The Plot Of The Week - The 327 GeV ZZ Anomaly
    By Tommaso Dorigo | October 14th 2011 08:13 AM | 5 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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    Higgs boson hunters often catch themselves dreaming of the boson having a mass high enough to give rise to the spectacular decay into two Z bosons, and then four charged leptons in the final state. At a hadron collider -let's talk of the LHC to be specific- such a signature is the only one providing events which, once properly selected, are more likely signal than background.

    The situation of observing an event display being able to tell for sure what it represents, among the infinite possibilities and the intrinsic indetermination of quantum processes, is reassuring and gives a physicist a feeling of power. God did not play dice this time: those two are 100% Z bosons, and their combined mass is exactly the one of the Higgs.

    Now look at what the CDF search for four-lepton decays of ZZ pairs turned out, in the graph above. CDF is one of the two experiments working with the proton-antiproton collisions that the Tevatron collider has provided for nearly 25 years. The accelerator has stopped working last month, but the data will continue to be analyzed for much longer. This measurement, in particular, will be updated with the total statistics collected by CDF; so far it shows data equivalent to an integrated luminosity of 6 inverse femtobarns -little more than half the total collected.

    I have discussed this measurement already in the past, but for some reason I decided to post this one graph again today. The figure shows the invariant mass of the four leptons, which must come from the decay of Z pairs, backgrounds from other processes being really negligible. If a Higgs boson existed, it could show up in such a distribution as a clustering of events all at the same reconstructed mass...

    Surprisingly, of the eight candidate events four do cluster at nearly the same mass, 327 GeV; the other four more closely follow the expected distribution of Standard-Model production of ZZ pairs. What is then that funny 327 GeV clustering: a Higgs decay ?

    Most likely not: the cross section of such a Higgs boson would have to be much higher than predicted. What's more, no similar thing has appeared in the by now similarly-sized data samples of the LHC experiments. Check for instance the CMS ZZ mass distribution above, also discussed in more detail in the link above. In it, the data are compared to backgrounds (the total being shown in purple) and different Higgs mass hypotheses (in different shades of red). Or check the DZERO result on ZZ searches shown on the right below, performed with 6.4 inverse femtobarns of data and reported in this article. Here, they call "Signal" the ZZ process from regular SM processes not including the Higgs.

    Or see the ATLAS result below, obtained with a luminosity of 2.1 inverse femtobarns (and reported here); there, again three different (and mutually exclusive) Higgs mass hypotheses are placed on top of expected SM backgrounds for comparison.

    If you are a believer, you might argue that all three experiments do see events in the 325 GeV ballpark... Exactly one event each. But if the CDF signal were true, these experiments (especially CMS and ATLAS) should have seen a larger number of such events, all piling up within 10 GeV or so of the 325 GeV region. At least a score each, in the case of ATLAS and CMS.

    Alas, a fluctuation. A very suggestive one, to be sure; one which might be called an observation of a new physics process, if the threshold for such claims were not placed at very high level of significance. In fact, the probability of the observed clustering (and of additional remarkable properties of those four high-mass events) is estimated to be of the order of 0.0001: "only" three-point-something standard deviations, nothing to worry too much about.

    The lesson to take home is one which affectionate readers of this blog already know by heart: in High-Energy Physics, 2- and 3-sigma effects are not uncommon, and mean usually very little. We test hundreds of distributions, and anomalies can show up any place in these distributions. 3-sigma effects are bound to occur!


    Nice and interesting post. You nailed it with your last sentence "3-sigma effects are bound to occur!".

    As a non-member of cern I am wondering, when I may reckon that the first 5/fb plots on higgs-searches will be shown. Are there some important conferences anytime soon? Cheers.

    Winter conferences. They occur in February and March 2012. Before that, I doubt we will see real new updates. But you could stay tuned to a conference in Bormio in January too. There, I will be showing the most recent CMS results... Hopefully we'll have something to spice it up by then already.

    Since comments are appearing now,
    this is a repost of a comment that disappeared a few days ago.

    Of course, none of the Golden Channel (Higgs to ZZ to 4 leptons) histograms shown above indicate anything close to 3 sigma evidence-level significance
    it is interesting that the above histograms indicate that all four experiments seem to have some sort of excess in the 190 GeV to 250 GeV range.
    Based on my likely imperfect eyeball looking at the above plots,
    it seems to me that:
    CDF sees about 4 events
    CMS sees about 8 events
    D0 sees about 6 events
    ATLAS sees about 13 events
    Of course background fluctuations could account for all of them,
    but maybe they might be the beginnings of signals
    and they do appear in all four experiments
    it is fun to speculate about what the Golden Channel histograms for the 5/fb that CMS and ATLAS will most likely
    show for the 2011 Halloween Higgs search run
    to think about whether we should expect 3 sigma evidence if Higgs lives in that 190-250 GeV region.
    Based on slide 16 of Roberto Vega-Morales at PHENO 2011 last May,
    the expected significance of finding a Standard Model Higgs for 5/fb at 7 TeV
    is over 3 (almost 4) sigma
    the uncertainty band ranges from around 2 to around 5 sigma
    it may be a fair bet that a Standard Model Higgs living in the 190-250 GeV range might show up in the Halloween data as 3-sigma evidence
    it could also well be that no more that 2 sigma would be seen
    it could also well be that 5 sigma discovery would be seen.
    Such possibilities are fun to contemplate especially when it is likely that actual experimental observational facts will become known in the near future.


    PS - Another interesting set of data to await is the analysis of possible Higgs around 120 GeV and around 140 GeV.
    The Golden Channel may not be much help there. From the slide 16 mentioned above, it seems that only 2 sigma should be the expectation for 140 GeV, and probably less than that for 120 GeV,
    so more complicated analyses might be needed there.

    PPS - Of course, the expectations shown on the slide 16 are for a simple Standard Model Higgs, and more complicated models might have somewhat different expectations, but it seems to me that the simple Standard Model Higgs calculation may be a useful baseline to use in interpreting the LHC Higgs data.

    Tommaso, you said that you "... will be showing the most recent CMS results ...[at]... a conference in Bormio in January [2012]...".

    Over on the viXra log blog, Chris commented:
    "... there are indeed people within atlas and cms that claim a higgs at ~120GeV. there are also people shaking their heads over these claims.
    we’ll just have to wait and see what they will come to agree upon. ...".

    Why do they have to agree ?

    If there is no agreed consensus by January 2012,
    will you be told to not talk about the situation
    will you be able to present the underlying data plots
    describe all the competing interpretations
    thus letting the audience members make up their own minds
    as if they were grown-up adults
    (as opposed to children to be "educated" to follow some "consensus") ??

    What is wrong with just releasing the basic data plots
    then letting each group of "people within atlas and cms" state publicly their own interpretations of that data,
    perhaps with disclaimers that no interpretation has yet been agreed to be a consensus viewpoint of ATLAS, CMS, and/or CERN ???


    Hi Tony,

    nothing wrong with releasing the data and letting people make up their opinion. In Bormio, lacking new results, I will shortly comment the combined results of Atlas and CMS (soon to be released). I do think that if the Higgs exists it has a mass in the 120 GeV ballpark, and will make the point in Bormio, using all the statistical information in my hands, and all the caution I am getting used to use.