ATLAS: 5.9 Sigma For A 126 GeV Higgs !
    By Tommaso Dorigo | July 31st 2012 10:53 AM | 10 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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    ATLAS has just released a note which summarizes the searches for the standard model Higgs boson in 7-TeV and 8-TeV data. Since July 4th the main improvement is the addition of the WW channel, which had not been shown back then. With it, the combined local significance of the 126 GeV Higgs boson excess in the WW, ZZ, and γγ channels grows to 5.9 standard deviations. In the words of a Facebook friend who's in ATLAS: "if this is not a discovery, I don't know what is".

    So, due congratulations to my ATLAS colleagues for this new important document. The paper is indeed full of detail about the searches, answering many of the questions that the format of the July 4th event did not allow to be asked.

    The most important measurements, those of mass and cross sections, are summarized in the figure on the right, which shows the 1-sigma contours for the different final states: the WW measurement is the one which extends the most in the horizontal axis, because of the large indetermination in the mass due to the escaping neutrino pair.

    The three measurements are consistent with each other, and the global signal strength is measured at 1.4+-0.3 times the standard model predictions for a 126 GeV Higgs boson.

    On the left you can see the reconstructed WW transverse mass for Higgs candidates found by ATLAS, with the data (black points) compared to sum of backgrounds and predicted Higgs signal (in red). As you can see, the data has a strong excess, which contributes sizably to the global combined significance.

    Finally, a very interesting plot similar to the one that CMS had produced on July 4th is the one shown below, which shows the signal strength modifier for the diphoton channel, calculated with respect to the different Higgs production modes: gluon-fusion and top pair higgsstrahlung on the horizontal axis, and vector boson fusion plus vector boson higgsstrahlung on the vertical axis.

    The production modes may be differently affected by non-standard-model physics, so this is a very nice check of consistency that Higgs production occurs (roughly) as we expect. It is shown below.


    Thanks a lot for the discussion. I did not understand one claim of the 4th July announcement. From the ATLAS data they found a confidence level of 4.1 sigma for the diphoton decay. However, the overall confidence level was of 4.9 sigma. Why should we look for the total one? Shouldn't we look for a 5 sigma for the diphoton decay individually?

    Also I mentioned an arxiv paper in my blog (, which talks about Higgs boson imposters. But since I'm not an expert in this field, couldn't grab much. Would be nice if you can make some comments to post on my blog. That may helps the readers as well.

    After clicking on the link I see the title still called the "new particle" rather than the SM Higgs. Does that mean we still don't know if this new particle is "the" higgs?

    Also, I'm a bit of a layman when it comes to this. If you could, what exactly is the significance of the addition of WW readings? Does that one show the spin or the tau-tau results? If not, do you know when would be the best time to expect those results?

    1+1+1 != 5

    Please restate the statistics after taking into account the 'signal' in the other two unmentioned channels.

    Not sure if this has been discussed yet: Will the Higgs result influence the long-stop at the end of this year? I'm guessing most of the planning for that stop has more or less already completed and things are already in motion. Is there any chance that the plans and equipment would be tweaked to gain better insight into that region of mass to perhaps discover more about the higgs without having to build a linear collider for instance?

    Hi Anon,

    the CERN management announced that the pp running will continue until december, and the heavy ion running will be done in jan/feb 2013. Then the shutdown will start. This way, CMS and ATLAS will likely get as much as 30/fb total each from the last two days of run.

    ... from the last two YEARS you mean ?

    yes, years, goddam wishful thinking.
    I find the last chart very interesting.
    Where would a mark be put for Super Symmetry?

    Hi Martin,

    probably all over the place. By tweaking tanβ, mA, etcetera you can get large cross section enhancements in various production modes. But I guess some expert could comment more
    meaningfully - e.g. if only Sven read this.

    Too late after my email-free holidays... Next time! :-)