Chiral condensates in a magnetic field: Accepted!


As my readers could know, I have had a paper written in collaboration with Marco Ruggieri (see here). Marco is currently working at Yukawa Institute in Kyoto (Japan). The great news is that our paper has been accepted for publication on Physical Review D. I am really happy for this very good result of a collaboration that I hope will endure. Currently, we are working on a proof of existence of the critical endpoint for QCD using the Nambu-Jona-Lasinio model. This is an open question that has serious difficulties to get an answer also for a fundamental problem encountered on the lattice: The so-called sign problem. So, a mathematical proof will make a breakthrough in the field with the possibility to be experimentally confirmed at laboratory facilities.

Marco himself proposed a novel approach to get a proof of the critical endpoint to bypass the sign problem (see here).

So, I hope to have had the chance to transmit to my readership the excitement of this line of research and how it is strongly entangled with the understanding of low-energy QCD and the more deep question of the mass gap in the Yang-Mills theory. Surely, I will keep posting on this. Just stay tuned!

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

Philippe de Forcrand (2010). Simulating QCD at finite density PoS (LAT2009)010, 2009 arXiv: 1005.0539v2

A simpler explanation for the CDF bump


A lot of fuss arose about the recent almost finding of a new particle at Tevatron (see here). Several exotic hypotheses were put forward mostly looking for physics beyond Standard Model. Of course, being there such a bump at about 3\sigma, we cannot yet cry out for a discovery and more mundane explanations could exist.

Indeed, this is the content of this paper appeared on arXiv. These authors point out some weak points in the analysis done by CDF that amount in the end at an imperfect estimation of the background. This is also my claim as strong interactions are not completely under control. I give here authors’ conclusions for your considerations:

In conclusion, we observe that the dijet invariant mass peak seen in the recent CDF Wjj cross section is completely consistent with the excess observed in the CDF single-top-quark analysis. Both may be explained by an upward fluctuation in the CDF data set of s-channel single-top-quark production, and t-channel production accompanied by an additional low-energy jet. The latter process is poorly modeled by Monte Carlo, and the apparent t-channel excess could simply be an artifact of theoretical uncertainty. Given the modest excess observed by the D0 Collaboration in their single-top-quark data set , we predict the D0 Collaboration would not see a significant dijet invariant mass peak if they follow the CDF procedure.

So, Standard Model strikes back again.

CDF Collaboration, & T. Aaltonen (2011). Invariant Mass Distribution of Jet Pairs Produced in Association with a
W boson in ppbar Collisions at sqrt(s) = 1.96 TeV arXiv arXiv: 1104.0699v1

Zack Sullivan, & Arjun Menon (2011). A standard model explanation of a CDF dijet excess in Wjj arXiv arXiv: 1104.3790v1

A physics software repository


Scientific publishing has undergone a significant revolution after Paul Ginsparg introduced arXiv. Before this great idea, people doing research used to send preprints of their works to some selected colleagues for comments. This kind of habit was costly, time consuming and reached very few people around the World until the paper eventually went through some archival journal. Ginsparg’s idea was to use the web to accomplish this task making widely known papers well before publication to all the community. This changed the way we do research as it is common practice to put a paper on arXiv before submission to journals. This has had the effect to downgrade the relevance of these journal for scientific communication. This is so true that Perelman’s papers on Poincaré conjecture never appeared on literature, they are just on arXiv, but the results were anyhow generally acknowledged by the scientific community. This represents an extraordinary achievement for arXiv and shows unequivocally the greatness of Ginsparg’s idea.

Of course, research is not just writing articles and get them published somewhere. An example is physics where a lot of research activity relies on writing computer programs. This can happen on a lot of platforms as Windows, Mac, Linux or machines performing parallel computations. Generally, these programs are relegated to some limited use to a small group of researchers and other people around the World, having similar problems, could be in need of it but are forced to reinvent the wheel. This happens again and again and often one relies on the kindness of colleagues that in some cases could have not the good will to give away the software. This situation is very similar to the one encountered before arXiv come into operation. So, my proposal is quite simple: People in the scientific community having the good will to share their software should be stimulated to do so through a repository that fits the bill. This could be easily obtained by extending arXiv itself that already contains several papers presenting software written by our colleagues that, aiming to share, just put there a link. But having a repository, it could be easier to maintain versions as already happens to paper and there would be no need to create an ad hoc site that could be lost in the course of time.

I do not know if this proposal will meet with success but it is my personal conviction that a lot of people around the World has this need and this could be easily realized by the popularity of certain links to download programs for doing computations in physics. This need is increasingly growing thanks to parallel computation made available to desktop computers that today is a reality. I look forward to hear news about this.

Today in arXiv


After the excitation for the findings at Tevatron, we turn back to routine. Of course, I have never forgotten to cast a glance at arXiv where it is crystal clear the vitality of the physics community. I want to put down these few lines to point to your attention a couple of papers appeared today on the preprint archive. Today, Nele Vandersickel uploaded her PhD Thesis (see here). She has got her PhD on March this year. Nele was one of the organizers of the beautiful and successful conference in Ghent (Belgium) where I was present last year (see here, here and here). But most important is her research work with the group of Silvio Sorella and David Dudal that is the central theme of her thesis. Nele does an excellent job in presenting a lot of introductory material, difficult to find in the current literature, beside her original research. Sorella and Dudal have accomplished an interesting research endeavor by supporting the Gribov-Zwanziger scenario, at odds in the initial formulation with lattice data, with their view that condensates must be accounted for. In this way, Gribov-Zwanziger scenario can be taken to agree with lattice computations.  These theoretical studies describe a consistent approach and these authors were able to obtain the masses of the first glueball states. I would like to conclude with my compliments for the PhD reached by Nele and for the excellent wotk her and the other people in the group were able to realize.

The other fine paper I have found is a report by a group of authors, “Discoverig Technicolor”, giving a full account of the current situation for this theoretical approach to the way particles acquire their masses. As you know, the original formulation of the Higgs particle that entered into the Standard Model contains some drawbacks that motivated several people to find better solutions. Technicolor is one of these. One assumes the existence of a set of Fermions with a self-interaction. We know that this kind of models, as Nambu-Jona-Lasinio is, are able to break symmetries and generate masses to massless particles. Indeed, one can formulate a consistent theory with respect to all the precision tests of the Standard Model as also discussed in this report. This means in turn that in accelerator facilities one should look for some other Fermions and their bound states that can also mimic a standard Higgs scalar boson. It is important to note that in this way some drawbacks of the original Higgs mechanism are overcome. Of course, the relevance of this report cannot be underestimated in view of the results coming out from LHC and we could know very soon if an idea like Technicolor is the right  one or not. For sure, this is time for answers in the end.

Nele Vandersickel (2011). A study of the Gribov-Zwanziger action: from propagators to glueballs arXiv arXiv: 1104.1315v1

J. R. Andersen, O. Antipin, G. Azuelos, L. Del Debbio, E. Del Nobile, S. Di Chiara, T. Hapola, M. Jarvinen, P. J. Lowdon, Y. Maravin, I. Masina, M. Nardecchia, C. Pica, & F. Sannino (2011). Discovering Technicolor arXiv arXiv: 1104.1255v1

A new particle at Fermilab?


I am a registered reader at New York Times and subscribed Dennis Overbye‘s articles. So, this morning I received the mail from the journal with a new writing from Dennis. The title is “At Particle Lab, a Tantalizing Glimpse Has Physicists Holding Their Breaths”. I just jumped on my chair and then, eagerly, read the article. Indeed, CDF Collaboration posted a paper on arXiv last night (see here). The kind of process that they studied is the one with a final diboson (WW or WZ) from a lepton plus jets. They analyzed the invariant mass for masses higher than 100\ GeV/c^2. They get a 3\sigma excess in the region 120-160\ GeV/c^2 that, if confirmed, should correspond to a new particle. Some tests seem  to point toward a non-Standard Model particle having a mass of 150\ GeV/c^2 that cannot be identified with Higgs.

As always, it is important to emphasize the this is a 3\sigma evidence and further analysis is needed to confirm or disprove the discovery. But it is important to emphasize that Tevatron is paving the way to a large number of discoveries to be seen soon at LHC.

Update: CDF will present these results at a seminar. We can follow it on the web here.

Another update: A post by Tommaso Dorigo, one of the authors of the CDF paper, is here.

CDF Collaboration, & T. Aaltonen (2011). Invariant Mass Distribution of Jet Pairs Produced in Association with a
W boson in ppbar Collisions at sqrt(s) = 1.96 TeV arXiv arXiv: 1104.0699v1

QCD at finite temperature: Does a critical endpoint exist?


Marco Ruggieri is currently a post-doc fellow at Yukawa Institute for theoretical physics in Kyoto (Japan). Marco has got his PhD at University of Bari in Italy and spent a six months period at CERN. Currently, his main research areas are QCD at finite temperature and high density, QCD behavior in strong magnetic fields and effective models for QCD but you can find a complete CV at his site. So, in view of his expertize I asked him a guest post in  my blog to give an idea of the current situation of these studies. Here it is.

It is well known that Quantum Chromodynamics (QCD) is the most accredited theory describing strong interactions. One of the most important problems of modern QCD is to understand how color confinement and chiral symmetry breaking are affected by a finite temperature and/or a finite baryon density. For what concerns the former, Lattice simulations convince ourselves that both deconfinement and (approximate) chiral symmetry restoration take place in a narrow range of temperatures, see the recent work for a review. On the other hand, it is problematic to perform Lattice simulations at finite quark chemical potential in true QCD, namely with number of color equal to three, because of the so-called sign problem, see here for a recent review on this topic. It is thus very difficult to access the high density region of QCD starting from first principles calculations.

Despite this difficulty, several work has been made to avoid the sign problem, and make quantitative predictions about the shape of the phase diagram of three-color-QCD in the temperature-chemical potential plane, see here again for a review. One of the most important theoretical issues in along this line is the search for the so-called critical endpoint of the QCD phase diagram, namely the point where a crossover and a first order transition line meet. Its existence was suggested by Asakawa and Yazaki (AY) several years ago (see here) using an effective chiral model; in the 2002, Fodor and Katz (FK) performed the first Lattice simulation (see here) in which it was shown that the idea of AY could be realized in QCD with three colors. However, the estimate by FK is affected seriously by the sign problem. Hence, nowadays it is still under debate if the critical endpoint there exists in QCD or not.

After referring to this for a comprehensive review of some of the techniques adopted by the Lattice community to avoid the sign problem and detect the critical endpoint, it is worth to cite an article by Marco Ruggieri, which appeared few days ago on arXiv, in which an exotic possibility to detect the critical endpoint by virtue of Lattice simulations avoiding the sign problem has been detected, see here . We report, after the author permission, the abstract here below:

We suggest the idea, supported by concrete calculations within chiral models, that the critical endpoint of the phase diagram of Quantum Chromodynamics with three colors can be detected, by means of Lattice simulations of grand-canonical ensembles with a chiral chemical potential, \mu_5, conjugated to chiral charge density. In fact, we show that a continuation of the critical endpoint of the phase diagram of Quantum Chromodynamics at finite chemical potential, \mu, to a critical end point in the temperature-chiral chemical potential plane, is possible. This study paves the way of the mapping of the phases of Quantum Chromodynamics at finite \mu, by means of the phases of a fictitious theory in which \mu is replaced by \mu_5.

Rajan Gupta (2011). Equation of State from Lattice QCD Calculations arXiv arXiv: 1104.0267v1

Philippe de Forcrand (2010). Simulating QCD at finite density PoS (LAT2009)010, 2009 arXiv: 1005.0539v2

M. Asakawa, & K. Yazaki (1989). Chiral restoration at finite density and temperature Nuclear Physics A, 504 (4), 668-684 DOI: 10.1016/0375-9474(89)90002-X

Z. Fodor, & S. D. Katz (2001). Lattice determination of the critical point of QCD at finite T and \mu JHEP 0203 (2002) 014 arXiv: hep-lat/0106002v2

Marco Ruggieri (2011). The Critical End Point of Quantum Chromodynamics Detected by Chirally
Imbalanced Quark Matter arXiv arXiv: 1103.6186v1


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