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    CMS And ATLAS: Higgs To Tau Pairs!
    By Tommaso Dorigo | November 14th 2012 03:23 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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    Maybe more interesting than the just reported result of searches for the Higgs boson into four-lepton final states (ee, eμ, μμ) are the result presented by CMS and ATLAS on the searches for the Higgs decay into tau-lepton pairs. The reason for the interest comes from the fact that last July the rather high rates of Higgs decays to photon pairs had suggested to some that this new particle might have reduced couplings to fermions, and could thus be a non-Standard Model particle after all.

    [ Remember, though: if the Higgs boson is observed in direct production, it is very hard to assume it does not couple to fermions: it is in fact the loop of virtual top quarks that emit the particle, unless other much more complicated mechanisms (which would have very, very low rates in any conceivable model) are at work.  ]

    So it is with some expectation that the results of LHC searches for tau-lepton pair decay modes must have been met at the Hadron Collider Physics symposium in Kyoto today. CMS appears at present a bit more sensitive to detect tau leptons than ATLAS, and the results shown confirm this. To make a long story short, see the following graph, which details five independent measurements of the Higgs to tau tau decay rates. The measurement can be split in different run ranges or, even more interestingly, in different final states (top part of the graph). That is because the tau pair can be sought in combination with one or two additional hadronic jets, or with a vector boson. These three are independent production modes, and it is interesting to compare the relative signal strengths: a discrepancy might occur in just one of the three, indicating maybe some departure from the Standard Model...


    No such luck: the data are in very good agreement! Indeed, by combining the measurements, CMS measures a production times BR rate of 0.72+-0.52, in good agreement with the expectation of 1.0.

     ATLAS had not shown any results on tau decays at ICHEP, and now produced their first measurements based on 17.6 inverse femtobarns of 2011+2012 data. Their combined measurement is best shown in a graph where the signal strength is displayed for the two production modes separately: Higgs bosons produced by gluon-fusion mechanisms, and Higgs bosons produced via mechanisms involving vector bosons (what in the graph above were separated in "VBF" and "VH"). The result is a signal strength larger than expected in the gluon-fusion mode (2.4 times the SM), and negative for the other mode (-0.4). Overall, there is however consistency with the Standard Model within the still large uncertainty: the global fit to the signal rate yields 0.7+-0.7. The SM is shown as a blue cross in the graph, and the best-fit solution is the red marker. As you can see, the SM is within the 1-sigma ellipse.

    In conclusion, this is a very hard blow to anybody who speculated on the exotic nature of the new particle announced last July by ATLAS and CMS. If it is a door to new physics, it is still very well closed!

    Comments

    Ciao Tommaso,
    did you mean "photon" instead of "fermion" here: "it is very hard to assume it does not couple to fermions: it is in fact the loop of virtual top quarks that emit the particle" ?
    Higgs to Tau pair decays are on the contrary a direct probe of the lepton yukawa coupling I believe.

    dorigo
    Hi Julien,

    no no I meant fermions. What I mean is that to produce a Higgs directly in hadron collisions, the Higgs must be coupling to quarks (the top quark loop from a gg interaction is the most effective way to do so). So observing the H->gamma gamma or H->ZZ direct production and decay, one is effectively also observing the coupling to fermions at production.

    Cheers,
    T.
    Hi,
    what happened to the VBF channel?
    This was (is?) the cleanest channel, where in the 7 and 8 TeV data, as presented at ICHEP, CMS did not even see the background. This cannot be seen in the first plot.

    Cheers, Sven

    dorigo
    I am not sure I understand the question Sven. The channel is indeed the one which provides the best sensitivity, and is the second bar from the top. The refinements in the analysis and the addition of 70% more data at 8 TeV got back things in line with SM expectation...

    Cheers,
    T.
    Hi Tommaso,
    you are right that the previous data is not clearly visible in fig. 1. But as I said, the VBF channel at 7 and 8 TeV had a mu value of about -1. Now the total 7 TeV data, the total 8 TeV data, and the total VBF data have all values larger than +1.
    For the 7 TeV data the VBF channel should dominate the average, but this does not seem to be the case. The 8 TeV data should again be driven by the lower than -1 VBF channel data from the first 40%. The VBF channel as such had about 10/fb at mu lower than -1, now with about 17/fb it is above +1. I know that I did some forbidden averages here, but the main trend should still be there - but it is not.

    Of course it could be that the new 7/fb at 8 TeV overcompensated all the previous results, but this would look to me like a "very strong" (not to say "unlikely") overcompensation.

    Cheers, Sven

    dorigo
    Hi Sven,

    you seem to ignore that analyses were re-optimized in most cases,
    so old results are superseded not just by the added data but also
    by the improved methodologies. I think that bearing that in mind
    the results reported here make more sense.

    Cheers,
    T.
    Hi Tommaso,
    ok, I understand that the effects I was looking for are not visible (at least to some extent) ue to the reprocessing of the "old" data. That is fine, of course. I was just surprized, because the "old" data caused so much attention.

    Cheers, Sven

    Hi,
    Is spin 2 pretty much ruled out now?
    Thanks,
    Paul

    dorigo
    Not yet, Paul. The data do not allow to discriminate the two hypotheses 0 vs 2 yet.

    Cheers,
    T.
    These findings are fascinating! As a fellow science-lover, I thought you might be interested in theoretical physicist Sean Carroll’s new book about the Higgs Boson, The Particle at the End of the Universe. I’m happy to share more info with you and send a review copy if you’re interested.

    I'm sure Sean will be pleased that his publisher sends spammers to science blogs to hawk his book to colleagues :)

    dorigo
    Hi Kelsey,

    I'll be happy to review the book.

    Best,
    T.