Light-Emitting Top Quark Pairs
    By Tommaso Dorigo | June 21st 2011 12:56 AM | 12 comments | Print | E-mail | Track Comments
    About Tommaso

    I am an experimental particle physicist working with the CMS experiment at CERN. In my spare time I play chess, abuse the piano, and aim my dobson...

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    A paper describing the first evidence of top pair production in association with a energetic photon has just appeared on the Cornell Arxiv. This search has been performed by the CDF collaboration in a sample of 6 inverse femtobarns of proton-antiproton collisions.

    There is nothing strange or particular about the fact that any hard production process at a hadron collider can produce, in addition with a massive state such as a top pair, additional energetic photons. That is because any charged particle involved in the process will have a small but finite chance of radiating electromagnetic energy, with a strength governed by our good-old fine-structure constant.

    However, the process involving top quarks and a photon may reveal interesting features not yet thought of. In addition, the final state where this signal has been extracted is one which CDF has looked at very carefully in the last 15 years: that is because events with leptons, missing energy, and photons are quite rare in the Standard Model, and the observation of the unexplicable "ee-gamma-gamma-met" event in Run I left everybody speechless back then.

    Ever since the observation of that impossible event, researchers in CDF -in particular a group led by CDF old-timer Prof. Henry Frisch (right)- have pursued the investigation of these final states (see here a short list of works related to photon final states by the group). The paper you can read today is just one among many produced by that line of research. Needless to say, there were never any hints of new physics in the CDF data samples of leptons, photons, missing energy, b-jets, and similar others; but we have learned a lot in the process.

    Below you can see the distribution of a few kinematical quantities which distinguish top production from background processes, for events containing a energetic (Et>10 GeV) photon candidate. From top to bottom and from left to right you find the primary lepton transverse energy, the missing transverse energy, the transverse energy of the leading b-tagged jet, and the photon transverse energy.

    Predicted backgrounds excluding ttgamma production amount to 13 events, but 30 events are seen. The predictions for the signal amount to 13.9 events, so the total data is in good agreement with being the sum of signal and backgrounds. What's more, the same top-pair selection with no photon requirement extracts a sample of 4429 events, where the same background estimation technique provides a prediction of 4420+-340: the obscenely good agreement of counts in this super-set confirms that things are in good order.

    The statistical significance of the excess above 13 events expected, after all is said and done, amounts to exactly three standard deviations. Unlike mass bump hunts, these three-sigma are a genuine signal, and the claim that this is indeed the first evidence of ttgamma is well founded. So, congratulations to the CDF colleagues for this important new result!


    Great, CDF has found another 3-sigma evidence. Should we really be trusting their 3-sigma papers anymore? :)

    Dear Joseph, Lubos,

    the fact that you seem unwilling to discern the tentative discovery of a new particle from the observation of a process that must happen (if electrodynamics isn't an opinion) betrays your theoretical mindset (at least in Lubos' case).

    Of course in case of a new particle, the prior probability that it is a true claim is infinitely smaller than the prior probability that ttgamma does exist more or less along the SM predictions... If you are a Bayesian you must at least understand this. If not, well... Just sit and wait: the resonance will go, the tt-gamma will stay.


    You should not be so hard on Joseph or Lubos. It's indeed refreshing to see that tt-gamma amounts to what appears to be a genuine signal with good agreement in counts. But you should also realize that many theorists have grown increasingly skeptical over three sigma reports after the stir created by the CDF resonance...



    No, Tommaso should be so hard on them.

    Out of probably more than a hundred claims of evidence by the CDF collaboration as a whole (and endorsed by most of the collaboration), there are a handful, that vanished. Given that there are as well uncertainties on the uncertainties, reliance on external tools like MonteCarlo genrators, and other libraries, there is absolutely zero evidence by this statistic, that the CDF collaboration as a whole is not very conservative in claiming anything.

    Of course it is true, that some theorists are skeptical about three sigma evidences, and some have recently become more skeptical, mainly because of their own stupidity, as they simply don't get statistics, especially as Tommaso mentions, Bayesian statistics. Of all my colleges (experimental particle physicists)
    a) nobody believed the CDF result indicates new physics or got the slightest bit excited.
    b) nobody thought, the people doing the analysis did anything grossly wrong, despite you see already some MC/data discrepancy looking in some of the control samples.
    c) nobody doubts, that this result about tty is really tty.

    One would expect, that theorists are somewhat more confident in their theories, that they have as well a low prior for results, that don't seem to make a lot of sense. Obviously not.

    If you feel so strongly about some theorists, would you care to disclose your identity and to stand by your statements in an open discussion?


    If someone says that he's just observing the normal t-t-gamma as predicted by the Standard Model, then great. I have no problem with that. I don't care, either.

    At any rate, I don't trust CDF even when it comes to this issue - which would otherwise be uncontroversial and unspectacular due to the recent stories that have assured me that there is something seriously wrong about CDF. I mean the spurious Wjj bump at 150 GeV and the huge spurious top-antitop mass difference which is clearly zero but they "measured it" at 3.3 GeV or so.

    CDF is just dead for me. I have discarded it as a source of noise that has informally died and that will be formally disbanded in September, thank God.

    Coming from a high-falutin theoreticist, this is pretty rich.

    It is rich and it is true, too. And the validity of the proposition is independent of the author of the proposition in general - even though yes, one may notice that the most stringent and careful theorists are more likely to avoid wrong propositions and it is no coincidence.

    Also, I am confident that this is not just my viewpoint. After two major new physics claimed by CDF were refused by D0 - the top-antitop mass and the Wjj 150 GeV bump - most phenomenologists and theorists will pay little attention to other "outstanding" claims by CDF such as the top-antitop asymmetry and any new one that will be added. This is an inevitable, rational reaction to previous failures - whether the reason is the people's mistake or other people's mistake or a flaw in the detector.

    It's unlikely that CDF has produced these two failed bombastic claims in such a short time span by a "complete accident".

    Hi Ervin,

    I think what Anon said is what most of my CDF colleagues feel. So you can attach any name out of 600 on the comment above, and it will still sort of hold, in some sense ;-)

    Tommaso, thanks for your reply.

    It is not my intention to lecture here, but I believe that broad accusations against theorists taking a skeptical attitude are not helpful. The HEP community is in pins and needles on what LHC will unfold in the post Tevatron era and a lot of unknown territory awaits exploration. In my opinion, many surprises are bound to occur on both experimental and theoretical sides and the blame game is simply destructive.



    All I wanted to point out is that a measure of significance of a deviation from known physics means nothing without an alternative hypothesis. If you are three sigma away from the null hypothesis, and zero sigma away from the hypothesis that there's something new, is quite different from being three sigma away from the null when any new physics is tens of sigmas away.