Higgs: New ATLAS And CMS Results
    By Tommaso Dorigo | November 14th 2012 02:53 AM | 27 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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    Today new results of Higgs searches have first been shown in Kyoto, at the Hadron Collider Physics conference. Let us see the CMS and ATLAS updates of their measurements in the ZZ-> 4 leptons final state, which constitutes the best signal-to-noise channel to study Higgs properties cleanly and measure mass and spin-parity of the new found particle.

    CMS first. CMS updated their ICHEP result by adding statistics from the 2012 run, for a total of 17.3 inverse femtobarns of proton-proton collisions at 7+8 TeV center-of-mass. The four-lepton invariant mass is shown on the right. As you can see, the peak at 125-126 GeV has become more prominent, and is in great agreement with Standard Model predictions (the arrow shows the Z boson peak reconstructed from 4-lepton decays). Note that the data follow very well the background prediction over all the spectrum, with a bumpy departure at the mass of the new particle.

    The significance of the peak in the ZZ->4l final state alone is quoted by CMS at 4.5 standard deviations. The signal strength is fit at 0.8 +0.35-0.28 times the Standard Model.

    The events allow a beautiful measurement of the boson's mass. A three-dimensional fit is employed, which uses not only the four-body mass, but also the kinematics of the final state as well as the individual per-event mass resolutions (muons and electrons have different momentum resolution depending on how they are measured in the apparatus). This allows CMS to produce the figure below, which shows the likelihood contours in the mass versus signal strength plane. As you can see, the signal strength is bang-on the expectation, and the mass is measured as M_h = 126.2+-0.6+-0.2 GeV.

    Another nice measurement is the one of spin and parity of the state. The Higgs is expected to be a scalar particle: zero spin, positive parity. Having observed the diphoton decay implies that this new particle, if it is a single state decaying both into diphotons and ZZ pairs, has even spin. So a test is to see whether angular distributions are more compatible with the 0+ or 0- hypothesis. The separation between the two is provided by the angular distribution of the decay products. A discriminant (pseudo-mela, where ME stands for matrix element) is cooked up, and on the left you can see the different expected distributions for the two alternatives. The data distribute more according to the full red line, albeit the separation is not yet very strong.

    In the end with pseudoexperiments one may determine the test-statistics distribution of the two hypotheses, as shown on the right. The data favor the 0+ one, with a CLs value of 0.024 disfavoring the 0- alternative. Beware of the typical probability inversion statement that many will let go "the probability of the 0- hypothesis is xxx": it is the probability of the DATA which was tested, under that hypothesis!!

    As for ATLAS, they did not add statistics in their H->ZZ->4l search since ICHEP. The money plot is therefore the one shown below, which is still based on 10/fb of luminosity. What they improved on was in background studies and lepton identification performance. They are still quoting a signal strength of 1.4+-0.6 times the SM expectation, and a significance from this channel of 3.6 standard deviations. We will have to wait for Moriond to see their new data.

    So in conclusion what we can say is that the four-lepton decay mode confirms the standard model interpretation of the new found object. CMS has furthered their studies quite a bit since last July, while ATLAS is lagging behind a bit. One wonders whether they have something cooking... Maybe we can expect surprises ?
    I personally doubt it. As this measurement shows, and as others will demonstrate in the next presentations at HCP, the particle behaves in rather disappointingly good agreement with what the vanilla Standard Model expects. Stay tuned for more results!


    Why are there no CMS gamma gamma updates with the larger dataset?

    Because the collaborations have decided to wait until Moriond for those.

    Last year in december the statistics on the Higss particle was 3 sigma. That is not enough to call it a scientific discovery. The July verdict was a 5 sigma, which is a close to 100 % discovery. But that 5 sigma was only that there is a particle, not on the qualities that is necessary to call it the Real Higgs particle. I don't think that statistics on the qualities has changed much since last december, but I cannot say, because the media does not write about it.

    My prediction is still that the qualities of the Higgs particle, necessary to prove the standard model right, will not reach 5 sigma. The simple argument is that these qualities do not exist.

    The standard model in its formative states, concluded that there is no mass. That sounded weird to Peter Higgs, and therefore he mathematically predicted the Higgs particle.

    I think that the standard model is right, when it comes to the conclusion of no mass. I, as Einstein did in his older days, think that everything, including gravity, can be explained through electromagnetism. On top of that I think that above the force of electromagnetism there must be a force that we do not know, yet.

    I have a videopresentation on my blog, that explains this in details. But beware I am no physician, I am a philosopher using physics to make my point.

    Dear Otto,

    this is a mix of conspiracy theory ("because the media does not write about it." - what exactly do you mean: we publish all results in every detail and I myself am part of the "media" and write about Higgs properties measurements) and misconceptions. The mass of elementary particles is a fact, and indeed the Standard Model (which includes the Higgs mechanism -without that there is no Standard Model you can speak of) predicted the mass of W and Z bosons very accurately, way before they were even proven to exist. Also, the SM has predicted the mass of the Higgs boson with 30%-ish accuracy based on measurements of electroweak properties and calculated radiative corrections.

    So I do not know what you are speaking about, but it sounds a lot like pseudoscience (you yourself claim to not be a physician, well I imagine you are no physicist either). And as open-minded as I am with respect to off-the-mainstream theories, I have to warn you - pseudoscience is not welcome here.

    Trust me my friend, It is all high class physics drama. I remember the day, I went to a girl working on carbon nanotubes in Queen Mary Uni during my research days. Actually, I want to use few technique they are using for carbon nano tubes. After looking at what she was doing and stuff, I can't believe that it is called science. After 1-2 years, all hydrogen storage results of major contributor in carbon nano turns out fake. Higg particle is like indian riddle: Who created earth? answer: God, Well, then who created God ? As per your argument, someone is needed to create someone. So, nothing solved yet.
    john from

    Amir D. Aczel
    Thanks for a very informative article. Could you explain what the 0+ and 0- mean?

    Amir D. Aczel
    0+ means scalar, 0- means pseudoscalar.

    Amir D. Aczel
    Thanks!! So we want 0+, right?
    Amir D. Aczel
    The SM wants scalar, so we all want pseudoscalar. :)

    Just got this in time for a talk I'm giving on the Higgs tonight, many thanks!

    The Atlas col is underestimating the bckg as visible in the 200 GeV region,
    eroding the statistical significance of any peak

    Hi Felipe,

    you got it wrong - an underestimate of backgrounds increases the significance of any peak,
    not the opposite. The reason is that the signal that can be fit will take advantage of the excess of data. In any case, the effect, if any, has no bearing on the 125 GeV bump.

    let me ellaborate
    "The Atlas collaboration underestimates the background as can be seen by noting that in this plot there are many
    more points above than below the background, with too large a difference. Looking at the high energy sideband where there is no claim of a resonance this is evident. Correcting this systematic error raises the background normalization everywhere and thus, erodes the claimed significance of the peak at 125"
    it just takes double time to type all that

    Hi Tommaso,

    gossip told us that ATLAS did not unleash data on gamma-gamma decay because they "see double". Another peak around at lower masses?



    Hi Marco,

    unlike others, I do not sleep with any ATLAS collaborators, nor do I go out for dinner to chat as some of my own collaborators do. In fact it is hilarious that people are "afraid" of blog leaks, when the leaks come from all other sources.

    In any case I cannot discuss these matters here, for obvious reasons.


    There is no problem of course, but your answer is really humorous. Thanks.


    Moriond is not until March 2013
    Jester says on Resonaances that his "... guess is
    {ATLAS} will release the 13fb-1 diphoton results in a few weeks ..."
    "... in CMS ... the ... h ->diphoton ... signal strength went down ....
    the new results are not very consistent
    with what was presented on the 4th of July
    ... Most likely, all these analyses will be released before
    the end of the year ...".

    So will there be a December seminar
    with full details and combinations following in March ?


    Hi Tony,

    don't think either will happen. Most likely we'll get updates for winter conferences
    and that'll be all.

    The two photon signal was also suapect because the background was not calculated in the SM
    but fit to some ad hoc polynomials (when in most exclusion analysis for MSSM for example they do use the SM backgrounds)
    let us hope that this did not bias the analysis and the claim has to be retracted like the quick neutrinos, this would be a field killer

    Can someone help a non-particle physicist? I read in a related report that these results mean gamma particles tended to be found instead of the muons predicted by Standard Model. By "gamma particles," do they mean "gamma rays," like xrays ... and if so, what is the frequency of the observed gamma rays? I looked "gamma particles" up on google and it seems to be a very general term used for short wavelength radiation ... UV, X-ray, conventional gamma waves ....

    Hi BB,

    first of all, that report is wrong, because there is no such discrepancy (gamma instead of muons): everything is as predicted by the Standard Model.

    Then for your question.
    Gamma ray = gamma particle = photon of high energy (several MeV and above)
    X ray = photon of medium-high energy (keV to MeV)
    light quantum = photon of any energy.
    radiation, visible light, microwaves, electromagnetic waves, etcetera: all of them are
    different representation of the same physical phenomenon, the one of electromagnetic
    radiation with varying wavelength.

    Rather than the frequency we measure the energy directly. The two are proportional,
    the constant of proportionality being the Planck's constant: E = h v where v is the frequency and
    h is the Planck's constant.

    Thank you very much, yes I'm familiar with Planck's equation. The report I read is here:
    The part I'm referring to is near the end: "The Higgs boson is naturally very unstable -- one of the reasons why it has been so elusive. When it decays, the Standard Model predicts that it should produce a certain number of tau particles (heavy cousins of the electron). But the data suggest an excess of gamma particles were being generated." ...
    To me (working in another branch of physics), this reads like gamma rays are being found instead of predicted muons ... I wondered what the energy(frequency) of the observed gamma rays is???
    Maybe I am reading the report wrong, or something.

    I suspect you are reading the wrong report, not the report wrong ;-)
    The energy of the two gammas emitted in a Higgs decay is 63 GeV, just as the energy
    of the two tau pairs. TWO, not "a certain number".

    Thanks, knowing the energy will let me work out what I don't understand! So glad to find this blog, cheers :-)

    Sorry posted the wrong link, can't seem to edit it: the report I just read is here:

    It shouldn't be a "disappointment" because it is maybe indicative that this is all that there is, so something would have to have been missed along the way, and some serious soul searching is in order before racing off on another tangent to higher energies in search of the some pseudo-scientific fantasy of absolute symmetry.

    There were a number of very important questions that never got answered back when physicists understood that the planck scale was the end of the road, and I believe that they all have the same answer:

    1. Why couldn't Dirac unify GR and QM with the Dirac equation like he did with SR and QM?

    2. In 1928 Dirac wrote that every elementary particle with positive energy, e=mc^2, has its antiparticle with
    negative energy, –e=mc^2.

    So why doesn't antimatter have negative mass?

    3. Why don't Positrons fall "up"?

    Because antimatter has negative pressure until it becomes a real or virtual particle.

    I hope that the ongoing collapse of the world economies forces some very bored theorists to do a long overdue rethink. I get so disheartened whenever I hear about disappointment with the "vanilla" model. The belief that a "grander" theory will fix all the problems that failures like the above illustrate is the wrong way to do science.

    Something got missed... is all...