Discovery of a New Particle?

Except for the excitement surrounding first beams in the LHC, particle physics has been an all-too-quiet subject recently. It looks like that may be about to change, with a dramatic new result announced by the CDF experiment this evening, in a preprint entitled Study of multi-muon events produced in p-pbar collisions at sqrt(s)=1.96 TeV.

The CDF result originates in studies designed to determine the b-bbar cross-section by looking for events where a b-bbar pair is produced, each component of the pair decaying into a muon. The b-quark lifetime is of order a picosecond, so b-quarks travel a millimeter or so before decaying. The tracks from these decays can be reconstructed using the inner silicon detectors surrounding the beam-pipe, which has a radius of 1.5 cm. They can be characterized by their “impact parameter”, the closest distance between the extrapolated track and the primary interaction vertex, in the plane transverse to the beam.

If one looks at events where the b-quark vertices are directly reconstructed, fitting a secondary vertex, the cross-section for b-bbar production comes out about as expected. On the other hand, if one just tries to identify b-quarks by their semi-leptonic decays, one gets a value for the b-bbar cross-section that is too large by a factor of two. In the second case, presumably there is some background being misidentified as b-bbar production.

The new result is based on a study of this background using a sample of events containing two muons, varying the tightness of the requirements on observed tracks in the layers of the silicon detector. The background being searched for should appear as the requirements are loosened. It turns out that such events seem to contain an anomalous component with unexpected properties that disagree with those of the known possible sources of background. The number of these anomalous events is large (tens of thousands), so this cannot just be a statistical fluctuation.

One of the anomalous properties of these events is that they contain tracks with large impact parameters, of order a centimeter rather than the hundreds of microns characteristic of b-quark decays. Fitting this tail by an exponential, one gets what one would expect to see from the decay of a new, unknown particle with a lifetime of about 20 picoseconds. These events have further unusual properties, including an anomalously high number of additional muons in small angular cones about the primary ones.

The exciting possibility here is that a new, relatively long-lived particle has been observed, one that decays in some way that leads to a lot more muons than one gets from Standard Model states. It should be remembered though that this is an extraordinary claim requiring extraordinary evidence, and the possibility remains that this is some sort of background or detector effect that the CDF physicists have missed. It should also be made clear that this paper is not a claim by CDF to have discovered a new particle, rather it is written up as a description of the anomalies they have found, leaving open the possibility that these come from some standard model processes or detector characteristics that they do not yet understand.

The overwhelming success of the Standard Model during the past 30 years has meant that essentially all claims from accelerator experiments to see some new, non-SM physics have turned out to be mistaken. As a result, collaborations like CDF are now extremely careful about making such claims and will only do so after the most rigorous possible review. It’s a remarkable event that this one has gotten out, signed off on by the entire collaboration (although from what I understand, people can drop their names from the publication list of a specific paper if they disagree with it, maybe one should check this author list carefully…).

What would really be convincing would be a confirmation of this from D0, the other Tevatron detector. The D0 collaboration would not only be working with a detector that has somewhat different characteristics, but would also have some motivation to find a problem with the result from their competition. If they also see it, that would be pretty extraordinary evidence. Another sort of extraordinary evidence would be to see evidence for the same kind of new particle in other channels.

This will undoubtedly unleash a flood of papers from theorists promoting models that extend the Standard Model in ways that would produce something with the observed experimental signature. This is not a signature characteristic of supersymmetry or any of the other known heavily-studied classes of models. If real, as far as I’m aware it’s something genuinely unexpected. Perhaps phenomenology experts can point to some less well-known models with this kind of signature. The only such thing I’m aware of is a very recent paper from three weeks ago by Arkani-Hamed and Weiner, entitled LHC Signals for a SuperUnified Theory of Dark Matter. They discuss a theory of dark matter involving a new hidden gauge symmetry, broken near the GeV scale, saying that this is “motivated directly by striking Data from the PAMELA and ATIC collaborations”. In these models there can be Gev-scale Higgs and gauge particles decaying to an anomalously large number of leptons. They discuss the question of whether the parameters of such models can be adjusted to give large decay lengths, and predict the observation of events that “contain at least two “lepton jets”: collections of n > 2 leptons, with small angular separations and GeV scale invariant masses”, pretty much just what CDF sees . Since the CDF paper undoubtedly has been the topic of intense discussion among the 450 or so physicists in the collaboration for many months now, the most likely explanation for the appearance of a new theory paper a few weeks ago discussing exactly the signatures in question is that news of what’s in the paper got out to some theorists early. Even if this particular result goes away, this gives some indication of what sorts of things are likely to happen once LHC data starts being collected and analyzed.

The bottom line though is that for the first time in quite a while, there’s some very exciting and potentially revolutionary news in particle physics. It’s coming not out of the LHC, which is still a hope for the future, but from a currently functioning machine which is producing more data every day. If this result holds up, this data contains a wealth of information about some new physics which will likely revolutionize our understanding of elementary particle physics. Particle physics may already have started to move out of its doldrums.

Update: This evening I came across an unexpected source of information about this, one which might explain why news of this result may have leaked out a while ago. More about this later.

Update: The results from PAMELA mentioned here, and which are listed as motivation for the Arkani-Hamed/Weiner paper, are now officially out. For discussion of this from someone much better informed than me, see this posting at Resonaances.

Update: There’s an excellent detailed posting about the paper from CDF’s own Tommaso Dorigo. If you’re interested in understanding exactly what is going on here, that’s where you should start.

Update: For entertainment, there’s always Lubos.

Update: I’m now free to explain what I was alluding to when I earlier mentioned an “unexpected source of information”. Yesterday evening while I was trying to find out more about the CDF result, its relation to previously published experimental results, and to possible phenomenological models, I was running some Google searches on relevant terms. One of these turned up something very surprising: the first result was a summary of CDF’s internal review of drafts of PRL and PRD papers on the subject, the second was the PRL draft. Both of these documents were from early July, and part of a publicly accessible directory containing all the materials from a review of the draft at that time. Investigating further, it became clear that the CDF web-server was seriously misconfigured, allowing directory listings and public access to a wide array of their work materials.

I wrote an e-mail to people at CDF warning them about the problem, heard back quickly, and have just checked that they have fixed it, so I don’t think I’ll cause them a problem other than embarassment by telling this story here. It seems likely that these materials have been publicly accessible and indexed by Google probably for several months now.

I confess to reading these documents, figuring that anything really interesting that is Googleable is fair game. Since they clearly were not intended for public consumption I won’t disseminate information about them beyond this story about their existence and the fact that the PRL draft contained tentative material interpreting the data in terms of new physics, the sort of thing the released paper avoids. One thing I can say is that it is very impressive to see the amount of effort and very serious scientific work behind a review of this kind. A lot went into this, and presumably a lot more has gone into it since last July. I understand why CDF does not make this kind of thing public, but it actually would be a wonderful example for the public of how science is done to do so.

Update: See Resonaances for a discussion of the dark sector model building.

Update: Another CDF blogger heard from: John Conway.

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42 Responses to Discovery of a New Particle?

  1. Kea says:

    Heh, heh, heh!!!

    And wow, Peter. Such a long blog post so soon after the paper appearing. One might think you knew this was coming.

  2. Peter Woit says:


    There had been some chatter on the intertubes indicating that it might be a good idea to check the arXiv this evening….

  3. Amos says:

    If this pans out, today is truly a day of joy.

  4. Neal Weiner says:

    I can tell you officially we had no word on this. This blog is, in fact, the first I’d heard of it. (But have now looked at the paper.) What we predicted was just what was needed to explain PAMELA we thought.

  5. Neal,

    that is pretty hard to digest. Lepton jets with lifetimes. Come on. I think you owe it to the physics community to let us know where the leak came from.


  6. Pingback: CDF publishes multi-muons!!!! « A Quantum Diaries Survivor

  7. Daniel de França MTd2 says:

    Halloween Particle!

  8. Daniel de França MTd2 says:

    “This is not a signature characteristic of supersymmetry or any of the other known heavily-studied classes of models.”

    But Arkani-Hamed/Weiner uses a SUSY model to explain that, in fact, with 2 aspects of a certain SUSY, which he calls, SUSY(I) AND SUSY(II). Check page 3.

  9. Daniel de França MTd2 says:

    Maybe it’s this other Arkani-Hamed’s/Wiener article that you are thinking of: . It was published in that same day you cited before, and it doesnt use SUSY.

  10. onymous says:

    Some comments:

    On a sociological note, although the paper doesn’t specify which people within CDF did the analysis, my understanding is that it is a group which has some history of finding difficult-to-understand anomalies in the data. Tommaso has documented some of these on his blog (search for “superjets”); they are an interesting story, which one might take into account when deciding what to think of this.

    In Nima and Neal’s model, as I understand it, one only expects lepton jets in events with SUSY, which you would think would stand out in some other way.

    My take on this is that it’s probably nothing. Maybe punchthrough of hadrons into the muon detector. They claim to analyze all these things and quantify them, and I haven’t digested the 70 pages, but these sound like very difficult problems, and I wouldn’t bet on them being fully understood. If you want exciting things in data right now, dark matter is where it’s at.

    (And dark matter / positron excesses is clearly what motivated the “lepton jets”; it’s offensive for Tommaso to publicly imply otherwise.)

    In short: calm down!

    — A phenomenologist who would love to see new physics and thinks this isn’t it

  11. Nothing new to me but two excellent birthday gifts at my birthday October 30;-).

    TGD has predicted the existence of colored excitations of leptons explaining CDF anomaly already for fifteen years ago.

    One of the basic predictions of TGD indeed is that leptons should have colored excitations. Already at seventeens a lot of evidence for colored electrons, or rather their pion like bound states, came from anomalous production of electron positron pairs in heavy ion collisions. For some mysterious reason it was put under the carpet. I have tried to tell about this in blogs but in vain.

    For year ago evidence for muo-pions came. Again it was forgotten although Lubos saw the trouble of ridiculing the experimenters.

    CDF gives evidence for tau-pion. The lifetime predicted for charged tau-pion obtained by scaling the prediction for pion life-time is correct if one scales down the parameter x in the parameter f(pi)= xm(pi) characterizing pion coupling to axial current by factor .41. To my opinion the case is now closed.

    The positron and electron positron cosmic ray anomalies can in turn be seen as evidence for M_89 copy of hadron physics.

    See my blog.

  12. Dear ominous (same as the one who left a comment in my blog a while ago, I guess):

    punch through is a possibility, but it has been investigated in depth, by people not exactly enthusiastic about the signal, and found inconsistent with the signal, its size, and characteristics. I would rather lend towards secondary nuclear interactions instead, but it is a tough call. The fact that there is a chance – what, 1% ? 0.5%? – that it indeed constitutes new physics, is extraordinary to me nonetheless.

    As for the theorists and their claims: we are adults and we know physics. We have read thousands of papers and published hundreds. We know the history of HEP. We know how people chats in the corridors. And I personally know that in this particular case some people in CDF have been less careful than they should have with this paper before publication.

    On the other side, there is this pair of papers coming out of the blue, which casually get published a few months after the CDF analysis emerges from the authors’ computers and is in internal review, and less than a month before that analysis sees the light in the arxiv. These two phenomenological -but I would rather call them phenomenal- papers discuss a signature almost never heard of before. It entails lepton jets. Many leptons, with small invariant masses. With long lifetimes.

    I say, you can believe in SUSY or you can believe in the Yeti, or whatever you want. I can believe a lot of things, but I do not buy that it is their own cooking. Especially since the paper is rather handwaving in tone and conclusions.

    So, to summarize: it may sound offensive to be inquisitive with respectable theorists about where the hell they got that idea, but it is no less offensive to act nonchalant and pretend we buy this incredible coincidence without a detailed explanation.

    Finally, about the authors of the analysis: be very careful. They are among the best physicists I know in CDF, and I know all of them. They have the vice of searching for the unknown rather than being happy with measuring something with twice better accuracy than others. I would like it if there were more like them around.


  13. woit says:


    The Arkani-Hamed/Weiner paper doesn’t use SUSY in any crucial way. Their proposal is that the observed signature comes from a “Dark Sector”, this can be embedded in a SUSY model, but that’s not necessary. From their paper:

    “While our discussion is framed within the context of low-energy SUSY, some of the conclusions hold in a wider class of theories for new physics. The signals associated with decays into the dark sector and back follow in any theory with a particle charged under the SM that is nonetheless stable in the absence of a small coupling to the Dark Sector, while the new colored states should be expected in any picture in which the Dark Matter is charged under the Standard Model and gauge coupling unification is taken seriously.”

  14. Pingback: News from the CDF and PAMELA experiments « Theorema Egregium

  15. Daniel de França MTd2 says:


    The article with Moose tries to argue for SUSY, since this new Sommerfeld force must be unified with a larger picture, that is, the SM through SUSY. What the other article, does the thing you want, that is, specificaly describing that phenomeny, but without saying that SUSY has anything to do with that, or trying to embed that phenomena in any bigger scheme…

  16. andy.s says:

    Didn’t Willis Lamb suggest that anyone discovering a new particle be punished with a $10,000 fine?

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  18. Yatima says:

    >> Didn’t Willis Lamb suggest that anyone discovering a new particle be punished with a $10,000 fine?

    That was before the quark model and QCD, right? These days, discovering new particles is what decade-in-building billion-dollar machines are all about.

    I applaud this New Kind of October Surprise. Here’s hoping something will come out of it.

  19. L. Carpenter says:

    This all seems very iffy, howver it still doesn’t rule out SUSY,
    You know, SUSY with R parity violation has displaced vertices, as per Dreiner, Grab, Banks, Carpenter, Kaplan, Rhee and probably many more etc and so on, you just make the coupling small enough and the intermediate sparticle heavy enough. However I’d bet that this might go away.

  20. “I would bet than this might go away”

    Lol… This is rather careful, isn’t it ? Of course odds are against the multi-muon signal being new physics! Would you bet 500 to 1 ? That would be a statement. For me, a >0.1% chance that this is new physics is a HUGE chance.


  21. Covariant says:

    I’m just wondering where the boundary between new and old physics is defined. When do we know enough of the old physics to know its new physics?

    This reminds of long range artillery in WWI, when for the first time the coriolis effect had to be taken into account in calculating trajectories (confirming the Earth was spinning).

    Do we know all the environmental effects well enough?

  22. mike says:

    It’s certainly wise not to call it a new particle until it’s been peer reviewed, but it’s also wise not to dismiss it immediately as a detector problem. It would be good to see the D0 provide some insight into the problem. Wouldn’t they have characterized the Si detectors in CDF pretty thoroughly by now?

  23. chickenbreeder says:

    Covariant: Foucault’s Pendulum (cir. 1850) was already definitive experimental confirmation of Earth rotation.

    A question from a casual reader: Could all this hope and positive outlook on the new particle be the result of desperation, given that there has been no breakthrough in high energy particle physics in decades? Also, was the experiment specifically designed with the expectation of finding a particle of this characteristic, or is it serendipity (assuming that there’s indeed a new particle in this story)?

  24. Peter Woit says:


    By “new physics” what is meant is something not in the Standard Model. One possibility is that what is being seen is due to some poorly understood effect of the Standard Model, e.g that this might somehow be just another bound state of quarks.


    The paper has undergone very extensive review at CDF, I doubt that a referee will find problems not identified in their internal review, but it’s not impossible. They are careful in the paper not to claim discovery of a new particle, just to be putting forward an anomaly in the data that is not understood, with a new particle one possible explanation. The behavior of these detectors and the data analysis are extremely complex, there’s a lot that can go wrong. It will be very interesting to see what D0 finds when they look for this.


    The lack of new discoveries in particle physics certainly makes any anomaly get a lot more attention. The detector was not designed to look for this.

  25. Luboš Motl says:

    “This is not a signature characteristic of supersymmetry.”

    That’s quite a breathtaking statement. The whole paper by Arkani-Hamed and Weiner is based on supersymmetry. There would be no lepton jets without it. The acronym LSP – lightest supersymmetric particle – appears 45+ times in their paper.

    They obtained the model by taking the model by these 2 and 2 other authors meant to match the PAMELA, ATIC, DAMA, INTEGRAL observations. Because the latter requires light scalars, SUSY is needed for naturalness. So they added it. Then it follows that there are several LSP-like particles and the lepton jets come from them. The model without supersymmetry cannot predict any lepton jets (although all models can be fine-tuned to get one particular prediction right, but others are likely to fail then).

    If this explanation were right, it would be a nearly complete experimental proof of SUSY as well as several other characteristically string-theoretical signatures, including a hidden gauge theory sector and a quiver spectrum of matter. It is likely that some compactifications of string theory predict exactly this thing and they will be located soon, leading to new predictions. This scenario was just unrealized by everyone because everyone kind of thought that all hidden sector particles have to be heavy.

    It would become the most direct proof in decades that the notion that these concepts are “untestable” or untrue or “not even wrong” and blah blah blah is just a gigantic and very stupid lie, indeed.

  26. Covariant says:


    I appreciate the answer. I think that was what I was unknowingly alluding to.


    I was cognizant of that fact (hence the word confirm and not proof). I think my intent was misunderstood.

    There is still a lot about our little blob of iron and mud that we are unsure about 🙂

  27. Peter Woit says:


    I think you missed this:

    Your claim on your blog that this is “An experimental proof of most low-energy signatures of string theory”, and speculation that Arkani-Hamed and Weiner will share a Nobel prize for this are among the funnier things I’ve ever seen on a physics blog.

    By the way, have you or anyone else asked Arkani-Hamed whether he knew of the CDF result before it was published?

  28. H-I-G-G-S says:


    If you buy into the string landscape explanation of the cosmological constant, as you seem to, then complaints about fine-tuning and appeals to naturalness carry very little weight. Without these there is no reason you can’t have a model with the main features of the Arkani-Hamed and Weiner model but without supersymmetry. Claiming this result, if it holds up, as nearly complete experimental proof of SUSY is absurd.


  29. Shantanu says:

    Peter, did I read correctly on cosmicvariance that one third of CDF collaboration have dropped their name off the paper? Do you
    know the reason?

  30. Esornep says:

    “Peter, did I read correctly on cosmicvariance that one third of CDF collaboration have dropped their name off the paper? ”

    You did. And what makes this utterly astounding is that the paper makes essentially no claims whatever to have found anything! All they say is, “we saw this weird signal in our data. Make of it what you will.” Or could it be that the recalcitrant 1/3 are protesting at this extreme wishy-washiness? 🙂

  31. Esornep says:

    BTW, Arkani-Hamed and Weiner have rushed out a second version of their paper. It raises many question marks. And I mean that literally!

  32. onymous says:

    What a circus! Anyone want to place bets on how long before we see the first “unparticle” explanation appear on hep-ph?

  33. Pingback: Not Even Wrong » Blog Archive » The Circus Begins

  34. witness says:

    to Esornep:
    The exact reasons for not signing vary but most range from “CDF can do much better in investigating this” to “this is clearly correlated hadronic punch-through of jets”.

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  36. Axolotl says:

    Latest from Arkani-Hamed at Dorigo’s blog: it’s lies, all lies, and anyway our model does *not* predict anything like what has been seen. Lubos Motl’s denunciation of NAH as a crackpot is expected momentarily.

  37. Covariant says:

    I think NAH is deserving of some sympathy.

    I am slowly making sense of the CDF paper, and being ignorant of how Tevatron runs are scheduled, I was wondering if CDF has done any time analysis on the “jets” to see if there was any variation based on time of day.

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  39. To whom may be concerned:

    the fact that the CDF paper carries only two thirds of the usual author names is due to the particular publication process this paper withstood. The authors wanted to see it out soon, because of the possibility (quite real, in fact, if you have read Peter’s update above) that it might leak out and be scooped by a quick parallel analysis by D0 (unlikely, given that D0 is under-manned these days, and the bounty of data on which to concentrate for several other important measurements so large). Some colleagues in CDF felt they wanted more analysis, and they decided not to sign. This is still a CDF publication, case closed.

    Let me add that CDF has published over 400 papers in its illustrious, 25-year-long career, and that we are publishing many, many other important measurements on a weekly basis. So, please do not concentrate too much on this one paper, which, I am sure, will be seen in perspective in a few months, one of two very different perspectives indeed. If electrons do not confirm, and D0 disproves the multi-muon signal, this will be just a honest attempt to explain to the scientific community what we saw and what we did not understand of muons in our data -which has some impact in past measurements. If there are confirmations, then the story is different of course, and those 1/3 of authors who took their name off will reconsider the matter for the future publications on the same topic.


  40. Pingback: CDF multi-muons, hmm « Charm &c.

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