An important hit from CDF at Tevatron

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.


4 Responses to An important hit from CDF at Tevatron

  1. Kea says:

    I believe this particular result is tricky for the LHC, which is a pp machine. No doubt they will sort it out eventually … but I wouldn’t want to miss my wine from Tommaso! Anyway, a great result from your point of view!

    • mfrasca says:

      Hi Kea,

      Thank you for your comment. I think that this is a very exciting time for doing high-energy physics and surely I will have a lot to write in this blog in the near future.



  2. Luboš Motl says:

    It is tricky for the LHC, indeed. The initial state – two p beams – has no preferred “backward” or “forward” direction at the LHC so the asymmetry is almost certainly zero. 😉

    But if there are new particles behind the asymmetry, the LHC should see them more directly -or otherwise.

  3. Joel Rice says:

    Just wondering if there is a similar asymmetry with the Tau lepton.

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