Banner
    Higgs Live Blogging 7 - The ATLAS Seminar
    By Tommaso Dorigo | July 4th 2012 02:55 AM | 6 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...

    View Tommaso's Profile
    [ The previous entries of this live blogging series are available here (part 1) , here (part 2) , here (part 3)here (part 4), and here (part 5) . The report of the CMS seminar is here.]

    Fabiola Gianotti now takes the stage for ATLAS. She also starts with a joke (this one about a german newspaper showing how Higgs bosons look like, quite unlike what we think they do).

    She also mentiones pile-up, higher by a factor of two.

    ATLAS is not ready with the publication of results of the "low-mass-resolution" channels, which in CMS were shown and decrease a bit the total significance...

    THe integrated luminosity of 2012 in ATLAS is 6.3 inverse femtobarns: one full inverse femtobarn larger than what CMS got ! This is due to the way the LHC operates... A bit unfair for CMS, but that's life. In some sense ATLAS is the CERN experiment, and CMS is the outsider...

    Fabiola is concentrating on pile-up and ways to handle it and maintain stable performance at all levels. She has spent some time on the triggers and now on computing.

    Talking about production modes, she mentions that the H cross section increases by 30% in going from 7 to 8 TeV. Some irreducible backgrounds grow by similar amounts, but some reducible backgrounds also increase. The increase in sensitivity with respect to previous data is thus at the 10-15%.

    Fabiola then describes the 2011 results in summary: a 3-sigma excess at 126 GeV and an exclusion almost everywhere else.

    Since 2011, improvements in the analyses have been brought by optimization and improved reconstruction and identification of physics objects.

    The gamma-gamma search in ATLAS is also performed by subdividing the data in many channels with different expected sensitivity. They also have an independent vector-boson-fusion category.

    170 signal events in a background of 6000 are expected by ATLAS in this decay mode.

    The mass resolution in the Higgs to gamma gamma signal is not affected by pileup, because ATLAS does not depend on tracking to reconstruct the photon angle.

    The gamma-gamma peak looks just as lovely as the CMS one. Unfortunately I do not have it available for immediate posting - will update this post later on with the graphs.

    The p-value distribution shows two similar 3.4-3.5 sigma significances for the 2011 and 2012 data. The combination produces a significance of 4.5 standard deviations. Expected for the SM Higgs would be only 2.4 sigma in this channel however!

    So both CMS and ATLAS see higher-than-expected Higgs rates in diphotons.... Quite interesting.

    Now Fabiola is talking about the H->ZZ-> four lepton signal.

    (Somebody next to me listening to Fabiola's talk mentions that she should use better colours for the slides. I concur - it's a awful mix of pastel greens and browns and indefinite colours, in overlaying boxes of text here and there... Maybe she is colour-blind).

    In the meantime Rolf Heuer leaves out this press release:

    CERN experiments observe particle consistent with long-sought Higgs boson

    Geneva, 4 July 2012. At a seminar held at CERN* today as a curtain raiser to the year’s major particle physics conference, ICHEP2012 in Melbourne, the ATLAS and CMS experiments presented their latest preliminary results in the search for the long sought Higgs particle. Both experiments observe a new particle in the mass region around 125-126 GeV.

    “We observe in our data clear signs of a new particle, at the level of 5 sigma, in the mass region around 126 GeV. The outstanding performance of the LHC and ATLAS and the huge efforts of many people have brought us to this exciting stage,” said ATLAS experiment spokesperson Fabiola Gianotti,“but a little more time is needed to prepare these results for publication.”

    "The results are preliminary but the 5 sigma signal at around 125 GeV we’re seeing is dramatic. This is indeed a new particle. We know it must be a boson and it’s the heaviest boson ever found,” said CMS experiment spokesperson Joe Incandela.“The implications are very significant and it is precisely for this reason that we must be extremely diligent in all of our studies and cross-checks."

    “It’s hard not to get excited by these results,” said CERN Research Director Sergio Bertolucci. “ We stated last year that in 2012 we would either find a new Higgs-like particle or exclude the existence of the Standard Model Higgs. With all the necessary caution, it looks to me that we are at a branching point: the observation of this new particle indicates the path for the future towards a more detailed understanding of what we’re seeing in the data.”

    The results presented today are labelled preliminary. They are based on data collected in 2011 and 2012, with the 2012 data still under analysis.  Publication of the analyses shown today is expected around the end of July. A more complete picture of today’s observations will emerge later this year after the LHC provides the experiments with more data.

    The next step will be to determine the precise nature of the particle and its significance for our understanding of the universe. Are its properties as expected for the long-sought Higgs boson, the final missing ingredient in the Standard Model of particle physics? Or is it something more exotic? The Standard Model describes the fundamental particles from which we, and every visible thing in the universe, are made, and the forces acting between them. All the matter that we can see, however, appears to be no more than about 4% of the total. A more exotic version of the Higgs particle could be a bridge to understanding the 96% of the universe that remains obscure.

    “We have reached a milestone in our understanding of nature,” said CERN Director General Rolf Heuer. “The discovery of a particle consistent with the Higgs boson opens the way to more detailed studies, requiring larger statistics, which will pin down the new particle’s properties, and is likely to shed light on other mysteries of our universe.”

    Positive identification of the new particle’s characteristics will take considerable time and data. But whatever form the Higgs particle takes, our knowledge of the fundamental structure of matter is about to take a major step forward.


    So finally CERN does not back off from announcing a discovery!

    Back to Fabiola: the H->ZZ peak of ATLAS is very nice too! They have been definitely luckier than CMS. I think the final word about who did better needs to be comparing the expected sensitivity, but taking into account the per-femtobarn yields.
    They observe 13 events in 120-130 GeV, with 5.1+-0.8 expected from backgrounds alone. This can be translated into a p-value of 0.004, so just short of three sigma. But if they fit the distribution their p-value may be smaller. What do they quote ?

    3.4 standard deviations is what they quote, when they expected 2.6 standard deviations. Lucky again! I can't help feeling a bit envious right now :)

    So the grand combination is only done using the gamma-gamma and ZZ->4l channels in ATLAS, together with all the channels analyzed in 2011. By doing the same thing, CMS gets 5.0 standard deviations, when they expect almost 6. What does ATLAS find ??

    5.0 standard deviations !!! Exactly like CMS ! Another ovation in the room!

    The funny thing is that the expected significance of ATLAS was 4.6 standard deviations, so a full 1.3 standard deviations less than CMS ! I will need time to digest this discrepancy and understand exactly where it comes from.

    Best-fit cross secttion: 1.2+-0.3 times the standard model prediction.

    Fabiola finally shows the evolution of p-value with time, since the summer of 2011 to this result. The significance went a bit down since December 2011, but now their result is quite solid. The strength of the signal is spread unevenly across the channels with respect to expectations, but overall the agreement with the standard model is good.

    Comments

    I am very sleepy still and just scanning your posts Mr. T but while I'll concede that it looks like they have something I find it disappointing that they still need to combine decay modes to go over 5 sigma. Was there a reason why they couldn't wait for more data? Nobody else was going to beat them to the actual discovery.

    vonankh
    4th of July!?
    I don't understand your comment on ATLAS having more luminosity because of an artifact of the accelerator -- isn't it just that CMS has had a lower data recording and certification efficiency this year?

    dorigo
    Not sure Josh - will try to find out. Probably the truth lays in the middle though.
    Cheers,
    T.
    For the gamma-gamma channel, "170 signal events in a background of 6000 are expected by ATLAS in this decay mode".

    However, the gamma-gamma channel had more than expected - so how many signal events were actually seen? Of the order 240?

    dorigo
    Yes, something similar. But I will need to update the post with the plots.
    Cheers,
    T.