New Higgs Limits From The Tevatron... New?
    By Tommaso Dorigo | March 14th 2011 04:55 PM | 7 comments | Print | E-mail | Track Comments
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    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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    ... Not really.

    What startled me most was that a colleague of mine at the University of Padova even sent a message to my departments' mailing list, saying that the new result is very important. But it clearly isn't! In fact, the exclusion at 95% CL in the range of Higgs boson masses that CDF and DZERO could put together from the analysis of additional data is almost exactly the same as the one that they published last Summer.

    But maybe I should make a step back and explain the matter from the start, to let you judge by yourself the relevance of the new Tevatron bounds on the rate of Higgs boson production in proton-antiproton collisions.

    CDF and DZERO are analyzing the proton-antiproton collisions at 2 TeV that the Tevatron collider is producing since 2001. They have refined their search tools to a really remarkable level, such that they in some cases even out-performed their own optimistic pre-run predictions (ones that I helped put together, so I know what I am talking about). Nowadays, every six months they update their results, adding further small improvements. What matters, however, is by now mostly the added statistics that the searches can study every time they make an update.

    The Higgs boson is searched in all the production channels and all the  decay modes to which the Tevatron experiments are even marginally sensitive; then, all search results are combined together, and a further combination occurs between results of CDF and DZERO. This allows the Tevatron experiments to produce combined search results with the highest possible sensitivity.

    All searches matter, but among the search channels that are most relevant, the direct production of Higgs boson with a subsequent decay to W boson pairs, and the latters' production of two lepton-neutrino pairs, is the one which provides most of the sensitivity in the only region where the Tevatron has produced a 95% upper limit below the predicted Standard Model rate, thus ruling out the existence of the Higgs at those particular mass values. This is shown graphically in the figure below.

    In this "brazil band plot" you can see a black curve as a function of Higgs mass. This curve determines what production rates of the Higgs are excluded: those above the curve. Since the y axis is normalized such that the expected standard model production rate is at unity (such that what is effectively drawn is the rate in units of "times the SM predicted rate"), this means that when the curve goes below 1.0 the corresponding mass values are disfavored, at 95% confidence level.

    Now, this means, for the graph you just saw, that the Tevatron is now excluding the mass region 158<M(Higgs)<173 GeV. Compare with the summer 2010 results: 158<M(Higgs)<175 GeV. The new result is actually excluding less than the old one! What's going on here ?

    Quite simply, the added data happened to "over-fluctuate" ever so slightly, so that, once added to the old data, pulled the limit up instead than down. This resulted in a narrower exclusion than the previous one!

    This is slightly unfortunate, but quite possible. In fact, the green band shows the range of where Higgs limits could be set with the available data and analyses. What I find embarassing is that one should "justify" oneself for this fluctuation, or plain claiming that the new result is very important! It is not, but it happens. The next one might be more important.

    Instead, what I see now happening in the press office at Fermilab is what I observed in other laboratories toward the end of life of experiments: overstatements, overhyping. Not good.

    What do I mean ? Have a look at the Fermilab Today site. The article there is slightly over the top for my very personal taste, for two or three reasons.

    One: they stress too much that the expected limit by the experiments was wider than the observed one. Come on, the black line is within the 1-sigma band throughout in the relevant region... It happens, get it over with! No need to stress such a detail! Here are the relevant quotes:

    "Statistical fluctuations in the number of observed particle collisions that mimic a Higgs signal, mixed with collisions that may have produced a Higgs boson, affect the actual range that can be excluded with 95 percent certainty. Combining their independent Higgs analyses, the two experiments now exclude a Higgs boson with a mass between 158 and 173 GeV/c2."

    And in the figure caption:

    "Combined the Tevatron experiments now are sensitive to a Higgs mass from 153 to 179 GeV/c2, but statistical fluctuation reduce the actual mass range that can be excluded so far."

    Two: they go as far as to quote, in addition to their ten-year standard 95% confidence level limit,  the limit at 90% confidence level (see graph below), which does extend further in the high-mass region. It shows clearly that they ache for the standard limit (at 95%CL) just kissing the 1.0 line from above (see figure above)... which means a narrower excluded region than it would have been with just a bit more "luck". But quoting a 90% CL  is a post-data decision and I strongly blame them for such practice! To explain why this is highly reproachable, ask yourself whether they would have quoted the 90% confidence level limit if it had not been wider... The question answers itself.

    Finally, in the above figure they include an "indirectly excluded region" on the right of the 185 GeV point. I think that information should not be mixed with direct search results on the same footing! It again shows their urge to overhype the new result.

    Ok, diatriba mode off. I am still quite willing to say that the Tevatron rocks, and that the games for the Higgs search are not over just yet!


    Hi Tommaso,
    Slightly playing Devil's Advocate here, but you could equally imagine that they are quoting the sensitivity to avoid the "embarrassment" of having a worse result than before - to prove that what they've done now really is better. That said, I agree with you that quoting it more than once in the story is offputting.

    I'm interested in your disapproval of showing direct and indirect limits together. I'm not sure I see a problem with that. Does it mean you don't like the famous plot with the LEP exclusion and the delta-chi2 from electroweak fits (where the best fit is inside the excluded region)?

    Hi Tommaso. Could you elaborate why you feel it is not appropriate to show indirect limits on the same plot as direct limits? Both pieces of information suggest a higher chance for a low-mass Higgs. As a low-mass Higgs is my thesis topic at CDF, I sometimes use this plot as a motivation for my analysis. If this argument is wrong, I would welcome some more explanations.

    Another comment/question I have is on this 185 GeV upper bond from the indirect electroweak fits. I understand that actually this number should be around 155 GeV if we used only the indirect electroweak fits. But when we combine this information with the lower bound from direct searches at LEP, then the 155 GeV becomes 185 GeV. If this is so, then the number shown on the plot here is not purely an indirect fit value after all.


    Hi Phil, Adrian,
    since you ask the same question, let me explain.

    Putting together indirect and direct limits is fine, as long as the presentation is clear. Here, the graphics are equal for direct and indirect info, and this is IMO a bad choice. So my criticism is basically on the graphics.

    Nowadays it is very easy to put together in a likelihood the direct and indirect info, as GFitter or ZFitter do. Having a single-dimensional plot like the one above is dangerous for that aim: it is simple -so uninstructed readers can get it - but it is too simple -so it is deceiving.

    Also, note that in the indirect limit all sorts of assumptions are used, implicitly. Choices are made on which experimental results to include in the indirect fits and which to exclude. Parameters are chosen. I am not criticizing that technology, but I am saying that it is very complicated to extract clean statements from a combination of direct and indirect info in a plot. Experts-only stuff.

    The `direct' bounds are valid for any SM-like Higgs boson, i.e. the model may well contain other new physics as long as the couplings of the Higgs boson (as well as production and decay modes) are (very) similar to the SM Higgs boson.
    The `indirect' bounds are valid for the SM Higgs boson only, i.e. new physics beyond the SM would (most probably) change this indirect bound (as it is the case, for example, in Supersymmetry or Z' models).

    In conclusion: showing both bounds in a `SM Higgs boson mass range plot' is in principle ok.

    Cheers, Sven

    Hi Tommaso, thank you for the prompt answer. It is clear now. Now that you mention it, I see that indeed that the Tevatron PR plot shows the old results in green (LEP's direct limitand indirect search using also LEP's direct limit) and the new results in brown (Tevatron's direct limit). Whereas from a scientific point of view, it is correct to put LEP's and Tevatron's results with the same color and the indirect limit with another color. Indeed, in science the old and the new result are equally valid and important, so they should have the same color, as they use the same technique (the trusted direct search). And the indirect limits should maybe have a question mark on top, just to show that they are not as "sure to be correct" as the direct limits are.

    Brazil band plots? I thought they were called cheese and lettuce plots. Anyway, I'm afraid it's been clear since the first that doubling Tevatron's dataset would not be sufficient to expose a Higgs signal. The curve has remained essentially the same, all that's happened is that the vertical scale has gradually expanded.

    I wonder why they cut this one off at 130 GeV?

    This happens to be a high mass combination, that's why cut on 130GeV.