CDF bump at 4.8 sigma!

01/06/2011

ResearchBlogging.org

In these days I am exceeding with exclamation marks but let me say that there are sound reasons for this. I will keep on staying on a prudence line as my more renowned colleagues are doing but there is a talk by Giovanni Punzi, the spokesman of CDF Collaboration at Tevatron, (see here), presenting the following picture


They increased the number of events and the bump is still there. They have consistently found 147\pm 5 GeV for the mass of this presumed particle. Of course, we have to wait for D0 and LHC to confirm this finding but this is becoming more and more a real discovery. But since the first inception, theoretical physicists have come out with possible explanations, the most reasonable of these seems a new U(1) interaction with Z’ leptophobic boson, where leptophobic just means that this particle has essentially strong decay modes. People at CDF has discarded several possible mundane explanations that emerged at the dawn of the first announcement but, as my readers know, it should be stated that our current understanding of low-energy QCD is somewhat in development and some modeling used by people at CERN or Tevatron can be not so accurate. My personal view is that is time to wait yet notwithstanding the good news.

For more relevant posts see here, here and here.

Kingman Cheung, & Jeonghyeon Song (2011). Baryonic Z’ Explanation for the CDF Wjj Excess arXiv arXiv: 1104.1375v3


A simpler explanation for the CDF bump

20/04/2011

ResearchBlogging.org

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 new particle at Fermilab?

06/04/2011

ResearchBlogging.org

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


The Tevatron affair and the “fat” gluon

25/01/2011

ResearchBlogging.org

Tevatron is again at the forefront of the blogosphere mostly due to Jester and Lubos. Top quark seems the main suspect to put an end to the domain of the Standard Model in particle physics. Indeed, years and years of confirmations cannot last forever and somewhere some odd behavior must appear. But this is again an effect at 3.4 sigma and so all could reveal to be a fluke and the Standard Model will escape again to its end. But in the comment area of the post in the Lubos’ blog there is a person that pointed out my proposal for a “fat” gluon. “Fat” here stays just for massive and now I will explain this idea and its possible problems.

The starting point is the spectrum of Yang-Mills theory that I have obtained recently (see here and here). I have shown that, at very low energies, the gluon field has a propagator proportional to

G(p)=\sum_{n=0}^\infty(2n+1)\frac{\pi^2}{K^2(i)}\frac{(-1)^{n+1}e^{-(n+\frac{1}{2})\pi}}{1+e^{-(2n+1)\pi}}\frac{1}{p^2-m_n^2+i\epsilon}

with the spectrum given by

m_n=\left(n+\frac{1}{2}\right)\frac{\pi}{K(i)}\sqrt{\sigma}

being \sigma the string tension being about (440\ MeV)^2. If we go beyond the leading order of such a strong coupling expansion one gets that the masses run with momenta. This has been confirmed on the lattice quite recently by Orlando Oliveira and Pedro Bicudo (see here). The interesting point about such a spectrum is that is not bounded from above and, in principle, one could take n large enough to reach TeV energies. These glueballs are very fat indeed and could explain CDF’s results if these should be confirmed by them, their colleagues at D0 and LHC.

It should be emphasized that these excitations of the glue field have spin zero and so will produce t-tbar pairs in a singlet state possibly explaining the charge asymmetry through the production rate of such very massive glueballs.

A problem can be seen immediately from the form of the propagator that has each contribution in the sum exponentially smaller as n increases. Indeed, this has a physical meaning as this is also what appears in the decay constants of such highly massive gluons (see here). Decay constants are fundamental in the computation of cross sections and if they are very near zero so could be the corresponding cross sections. But Oliveira and Bicudo also showed that these terms in the propagator depend on the momenta too, evading the problem at higher energies. Besides, I am working starting from the low energy part of the theory and assuming that such a spectrum will not change too much at such high energies where asymptotic freedom sets in and gluons seem to behave like massless particles. But we know from the classical theory that a small self-interaction in the equations is enough to get masses for the field and massless gluons are due to the very high energies we are working with. For very high massive excitations this cannot possibly apply. The message I would like to convey with this analysis is that if we do not know the right low-energy behavior of QCD we could miss important physics also at high-energies. We cannot live forever assuming we can forget about the behavior of Yang-Mills theory in the infrared mostly if the mass spectrum is not bounded from above.

Finally, my humble personal conviction, also because I like the idea behind Randall-Sundrum scenario, is that KK gluons are a more acceptable explanation if these CDF’s results will prove not to be flukes. The main reason to believe this is that we would obtain for the first time in the history of mankind a proof of existence for other dimensions and it would be an epochal moment indeed. And all this just forgetting what would imply for me to be right…

Frasca, M. (2008). Infrared gluon and ghost propagators Physics Letters B, 670 (1), 73-77 DOI: 10.1016/j.physletb.2008.10.022

Frasca, M. (2009). Mapping a Massless Scalar Field Theory on a Yang–Mills Theory: Classical Case Modern Physics Letters A, 24 (30) DOI: 10.1142/S021773230903165X

P. Bicudo, & O. Oliveira (2010). Gluon Mass in Landau Gauge QCD arxiv arXiv: 1010.1975v1

Frasca, M. (2010). Glueball spectrum and hadronic processes in low-energy QCD Nuclear Physics B – Proceedings Supplements, 207-208, 196-199 DOI: 10.1016/j.nuclphysbps.2010.10.051


An important hit from CDF at Tevatron

05/01/2011

I look around in the blogosphere to see if important news that escaped me were starting to come out. This is a typical reason to read blogs. In a few hours I saw the posts from Jester and Lubos informing me of a new paper from CDF (see here) appeared on arxiv. This paper contains a possible hint of new physics as they observe a 3.4 sigma effect of charge asymmetry in top-anti-top pair production that disagrees with the Standard Model prediction. They improve on preceding measurements by D0 and CDF due to the increased luminosity and now the effect begins to be an important real discover. As such, all this should be shortly confirmed at LHC.

The idea behind these measurements is to see charge asymmetry effects arising at the next-to-leading order by interference terms in QCD. This asymmetry translates into a forward-backward asymmetry. This effect should be really small as QCD is a charge symmetric theory and, indeed, at the leading order no difference is seen for top or antitop. As these top pairs are produced by proton-antiproton collisions this is a pure effect arising from strong interactions and so, any unexpected effect should be ascribed to problems with our current understanding of QCD. CDF improves significantly the result showing a large disagreement with the prediction of the Standard Model. But what is really interesting here is that they show inequivocally that this effect is mass dependendent. They measure the invariant mass of top-antitop pairs and find that their results are consistent with a resonance of a mass of 450 Gev. It is like a massive gluon entered into the process producing the asymmetry! It should not be forgotten that at this high energies QCD should settle at an asymptotic freedom regime and one could safely do a perturbative analysis of this process. Indeed, there is paper, cited in this CDF’s work, that do so assuming a massive gluon. CDF analysis relies on this paper to do a theoretical analysis of their results.

In the comments of Lubos’ post you can find an answer to the question if this massive particle could be supersymmetric. Anyhow, this particle is strongly interacting whatever its nature. With this in mind, we can try to do a simple analysis through the idea that Yang-Mills theory has a mass gap. As already said, if the coupling of the theory is finite, gluons and their excitations must be massive (see here and here). The spectrum of the theory is not bounded from above and so these massive excitations should be expected also at higher energies. This analysis is in agreement also with a recent work of Alexaner Dynin at Ohio State University (see here). Indeed, this spectrum is the same of a harmonic oscillator and so such massive gluons should be expected at any energy scale in principle. Of course, to confirm such a hypothesis, a full computation of charge asymmetry should be performed and eventually found in agreement with CDF measurements. But a mass gap in Yang-Mills theory should mean a massive spectrum and the absence of any bound from above could imply this (take this cum grano salis).

At 3.4 sigma we are in a fluke possiility yet. We have to stay tuned for this result to be finally confirmed in a near future.


The whisper of the universe

01/12/2008

Current experimental situation about dark matter is becoming increasingly exciting as results are coming out. I would like to emphasize the work of Piergiorgio Picozza that has been my professor teaching me the foundations of nuclear physics. He is currently the leader of PAMELA, an experiment that produced a lot of rumors and striking results as well. The New York Times has, as usual, published a beautiful article on the current situation about dark matter (see here).  It is worthwhile reading and makes quite clear that the situation is becoming stunning day after day but avoids anyway to cite the recent results of CDF Collaboration. Indeed, the link between dark matter and CDF finding is yet in its infancy. We are all awaiting for other confirmations as always happens to experiments. I am confident that these will not miss for too long. Whoever is interested about the relevant discussions about I just point you to the Dorigo’s blog being him one of the authors of the CDF’s paper. This provoked a lot of rumors in the blogosphere that sometime degenerated in hot discussions. We just stay tuned to learn more about.


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