Of Particular Significance

Theory Killers at the HCP conference

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 11/13/2012

There were many interesting results presented yesterday at the HCP conference in Kyoto, and they were both too numerous and too detailed for me to completely absorb as yet — a follow-up will clearly be needed.  But a few are obviously so important that I want to point them out now.

First, both ATLAS and CMS, the two general purpose experiments at the Large Hadron Collider [LHC], produced important new results on “multileptons”.  Based on a significant fraction of their 2012 data, they looked for signs of new phenomena that would appear as proton-proton collisions that produce at least three leptons or anti-leptons, or even (in unusual combinations and/or along with other unusual things) two leptons or anti-leptons.  (I’ll just summarize this class of studies as “multileptons” for the purpose of this brief post and be more specific at a later date.) ATLAS used about 50% more data than CMS, but CMS had a more intricate analysis of their data, so I believe the results were similar where they can be compared.  [By the way, the CMS result was approved to be shown at this conference under extreme conditions; at least two of the major players in the analysis had no power or internet for over a week following Hurricane Sandy!]

The bottom line is simple: neither CMS nor ATLAS sees any significant deviation from what is predicted by the Standard Model.  And this now kills off another bunch of variants of many different speculative ideas.  The details are extremely complicated to describe, but essentially, what’s dead is any theory variant that leads to many proton-proton collisions containing

  • two or more top quark/anti-quark pairs
  • multiple W and Z particles
  • two or more as-yet unknown moderately heavy particles that often decay to muons, electrons and/or their anti-particles
  • new moderately heavy particles that decay to many tau leptons

and probably a few others I’m forgetting. While multilepton searches (especially those for 3 or more leptons) are often touted as a great way to look for supersymmetry in particular, that description vastly understates their power — they are a great way to look for many different types of phenomena not predicted in the Standard Model.  (This is something that a number of scientists at Rutgers University have been emphasizing in talks and papers.)  And both experiments have demonstrated this with various interpretations of their results; CMS has over a dozen of them!

Each interpretation involves looking to see what specific variants of a particular subclass of theories is now excluded by the new data.  Many of these subclasses of are called “variants of supersymmetry”, but in fact the phenomena considered are so stripped-down, typically involving two or three new particles and one or two new physical processes, that they often have nothing specifically to do with supersymmetry at all; the interpretation given applies just as well, or almost as well, to variants of certain non-supersymmetric speculative theories.  [The use of these “simplified models”, as they are now termed (I used to call them “theory fragments”, see the last of my lectures from 2006; the Harvard group termed them “OSETs“) allows for results from data to be interpreted in a broad and simple fashion that applies to a wide variety of speculative ideas.  This comes at the cost of not being able to see transparently which variants of a particular speculative idea are excluded.]

ATLAS also produced another powerful result, using a significant fraction of their 2012 data, where they looked for an excess of events which contain multiple jets (at least 4), of which 3 or more appear to be from bottom quarks, along with signs of undetected particles (which appear through an imbalance of the momentum of the detected articles; this imbalance is called “missing energy”, a misnomer that has stuck for historical reasons.)  This is a very powerful way of ruling out any theory variant that produces both invisible particles and large number of bottom quarks.  No excess of events was found, and this allows excludes variants of theories that abundantly produce invisible particles along with two top-quark/anti-quark pairs, two bottom-quark/anti-quark pairs, or two Higgs particles. (Precise limits were only presented for a particular supersymmetric subclass, in which pairs of gluinos are produced, each gluino then decaying to a top quark/anti-quark pair and an invisible particle.)

CMS has presented the first measurement of the rate for something called Vector Boson Fusion production of a Z particle. Vector Boson Fusion is one of the ways to produce Higgs particles, and both ATLAS and CMS will be studying this process in great detail and attempting to make precise measurements of it. But to make sure we understand this process well, it’s important to measure similar processes and make sure that the Standard Model prediction matches the data. So this measurement, where the Higgs is replaced by a Z, is going to be an important test case to give us confidence that theory and experiment are in good agreement where we strongly expect them to be.

Finally (for now) ATLAS also presented a very interesting measurement — very interesting to me, anyway, since my collaborators (Mariangela Lisanti, Philip Schuster and Natalia Toro) and I have been pushing the experiments to make something like it — where they look for an excess of collisions with one lepton (electron or muon) or anti-lepton, and seven or more jets.  (I am not sure why they chose seven and not six, which I would have thought would have been enough; I have some reading to do.)  This is a very important search because it has the chance to fill a gap in the search strategies which has been around for a while — most (non-Higgs) searches have either been for

  • big signals that produce undetectable particles seen as “missing energy”) along with energetic quarks, gluons or leptons
  • big signals that produce small numbers of particles in special configurations or combinations
  • signals with an exceptional rate that produce tremendous amounts of energy and numerous quarks and gluons

There’s a gap there: signals that aren’t exceptional in rate, don’t produce tremendous amounts of energy or substantial amounts of missing energy, but produce numerous particles.  We pointed this out early last year, and suggest methods to fill the gap, but in my view CMS and ATLAS have been a bit slow to address the issue. And unfortunately, in this latest search, ATLAS yet again insisted on large amounts of missing energy — greater than 180 GeV.  I don’t know why they felt they had to do this, but they did.  So they still haven’t filled the gap.  But nevertheless, this measurement is a step in the right direction.

And guess what?  There’s a small excess in the number of events!   14 where 4 TYPO! 6 (sorry!!) are expected [that’s a crude statement; look at the talk for details].  But the ATLAS speaker warned: don’t get excited.  If there were a real excess from a real signal in 1 lepton plus 7 jets, one would at least expect a slight excess in 1 lepton plus 6 jets.  Instead, there’s a big deficit.  So the observed excess is likely just a statistical fluctuation.  Something to keep an eye on, of course, especially since this search was done with last year’s data, and this year we’ll have about 5 times as much, at higher energy.

Yet maybe ATLAS is looking in the wrong place? Maybe they should go back and look at whether there’s an excess of events when they lower  or eliminate the requirement on the missing energy? Because as far as I know no one has looked carefully at such events to see whether they agree with the Standard Model prediction.

[NOTE ADDED: Not So!  in another talk at HCP,  the following search by CMS was briefly mentioned: http://arxiv.org/pdf/1210.7471v1.pdf .  See in particular Figure 1; this is almost exactly the method that we recommended.  This search needs to be more widely studied and interpreted; it rules out many more variants of many more theories than the ones analyzed in the paper.]

This certainly doesn’t cover everything new that was presented… but it’s all I have time for today.  There’s more to come!

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20 Responses

  1. Thanks for the cool write up, We are also HCP – but we place Healthcare Professionals in Jobs across the US. Occupational Therapy, Infection Prevention, HIM, RN, Speech Therapy, Physical Therapy

  2. Errata: “analogy of Brownian noise” should be “analogy of Higgs field”. Supersymmetry appears broken heavily with extradimensions at the Higgs boson energy density scale. Actually the only particles similar to SUSY models are neutrinos, which are superpartner of photons.

  3. I can explain the nature of this problem illustratively with water surface model of space-time. At the water surface the particles are represented with Russel’s solitons, which are formed with mutual interference of transverse and longitudinal waves in different ratio. Supersymmetry considers the existence of solitons formed in similar way but inverted ratio. Such a particles correspond the Falaco solitons which are formed at the water surface, but in the underwater. But when you observe the smallest density fluctuations, like the Brownian noise (analogy of Brownian noise), then the character of both types of solitons converges mutually and these these solitons don’t differ each other anymore. It results into blow of theories, like the SUSY, which predict the dual version of each solitons at the water surface.

  4. There’s a small excess in the number of events! 14 where 6 are expected [that’s a crude statement; look at the talk for details].

    Which talk was it?

  5. Matt,
    I don’t think I make personal attacks on you, and it wouldn’t even occur to me to ever write anything like “Matt Strassler yet again can’t figure out what scientific concept X means”. I do regularly provide links to what you have to say here when I think it’s interesting. Sometimes I agree with you, sometimes not, but in any case the link is there so that anyone who is interested is encouraged to see for themselves what you have to say, fully, in your own words.

    About SUSY I think I understand your point of view, and roughly would guess that we don’t disagree much on the science, do disagree on the issue of SUSY/string theory zealotry and whether it’s a problem someone should do something about. About either the science or the sociology I think there could be interesting discussions to be had. If you’re not interested, fine. If you are, you need to get over the assumption that people who disagree with you are ignorant.

    1. Oh no :-(((

      why do you always have to pop up here to spoil this nice site and make any reasonable discussion and conversation impossible?
      Just go away and stop trying to spoil this nice site from time to time, you and your distructive scournfulness and unfairness are not welcome here !
      Go back to your site where you can do what you want.

      Prof. Strassler should ban you

      1. … you should not be allowed to talk or write about fundamental physics, particle physics, cosmololgy, and anything related. You neither a clue nor a real honest interest in it, and you dont know anything about the scientific method which Prof. Strassler had explained here repeatedly.

        Just go away

    2. What do you mean by

      “the issue of SUSY/string theory zealotry and whether it’s a problem someone should do something about. About either the science or the sociology …”?

      You should not even be thinking about trying to negatively influence physics and undermining the natural scientific process in the real world in such a manner as you obviously want to do.
      I hope all scientists and people who are in positions to make decisions are warned about you. Nobady is allowed to “do something” that disturbes the natural gain of knowledge in the course of the scientific process!

      As I told you many times elsewhere, you have NO right to patronize physicists and other people who are much more knowledgable and smarter than you are by telling tell them what they are allowed to research, to be interested in, to think about, etc … !!!

      In trying to negatively influence science in the real world you greatly overestimate yourself in a horribly arrogant manner !!!

        1. You are ignorant, Peter — ignorant of the people in the field. You are an anti-supersymmetry zealot; go fight with the supersymmetry zealots, and leave reasonable people alone.

          Not to mention that you make irresponsible personal attacks against me and other people in the field on a regular basis. Worse, you twist what I’ve written to make it sound like I support your point of view, or to make it sound as though I’m — for instance — a supersymmetry zealot.

  6. Can a similar idea as yours apply to jets alone (essentially the same idea except without any leptons)?

      1. Sorry for the vagueness. I’m referring to the idea talked about in your paper “Study of LHC Searches for a Lepton and Many Jets.” If I understand it correctly, it has to do with looking for excesses of events with a lepton and several jets with no missing energy. I was wondering it a similar idea could be applied to purely jet events – whether an excess of events of with around six or more jets could be searched for and if found indicate beyond the SM physics.

        1. OH! got it. [And actually yes this was my idea… whoops!]

          We were sure that you could do this with a rare object + jets, where the rare object could be a lepton, a Z, a photon (or two objects, such as opposite-flavor leptons). But the key to being able to do it is being able to model your backgrounds; in the case of a lepton, you are dominated by top quarks, which you can measure and model; in the case of a Z + jets , you can use a photon + jets, etc. With only jets, it isn’t obvious that a model of the backgrounds can be obtained. Maybe if you demand two b tags you can do it.

          Anyway, I wouldn’t discourage anyone from considering it, but we didn’t want to go into print claiming something that we weren’t confident of.

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