## Back from Paris

It is several days that I have no more posted on the blog but for a very good reason: I was in Paris for the Eleventh Workshop on Non-Perturbative Quantum Chromodynamics (see here). It has been a beautiful chance to see Paris with the eyes of a tourist and being immersed in a lot of physics in the area I am currently contributing. The conference was held at the Institut d’Astrophyisique de Paris. This week was indeed plenty of information for people in high-energy physics due to the release by D0 of their measurements on the Wjj data, showing that the almost 5 sigma bump of CDF was not there (see here, here and here). In the conference there has been room for talks by experimentalists too and it was the most shocking part as I will explain below.

The talks were somehow interesting with a couple of days mostly dedicated to AdS/CFT approach for QCD. So, string theory got a lot of space even if I should say that more promising approaches seem to exist. The first day there have been a couple of talks that were very near my interest by Dario Zappalà and Marco Ruggieri. They were reporting on their very recent papers (here and here). With Marco, I spent all the week together while with Dario we have had a nice dinner near Latin Quartier. The question Dario presented was about the existence of massive excitations (let me say “persistence”) also beyond the critical temperature for Yang-Mills theory. We discussed together with Marco this result and Marco claimed that massive excitations should have melted beyond the critical temperature while my view is that the residual of mass should be due to temperature corrections to the mass spectrum of the theory. Marco in his talk presented the idea of measuring the chiral chemical potential on the lattice as this could give plain evidence of existence for the critical endpoint without the annoying sign problem. A proof of existence of the critical endpoint is somehow the Holy Grail of finite temperature QCD and something under a lot of studies both theoretically and on the lattice. So, Marco’s proposal can turn out a significant shortcut toward the reaching of this goal.

The second day Carl Bender gave a very beautiful talk telling us about PT invariant quantum mechanics. PT stays for Parity and Time reversal. The point to start from is the Dirac postulate about the Hamiltonian being Hermitian self-adjoint. Differently from the other postualates of quantum mechanics, this one is too much a mathematical requirement and one could ask if can be made somewhat looser. The paradigm Hamiltonian has the from $H=p^2+ix^3$. The answer is yes of course and we were left with the doubt that maybe this is the proper formulation of quantum mechanics rather the standard one. I suspect that this could represent a possible technique useful in quantum gravity studies.

I have already said of the two days on string theory. I have just noticed the talk by Luca Mazzucato showing how, with his approach, my scaling with $\lambda^\frac{1}{4}$ for the energy spectrum could be recovered in a strong coupling expansion being $\lambda$ the ‘t Hooft coupling. Unfortunately, Gabriele Veneziano could not partecipate.

On Wednesday there was the most shocking declaration from an experimentalist: “We do not understand the proton”. The reason for this arises from the results presented by people from CERN working at LHC. They showed a systematic deviation of their Montecarlo simulations from experimental data. This means for us, working in this area, that their modeling of low-energy QCD is bad and their possible estimation of the background unsure. There is no way currently to get an exact evaluation of the proton scattering section. I am somewhat surprised by this as so far, as I have always pointed out in this blog, at least the structure of the gluon propagator at low energies should be known exactly from the lattice. So, modeling the proton in such Montecarlo models should be a mitigated issue. This does not seem to be so and these different communities do not seem to talk each other at all. After these shocking news, the evening we took an excellent social dinner and I have had some fine discussions with foreigners colleagues that were well aware of the books from Umberto Eco. One of these, Karl Landsteiner, suggested us to visit the Pantheon to look at the Foucault pendulum. I, Marco Ruggieri and Orlando Oliveira took this initiative the next day and it was a very nice place to visit. If you are a physicist you can understand the emotion of being there seeing that sphere moving like Newton’s equations demand and inexorably proving the rotation of the Earth. Karl gave an interesting talk that day where AdS/CFT is used to obtain transport coefficients in heavy ion collisions.

In the same day, Orlando Oliveira gave his talk. Orlando is a friend of mine and gave relevant contribution to our understanding of the behavior of low-energy gluon propagator. He has been the author of one of the papers that, at Regensburg on 2007, started the end of the so called “scaling solution” for the gluon propagator (see here). Orlando is going ahead, starting from the acquired form of the gluon propagator, to understand low-energy phenomenology of nuclear forces. In this work, he and his colleagues introduce an octect of scalar fields having the aim to produce the gluon mass through a non-zero vacuum expectation value (see here) producing chiral symmetry breaking. My work and that of Orlando are somewhat overlapped in the initial part where we have an identical understanding of the low-energy behavior of  Yang-Mills theory.

On Friday, there have been a couple of significant events. The first one was my talk. This is a report on my recent paper. I will not discuss this point further leaving this material to your judgement. The second relevant event was given in the talks by Thierry Grandou and our Chairman and Organizer Herbert Fried. The relevant paper is here. While Grandou made a more mathematical introduction with a true important result: the resummation of all gluon exchange diagrams realizing some dream of having completely solved QCD, Fried provided a more concrete result giving the binding potential between quarks analytically obtained from the preceding theorem. We were somehow astonished by this that seems just a small step away from the Millenium prize. Berndt Mueller, one of the Organizers, suggested to Fried to determine the mass gap and wait a couple of years to get the prize. Indeed, this appears a true striking exact result in the realm of QCD.

All in all, an interesting conference in a special place: Paris. For me, it has been a very nice period of full immersion in physics with the company of very nice friends.

Update: Mary Ann Rotondo put online the slides of the talks (see here).

P. Castorina, V. Greco, D. Jaccarino, & D. Zappalà (2011). A reanalysis of Finite Temperature SU(N) Gauge Theory arXiv arXiv: 1105.5902v1

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

Irene Amado, Karl Landsteiner, & Francisco Pena-Benitez (2011). Anomalous transport coefficients from Kubo formulas in Holography JHEP 05 (2011) 081 arXiv: 1102.4577v3

O. Oliveira, W. de Paula, & T. Frederico (2011). Linking Dynamical Gluon Mass to Chiral Symmetry Breaking via a QCD Low
Energy Effective Field Theory arXiv arXiv: 1105.4899v1

Marco Frasca (2011). Chiral symmetry in the low-energy limit of QCD at finite temperature arXiv arXiv: 1105.5274v2

H. M. Fried, Y. Gabellini, T. Grandou, & Y. -M. Sheu (2009). Gauge Invariant Summation of All QCD Virtual Gluon Exchanges Eur.Phys.J.C65:395-411,2010 arXiv: 0903.2644v2

### 12 Responses to Back from Paris

1. Tony Smith says:

Marco Frasca said “the most shocking declaration from an experimentalist: “We do not understand the proton”. The reason for this arises from the results presented by people from CERN working at LHC. They showed a systematic deviation of their Montecarlo simulations from experimental data. This means for us, working in this area, that their modeling of low-energy QCD is bad and their possible estimation of the background unsure. There is no way currently to get an exact evaluation of the proton scattering section.”.

Is that why D0 said in arxiv 11096.1457 that for multijet background calculations (particularly including the electron channel in the studies of the CDF Wjj bump) a “data-driven method” was used because “estimation of this background from Monte Carlo simulations is not reliable” ?

Could that explain at least some of the discrepancy between D0 and CDF analyses of the Wjj bump,
particularly in light of the fact that the QCD Multijets background
in the D0 electron channel data is centered around 105 GeV/c2
while
in the CDF analysis it is centered around 75 GeV/c2
so that
the D0 QCD Multijet background might be high in the relevant region of the Wjj bump (120 to 160 GEV/c2) thus obscuring a Wjj signal seen by CDF
because the CDF method of calculating electron-channel QCD Multijet background may have been significantly different ?

Tony

• mfrasca says:

Dear Tony,

Your question is rather interesting. I cannot say if your point is correct or not but, for sure, simulations representing low-energy processes are not properly represented and this is mostly due to our scarce understanding of the corresponding physics. My personal view is that is possible that CDF indeed uncovered a new state but at this stage only data from LHC can confirm. In this kind of reaction the correct evaluation of the background is crucial.

Marco

• Gordon Watts says:

The reason that CDF/DZERO/ATLAS/CMS have to use data driven methods is this a combination statistics problem and detector modeling problem. First, this QCD background is for a lepton faking a jet. It happens so rarely that the amoun of MC we would have to produce would be expensive and take a *very* long time. Second, this QCD background also covers various detector problems (imagine a heavy flavor jet with a lepton in it, jet measurement screws up, so it looks like a lepton on its own… ops). That is something our detector simulations are only so-so at… not enough for us to trust them for an analysis like this or one of the Higgs searches.

As to why the peak is where it is. I have no idea – and I would love to know. If you scale the results to luminosity and compare the results, you’ll see that DZERO’s lepton ID must be very different from CDF’s, so the reason could be hiding in there. [I am a member of DZERO].

2. Ulla says:

“mostly dedicated to AdS/CFT approach for QCD. So, string theory got a lot of space even if I should say that more promising approaches seem to exist.”

Can you tell us about these other approaches? Especially in their relations to Minkowski spaces and Ricci solitons.

Ulla.

• mfrasca says:

Dear Ulla,

The point is that AdS/CFT has no new previsions for QCD. This approach relies heavily on what is already known and so I would have preferred to spend more time on sum rules to give a successful example. But also techniques that rely on instantons and lattice results should be in more good position with respect to such a stringy approach. This does not mean that in a near future AdS/CFT methods could improve to a comparable level, but as for today they appear somewhat intangible.

Marco

• Ulla says:

del Duca, Vittorio: “Wilson Loops and Amplitudes in N=4 SYM”
In the planar N=4 supersymmetric Yang‐Mills theory, we discuss two‐loop amplitudes and Wilson loops at weak coupling, and compare them to Wilson loops at strong coupling, which are available in the literature.

Loops also inevitable in Wjj?

3. Could you give some link related “We do not understand proton” or perhaps explain a little bit what we do not understand. You mention also scalar gluons. Could they provide understanding?

• mfrasca says:

Dear Matti,

This is just a citation of a sentence from a CERN experimentalist at the Conference. Their plots show systematic deviation from standard Montecarlo simulations of events signaling that their low-energy models are failing. This is rather understandable in view of the fact that low-energy QCD is currently forefront research with a lot to be clarified also for experimental data. Until this matter is not properly assessed, background evaluation at CERN or Fermilab could be a serious problem.

Marco

4. Thank you. Experimental physics is really hard. Much easier to just build theories of everything;-).

5. […] Ruggieri, one in collaboration with Raul Gatto (see here) and the other is the contribution to Paris Conference proceedings (see here). Marco is currently using a Nambu-Jona-Lasinio model to understand the […]

6. […] in my blog (see here and here) and I presented its content in a conference in Paris this year (see here). The contribution to proceedings is […]

7. […] Professor Berndt Mueller from Duke University are the organizers. As it also happened in the precedent edition, the workshop was really interesting and rich of ideas for research. The first talk was given by […]