Of Particular Significance

Quick Summary of the Multi-lepton Situation at the LHC

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 10/21/2011

For those of you wondering about this new and interesting hint of a possible (but still very doubtful) new physical phenomenon in the data from the CMS experiment at the Large Hadron Collider (LHC), here’s the summary of the situation.  [UPDATE: 10/22/11 very slightly reworded and reordered for clarity and proper emphasis]

  • Proton-proton collisions that produce three or more charged leptons and/or anti-leptons (“multi-leptons” for short) have long been seen as an excellent but very tricky place to look for new physics.   I have written an explanatory article as to why they are so good (to the point of being sometimes called “golden”), but also why they are tricky; there’s some fool’s gold in there.
  • On Wednesday and Thursday the CMS experiment gave two presentations  (here are my detailed comments, showing some tables and plots, on talks #1 and on #2) at a workshop in Berkeley, CA, giving two perspectives on a single set of multi-lepton events, obtained from data collected through August. They finds a small excess — more events than expected — but too small for CMS to claim to have seen anything significant.  [And Nota Bene: CMS has made no claims there is an excess, because the evidence is very weak by the stringent standards used (for good reason) in particle physics.]
  • Indeed, it must be remembered that small excesses of this type occur regularly in science, just from statistical fluctuations or from errors by experimentalists or theorists (remember the Higgs hints from earlier in the year, which were stronger, but have so far gone unconfirmed.)  And the excess seen here does not look particularly different from a statistical fluctuation.  So we should not be surprised if this hint goes away with more data from CMS, or when ATLAS reports their own studies.
  • But the excess is still large enough to intrigue theoretical particle physicists and motivate some consideration of what new physics, if any, might cause such an excess, and how to look for it more efficiently.
  • Two moderate earthquakes occurred in Berkeley, before and after the second talk.  Draw your own conclusions.      [joke…!]
  • Some of the plots shown at the second talk suggest that any such excess would most likely have to come from rather lightweight particles (well below 500 GeV) that do not carry the strong nuclear force.  (The reasoning is explained in my post following the second talk.) The logic is not airtight, but a number of other simple possibilities clearly wouldn’t fit the data at all.
  • Supersymmetry theorists will be quick to point out that supersymmetry has particles of the right type (charginos and neutralinos) and many variants of supersymmetry predict such a signal, though typically a bit smaller.  (You can read about how supersymmetry does that here; note especially Figure 2.)
  • However, many other classes of models of new physics can have light-weight particles with no strong forces, and associated multi-lepton events; such events are very generic and do not depend on anything special to supersymmetry.
  • So far I am aware of nothing in the data that would point toward or away from any one of these classes of models, so one cannot draw any conclusions.
  • We will know a lot more by March, when analysis of about 2.5 times the current data set will be possible.

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18 Responses

  1. Matt,

    You said that neither experiment has enough data to have seen a 120 GeV Higgs yet. However, this is only true for three generations. If there is a fourth generation, then the production rate is enhanced and could possibly be observed with the current amount of data.

    1. True in principle. I was referring to a Standard Model (SM) Higgs; outside of the SM, the rate for Higgs –> two photons could be larger. Specifically for a fourth generation, it’s borderline:

      Tevatron: Summer 2011 Combined CDF and DØ upper limits on gg→H→W+W- and Constraints on Fourth-Generation Fermion Models with up to 8.2 fb-1 of data. Assuming a heavy sequential fourth generation of fermions, 95% C.L. exclusion for a Higgs boson with mass mH between 124 and 286 GeV/c2.

      CMS (with 1.1 inv fb): The observed CLs-based limits are shown by the solid symbols and black line and exclude the SM4 Higgs with a mass from 120 GeV/c² to 600 GeV/c² (maximum Higgs boson mass covered by the current analyses). Their bound with 2 inv fb should be better.

      ATLAS with something near 2 inv fb: in this model Higgs boson masses above 120 GeV and up to 600 GeV are excluded at the 95% CL.

      So probably you have only the window 115-118 (lower bound from Tevatron, upper bound has to be a bit below 120 or the 120 bound would have gone up to 121 or 122.) I have not looked at what happens if you roughly combine ATLAS and CMS data by eye… have you? I would bet the 95% combined bound would now be 118, which would leave at best 115-116, maybe 117 (?)

      Clearly 120 is out in this scenario.

  2. “There are enough people who enjoy and appreciate the amazing amount of work and time Prof. Strassler puts into keeping us all informed and explaining nicely a lot of fascinating things.”

    Agreed 100%.

  3. Dear Prof Strassler,

    Your candid and very patient sharing of real time stories from “the Physics kitchen”, are nothing less than spectacular and very highly appreciated.

    I am a passionate silent reader of your Blog, that strongly disagrees – as I am sure are many many others – with Paolo.

    I dearly hope you will get discouraged.


  4. You are on a best way to lose Your credibility. three blog entries and all of them about nothing. here is my advice- abandon this path unless there is something worth the effort

    1. I’m not too worried about my credibility. I’ve got plenty of highly-cited scientific papers to back it up. And who are you? What’s your full name?

      Besides, I’ve said again and again: this is a “minor discrepancy”; “this will probably go away”; “it must be remembered that small excesses of this type occur regularly in science”. What more do you want? I can’t stop people from saying what they want to say, but at least those who actually *read* what I say can understand what I am *not* saying. You say this is all “about nothing”. I say this is probably all about nothing, and I won’t be surprised if it is entirely about nothing, but the small probability that it isn’t about nothing makes it worth a look by theorists.

      And as for “abandoning the path”; I’m already done. I don’t expect to have anything more to say about it until we have results from ATLAS or an update from CMS with much more data… presumably months from now — unless one of my colleagues makes an interesting observation that merits a comment.

    2. Paolo,

      You are not forced to read this blog if You dont like it; just avoid clicking the corresponding link :-).
      There are enough people who enjoy and appreciate the amazing amount of work and time Prof. Strassler puts into keeping us all informed and explaining nicely a lot of fascinating things


  5. So I am confused by what ” new physics” suppose to mean – beside the grammatical sense- maybe I’m too new on my ” physics journey… All is new at this point!

    1. Sorry, and thanks for asking! — that was jargon that leaked in to my writing. It just means “a new physical phenomenon” which we particle physicists tend to shorten. I’ve reworded the sentence. [It’s not that you’re too new, it’s that I’m too old!]

  6. If you (JollyJoker) are talking about the Motl’s “Higgs mass poll” post on October 9, 2011 (about 2 weeks ago), it is only a blogger’s gimmick of attracting more clicks. If there is a genuine hint on Higgs, it will be leaked out not from a blogger, and there will be rumors all over the places by now.

    Higgs mechanism is a very bad idea even if the Nature did employ it, as there is a much better way to fulfill its intended function. Of course, no one wanted to hear this before now. Now, the final verdict on Higgs will be out in months. It will be a good time to make a betting poll. We should not let Hawking get all the fun. I will bet that all types of Higgs will be ruled out.

    1. Only nature will answer these questions. I see no point in betting. While we are at it, it is impossible to rule out *all* types of Higgs particles before at around 2020 or so — so you’d all better be patient.

  7. “remember the Higgs hints from earlier in the year, which were stronger, but the word is that they have gone unconfirmed.”

    Really? Motl seems to have heard the opposite:

    “While we’re patiently waiting for the official announcement of the discovery of the Higgs boson(s) by the LHC, and your humble correspondent is determined not to leak the information about the boson(s) and the mass(es) in advance”

    His first hints about this were more than a week ago and I haven’t seen them mentioned elsewhere though.

    1. Lubos is joking; he has no special information. And I shouldn’t have said “the word is”, since that sounds as though I am spreading rumors — so I have reworded it. The point is that there is enough data by now that a simple search strategy in the range where the hints were sitting [specifically, 140-145 GeV] should have seen something by now. [The simple strategy is looking for Higgs –> two lepton-antilepton pairs.] The lack of buzz *suggests* nothing’s there. But I also have no special information.

      1. “The lack of buzz *suggests* nothing’s there. But I also have no special information.”

        The conspiracy is spreading… Why do the physicists of the world keep this information from us mere mortals? A summer of data releases a few weeks after they were gathered and then, around 2/fb, suddenly a complete blackout! Something is afoot, I tell you! 😉

        1. I’d like to see 5000 people with competing interests manufacture a consipiracy.

          The whole business is easy to understand. It has to do with (a) the internal policies of ATLAS and CMS aimed at keeping their experimental processing of the data sensible and untainted, and (b) the data-taking and conference schedules. There’s no point in trying to do a proper analysis until LHC stops taking proton-proton data this year, which happens within the next few days — it would be a huge amount of work and would be out of date before it was finished. That’s the reason for the “blackout”, as you call it. The summer had major conferences so they worked to get some things finished for then. That’s the tradition in the field. There are no must-publish conferences in the fall. However, there will be an update on the Higgs with the full 2011 data set probably in December, unless there’s a serious problem. There will also probably be an update on the summer’s results no later than late November, but again that’s just further processing the data collected through summer. Such is my current understanding; of course they can always change it if they want, these aren’t promises.

        1. Oh, come on. As though “viXra” has special information.

          I would remind you that ATLAS does not see CMS’s data, and vice versa; and with the amount of data each experiment has right now, neither one could draw a conclusion separately at 120 GeV. So. Forget it.

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