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.