation. Leonardo, together with Paolo Cea, Alessandro Papa and Massimo d’Elia produced a lot of significant works in quantum field theory and a recent paper by Cosmai and Cea arose some fuzz also in the blogosphere (see here). Their forecast for the Higgs boson agrees quite well with my view about this matter. They were also part of the organizing committee. Of course, I was in Bari with my friend Marco Ruggieri that lived there for more than twelve years gaining a PhD in that university.

The scientific content was really interesting an I have had the chance to learn something more about lattice field theory. You can find all the talks here. About this, it should be said that people work with small lattices yet. While this has been a natural way to manage the QCD on the lattice due to missing computational resources, things are rapidly changing due to CUDA as I discussed a lot in my blog and was presented in some talks at this conference. Small groups will be able, with very few bucks of their budgets, to reach a significant ability to analyze increasingly lattice volumes. Besides, also large scale projects in this direction, mostly due to INFN and extending the APE project originated by Nicola Cabibbo and Giorgio Parisi, were presented (see talks by Francesco di Renzo e Piero Vicini). A typical situation in this kind of lattice analysis, improved using CUDA,  was also pointed out by Massimo D’Elia in his talk.  Thanks to this new technology they are increasing significantly the volumes. You can compare the content of his talk with that of his collaborator Francesco Negro, discussing a really interesting problem on the lattice (and a promise for the future with CUDA), with smaller volumes due to reduced computational resources. The interest for the activity of this group and Francesco’s work is strongly linked to a paper that I and Marco Ruggieri wrote together about the QCD vacuum in presence of a magnetic field (see here). The work by Francesco, even if for small volumes, provides interesting conclusions. It should be said that the Nambu-Jona-Lasinio model is there well alive and kicking.

Petruzzelli Theater

From a strictly theoretical side, I would like to point out the talks by Giuseppe Mussardo, with which I have had a nice mail exchange and is author of some beautiful books (e.g. see here), and the ones by Adriano Di Giacomo and Valentin Zakharov that seem to have some relevant contact points with my work. There was also a talk by Edward Shuryak, one of the proponents of the instantons liquid for the QCD vacuum that is strongly supported by lattice simulations and theoretical works like mine.

At the end of the social dinner, we have had some interesting discussions with Di Giacomo and Cosmai. There was some excitation about the announced seminar about neutrinos by CERN and INFN. In a pub after the dinner, I have had some interesting discussions about a proposal by Michele Pepe and others (see his talk) that holds the promises to improve significantly lattice computations removing artifacts. It was also the chance to hear the point of view of Owe Philipsen (see his talk) about the current situation on lattice simulations on QCD at finite temperature. As I have discussed in some posts in this blog, this kind of simulations are plagued by the infamous sign problem and most of the work turns back to try to get rid of it. My friend Marco expressed the somewhat pessimistic view that a critical endpoint will never be seen on lattice computations. Indeed, he is the proponent of the use of a chiral chemical potential that does not display this stumbling block on the lattice (see his talk). This approach holds the promises to reach the goal as he showed in a recent paper. His proposal is under scrutiny by the lattice community. The QCD critical endpoint is a Holy Grail for all of us working in this area as QCD displays a quite rich phase diagram and we have also a lot of experimental data in heavy ion collisions to understand. You should take a look both at the talks of Marco and Alessandro Papa.

I would like to have cited all the talks and I apologize for omissions. If my readers have some time to spend usefully just read it all, as the conference was well organized and with very interesting contents in a really nice atmosphere somehow excited by neutrino news in the last two days.

P. Cea, & L. Cosmai (2011). The trivial Higgs boson: first evidences from LHC arXiv arXiv: 1106.4178v1

Marco Frasca, & Marco Ruggieri (2011). Magnetic Susceptibility of the Quark Condensate and Polarization from Chiral Models Phys.Rev.D83:094024,2011 arXiv: 1103.1194v1

Marco Ruggieri (2011). The Critical End Point of Quantum Chromodynamics Detected by Chirally
Imbalanced Quark Matter Phys.Rev.D84:014011,2011 arXiv: 1103.6186v2


An interesting review

14/09/2011

ResearchBlogging.org

It is some time I am not writing posts but the good reason is that I was in Leipzig to IRS 2011 Conference, a very interesting event in a beautiful city.  It was inspiring to be in the city where Bach spent a great part of his life. Back to home, I checked as usual my dailies from arxiv and there was an important review by Boucaud, Leroy, Yaouanc, Micheli, Péne and Rodríguez-Quintero. This is the French group that produced striking results in the analysis of Green functions for Yang-Mills theory.

In this paper they do a great work by reviewing the current situation and clarifying  the main aspects of the analysis carried out using Dyson-Schwinger equations. These are a tower of equations for the n-point functions of a quantum field theory that can be generally solved by some truncation (with an exception, see here) that cannot be completely controlled. The reason is that the equation of lower order depends on n-point functions of higher orders and so, at some point, we have to decide the behavior of some of these higher order functions truncating the hierarchy. But this choice is generally not under control.

About these techniques there is a main date, Reigensburg 2007, when some kind of wall just went down. Since then, the common wisdom was a scenario with a gluon propagator going to zero when momenta go to zero while, in the same limit, the ghost propagator should go to infinity faster than the free case: So, the gluon propagator was suppressed and the ghost propagator enhanced at infrared. On the lattice, such a behavior was never explicitly observed but was commented that the main reason was the small volumes considered in these computations. On 2007, volumes reached a huge extension in lattice computations, till (27fm)^4, and so the inescapable conclusion was  that lattice produced another solution: A gluon propagator reaching a finite non-zero value and the ghost propagator behaving exactly as that of a free particle. This was also the prevision of the French group together with other researchers as Cornwall, Papavassiliou, Aguilar, Binosi and Natale. So, this new solution entered into the mainstream of the analysis of Yang-Mills theory in the infrared and was dubbed “decoupling solution” to distinguish it from the former one, called instead “scaling solution”.

In this review, the authors point out an important conclusion: The reason why authors missed the decoupling solution and just identified the scaling one was that their truncation forced the Schwinger-Dyson equation to a finite non-zero value of the strong coupling constant. This is a crucial point as this means that authors that found the scaling solution were admitting a non-trivial fixed point in the infrared for Yang-Mills equations. This was also the recurring idea in that days but, of course, while this is surely true for QCD, a world without quarks does not exist and, a priori, nothing can be said about Yang-Mills theory, a theory with only gluons and no quarks. Quarks change dramatically the situation as can also be seen for the asymptotic freedom. We are safe because there are only six flavors. But about Yang-Mills theory nothing can be said in the infrared as such a theory is not seen in the reality if not interacting with fermionic fields.

Indeed, as pointed out in the review, the running coupling was seen to behave as in the following figure (this was obtained by the German group, see here)

Running coupling of a pure Yang-Mills theory as computed on the lattice

This result is quite shocking and completely counterintuitive. It is pointing out, even if not yet confirming, that a pure Yang-Mills theory could have an infrared trivial fixed point! This is something that defies common wisdom and can explain why former researchers using the Dyson-Schwinger approach could have missed the decoupling solution. Indeed, this solution seems properly consistent with a trivial fixed point and this can also be inferred by the goodness of the fit of the gluon propagator with a Yukawa-like propagator if we content ourselves with the best agreement just in the deep infrared and the deep ultraviolet where asymptotic freedom sets in. In fact, with a trivial fixed point the theory is free in this limit but you cannot pretend agreement on all the range of energies with a free propagator.

Currently, the question of the right infrared behavior of the two-point functions for Yang-Mills theory is hotly debated yet and the matter that is at stake here is the correct understanding and management of low-energy QCD. This is one of the most fundamental physics problem and something I would like to know the answer.

Ph. Boucaud, J. P. Leroy, A. Le Yaouanc, J. Micheli, O. Péne, & J. Rodríguez-Quintero (2011). The Infrared Behaviour of the Pure Yang-Mills Green Functions arXiv arXiv: 1109.1936v1

Marco Frasca (2009). Exact solution of Dyson-Schwinger equations for a scalar field theory arXiv arXiv: 0909.2428v2

I. L. Bogolubsky, E. -M. Ilgenfritz, M. Müller-Preussker, & A. Sternbeck (2009). Lattice gluodynamics computation of Landau-gauge Green’s functions in
the deep infrared Phys.Lett.B676:69-73,2009 arXiv: 0901.0736v3

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