Of Particular Significance

New Higgs Result From the ATLAS Experiment

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 07/18/2012

[2:40 Paris time: UPDATED ]

The ATLAS experiment (in a talk by Olivier Arnaez at the Higgs Hunting workshop in Orsay, France that I’m attending) has made public its search for Higgs particles decaying through real and virtual W particles to a lepton (electron or muon), anti-lepton, neutrino and anti-neutrino.  [CMS made its result for this search public on July 4th at the big presentations at which discovery of the Higgs-like particle was announced.]  The importance of this search is now no longer to tell whether the Higgs-like particle exists — we are confident that it does — but to try to measure how often these particles are produced and then decay in this particular way.

This is a very difficult measurement, with a small signal and a complex and large background, and it only allows a measurement of a rate of the process; the mass of the particle producing an observed excess cannot be determined to better than 30-50%, because the neutrino and anti-neutrino are not observed, and the lepton and anti-lepton are not enough to figure out the mass of the parent particle.  Meanwhile, small amounts of data can play tricks.  After last summer’s data, ATLAS saw a sign of something.  By March, with the full 2011 data, they said they didn’t.  Now, including 2012 data, they see it — in fact, they see slightly more than expected for the simplest possible type of Higgs particle (“the Standard Model Higgs) of mass 125 GeV/c2.

More details to follow shortly, but the news is:

  • ATLAS observes in 2012 data a 3.1 sigma deviation from the no-Higgs hypothesis
  • Combined with 2011 data (which showed almost no excess) there is now a 2.8 sigma deviation from the no-Higgs hypothesis
  • the excess looks like a Higgs particle; it is quite consistent in its transverse-mass distribution with the hypothesis of a Higgs-like particle at a mass of 125 GeV/c2
  • the fit to the signal is 1.4+-0.5 times larger than the Standard Model prediction for a Higgs particle of mass of 125 GeV/c2 (though one should not forget that this measurement isn’t good for telling us the mass; just by itself, it would also be consistent with a lighter Higgs particle produced with an expected rate or a heavier Higgs particle with a much lower than expected rate.)
  • this in turn is in good agreement with what ATLAS observes in the two-photon and four-lepton searches, both of which are a bit higher than the prediction for the simplest Higgs with mass of 125 GeV/c2
  • this also is consistent specifically with the prediction, which holds for most expected types of Higgs particles, that the ratio of the strengths of the interaction of the new particle with W and Z particles is equal to the ratio of the masses of the W and Z.

These results are also roughly consistent, within the uncertainties, with those from CMS, which sees a slightly smaller production rate than expected, something like 0.6 +- 0.4 (don’t quote me, I’m reading it off a plot) times what is predicted for a simplest Higgs of mass 125 GeV/c2.

NOTE ADDED: Did I mention this is a difficult measurement? The excess in the 2012 data looks a lot like a Higgs particle signal, but a mis-estimated background would look pretty similar (though admittedly it would have to be a big mis-estimate — but then again, the signal in 2012 is itself more than twice as big as expected), and so we’re reliant on the estimates of the systematic uncertainties on the backgrounds given by the experimenters.  There are differences between the 2011 and 2012 analysis techniques (improvements, surely) that I don’t understand yet.  So… an impressive result, but my own view of it is still a little murky.  I’m glad this excess wasn’t essential for the claim of Higgs particle discovery.

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32 Responses

  1. There are hundreds of houses in my town and dozens of garbage trucks. I have made sample estimates of the mean and variance of collection intervals at my house. They happen to be anti-bunched, as my wife frequently reminds me.

  2. Is it meaningful to ask if the photons and weak bosons that have been detected at the LHC are antibunched? If so, are they?

      1. Matt: this is getting a bit late because I hate to snap back with a reply without thinking first, although that may the only feasible way to discuss on a blog. Are you saying that the intermittency of collisions makes it impossible to detect any anti-bunching of the bosons produced by the collisions?

        1. No. There’s no bunching at all. The bosons all come out in various directions after each collision — there’s no collimation, no beam, and no bunching in the outgoing particles.

  3. From the latest CMS analysis summary it appears a fermiophobic Higgs boson in diphoton decay mode is excluded with 95% LC between 110-127 Gev, and for the state found near 125 GeV at 99% CL. Can we therefore rest in peace that quarks and leptons acquired mass from Higgs field as well ? We do not need any more complications or do we ? 🙂

    1. Some of them got mass this way, it would appear… the top quark, at least. I don’t think we can yet rest assured that this particle’s field did the job for all of the quarks and leptons.

  4. Professor Matt.:
    In your opinion what kind of discovery if made would explain the value specifications of the SM including the constants of nature ?
    M-fantasy provided 10^500 ” explanations” !!!

    1. I have no idea how to answer this question; it seems to me that we do not yet have the information to answer it. I suspect if you had asked someone in the mid 19th century what kind of discovery would explain all of basic chemistry, it would have been hard for him or her to imagine quantum mechanics.

  5. Yes, I agree with that Matt. The gam-gam channel is most sensitive to the HWW vertex, and the other channel to the HZZ. Having these come close (I guess at the moment we can only talk about within an order of magnitude) to the EWSB prediction, is a bit like Mw/MZ coming close to sin(theta_w). Possible by accident, but most likely means there is a broken symmetry. I guess we have to wait for the experiments improve their precision to be sure.

  6. This is probably the best talk I’ve seen out of the Higgs experiments, very detailed in terms of what they do in their analysis, which you need in order to have confidence in the results (and they use the event generator HERWIG I worked on). There is clearly a signal in the 2012 data, but: a) it is a factor of a few too large for 2012, b) is completely absent in the 2011 data. If not for this I would be starting to believe in the Higgs, but the suspicion with the above is obvious – has their 2012 analysis tweaked the results?

    Its hard for me to watch the read the multitude of web reports (fortunately I havent seen it in print yet) calling this the Higgs boson. Even the Higgs-like boson is going too far. I think Higgs-candidate is more appropriate. To be Higgs like, we need proof that boson has a non zero vacuum expectation, more particular at the level of EWSB. And then to be Higgs all its cross sections need to be right – because with that there is no room to move in the SM.

    1. One does indeed have to understand better what was done in 2012 and how it differs from 2011. I know that the experimentalists involved worked very hard to make the 2012 analysis as blind as possible. I will try to learn more in coming days and weeks. More info will be in the note describing the result.

      As for “Higgs candidate”, I’d support that. What I’d say is that this Higgs candidate is “Higgs-like” because it has the right scale for the gamma-gamma/ZZ coupling ratio, which is not at all trivial; a random scalar boson could be off by factors of 100. And I think it is likely to be *a* Higgs (not *the* Higgs of the Standard Model, necessarily) because it is really quite difficult to write a theory of a scalar particle where the two-photon and four-lepton bumps both come out within a factor of 2 of the Standard Model Higgs expectation. It requires two lucky and independent accidents… remarkable accidents. Such accidents can happen — obviously we must check the properties of the particle in detail. But I view the possibility that this is not a Higgs of some type as a long shot… my personal view, shared by many but probably not everyone.

      Whether this likely-to-be-Higgs particle is the one predicted in the Standard Model, however, is anyone’s guess at this point. The big argument in favor of the Standard Model is the lack of any other discovery so far in LHC data, or elsewhere. But that’s much less convincing as an argument compared to the one in the previous paragraph. I know many types of physics that would alter the Higgs in a subtle but important way and wouldn’t yet have been discovered at the LHC (and some of these might only be discovered by studying the Higgs and its decays.)

      1. Hi Matt,

        A major difference is the 2012 analysis is done with emu/mue events while the 2011 analysis does have same flavor events where DY background needs careful treatment.

  7. 1. In light of this new measurement, the Higgs-like particle at 125 GeV, does the SM predict a “unit” mass for the lightest possible “particle”? And if so, is it the Higgs or does the Higgs boson also require a parent particle?

    2. I guess another way of phrasing the question would be, in light of the universal constant, c, what is the minimum energy density required to create a close (standing) wave with an infinite (or close to infinite) half life (cannot be scattered by any other particle, including one of the same characteristics)?

    3. Is it valid to say that the constancy of the speed of light limit, c, is because there is one and only one transition point where a scalar turns into a vector, i.e. the point where the primordial energy became oscillatory (waves)?

    1. 4. If the Higgs field do exist, a) must there be one unique particle associated with it or could there be b) one for each massive fundamental particle or c) an infinite number of Higgs particles?

      If a), then will it not indicate the possibility of a multiverse, one Higgs boson (mass-energy) per manifold and maybe even explain dark matter and dark energy. Dark energy being the summation of all the different Higgs.

      If b), then shouldn’t there be a theory (possibly and extension of SM) that associates all massive particle discovered thus far?

      If c), then wouldn’t it remove the need for SUSY?

  8. “[CMS made its result for this search public on July 4th at the big presentations at which discovery of the Higgs particle was announced.]”

    Proff: have you omitted “like” particle in a hurry!!

    1. Fair enough; I edited the line to put “-like” in there.

      The reason it happens in my prose is that I personally am convinced that this is a Higgs particle of some type, with all other options looking very implausible for theoretical and experimental reasons; I said a few words about this in http://profmattstrassler.com/articles-and-posts/the-higgs-particle/the-discovery-of-the-higgs/higgs-discovery-is-it-a-higgs/ . I’ll try to do a better job of keeping my personal views out of the posts.

  9. The fit to the signal is 1.4+-0.5 times larger than the Standard Model prediction for a Higgs particle of mass of 125 GeV (though one should not forget that this measurement doesn’t tell us the mass; it would also be consistent with a lighter particle produced with an expected rate or a heavier particle with a much lower than expected rate.)

    Matt: I don’t understand the what mass you are referring to when you say, “. . . this measurement doesn’t tell us the mass;”
    Could you explain or refer me to a more detailed explanation. Sorry for the simple minded question, but I’m just trying to understand.

    Don Murphy

    1. I’m referring to the mass of whatever particle produced the signal (we should be clear that saying the excess is from a 125 GeV particle is an assumption, not something that is obvious in the data.) I’ve updated the file to include a link to a place where I explained why this measurement is so hard and why the mass can’t be measured.

      1. By the way, is there a missing “higher than” in the quote statement? “it would also be consistent with a lighter Higgs particle produced with an [higher than] expected rate”

  10. Matt—completely off-topic, but a few hours ago XENON100 was to give a talk giving the results of the past year. But the talk, by Elena Aprile, isn’t yet posted. Any word?

    1. Google -> News -> “XENON100” gives me this page from today: http://www.innovations-report.de/html/berichte/physik_astronomie/xenon100_setzt_neue_beste_ausschlussgrenzen_dunkle_199153.html

      In english: from 13 months of data taking and after better background exclusion they’ve found 2 events while 1 event was expected due to background radiation. So nothing significant here except that they’ve lowered the threshold of possible matter dark matter interaction ratio again.

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