CDF publishes multi-muons!!!!

NB: This post is aimed at physicists.. However if you are not one, but you are really curious, you might find out that for once the annoying feeling of reading cryptic jargon is paid back by some real news!

I guess the most important message of the post you are about to read is: Do not check the arxiv today if you really cannot spend a couple of hours reading. Make it three. The  paper just released by CDF, titled “Study of multi-muon events produced in ppbar collisions at sqrt(s)=1.96 TeV” is guaranteed to have you fastened to the chair until you are done with its 70 pages.

The article reports on a very careful investigation produced by CDF, using Run II data collected by a trigger selecting events with two (or more) muons of low transverse energy. The study addresses two or three long-standing inconsistencies in measurements of bottom quark production and phenomenology at the Tevatron:

• the cross section for production appears in good agreement with next-to-leading order QCD predictions when b-quarks are tagged by a reconstruction of their decay vertex, while it is found to be significantly larger when the cross section is measured by identifying b-quarks through their semileptonic decay;

• the invariant mass spectrum of pairs of leptons produced in sequential semileptonic decays () is not well modeled by the simulation of b-flavored hadrons in CDF;

• the value of the time-integrated mixing probability of b flavoured hadrons is measured at the Tevatron to be significantly larger than that measured by LEP experiments.

The source of these apparently unrelated inconsistencies is traced back by the study to a sample of events where muons are originated several centimeters away from the primary interaction point (the proton-antiproton collision vertex), which makes b-quark decay as implausible a source as any other Standard Model process, no better than the other backgrounds which the study shows to be insufficient explanation for the observed events: punch-through from pions and kaons, or secondary hadronic interactions in the detector material.

Once a large sample of such weird events are statistically isolated -better say evidenced- in the sample, a further anomaly is found in the number of additional muons contained in narrow cones around the original ones, something which cannot be easily explained with conventional physics. The paper discusses the characteristics of these events, without falling in the trap of putting together an exotic explanation. Instead, what is made clear in the paper is that those measurements quoted above -lepton-based cross sections and phenomenology of b-quarks studied in high-energy hadron collisions- are affected by the findings described in this paper.

Below I offer two plots extracted from the preprint. The first one shows the impact parameter distribution of muons in the events constituting the anomalous signal (black points), compared to the impact parameter of muons attributable to QCD sources (in red). The impact parameter resolution for these tracks is 2.5 times smaller than the bin size. One observes a abnormal tail of muons with very large impact parameter. I recall that the impact parameter, which is measured in the plane transverse to the beam direction, is the distance of closest approach of the backward extrapolation of the track to the primary interaction vertex. A impact parameter of one centimeter is huge, given that the typical decay length of a B meson is of the order of a pair of millimeters.

On the right you see an exponential fit to the impact parameter distribution of the trigger muons for the anomalous events, for events with just two (top) or more than two (bottom) muons inside two narrow cones around the trigger muons. The distribution agrees with the decay of a particle with a lifetime in the 20 picosecond range.

To quote the paper, the first lines of the Introduction offer a quite clear picture of the situation:

“This article presents the study of events, acquired with a dedicated dimuon trigger, that we are currently unable to fully explain with our understanding of the CDF II detector, trigger, and event reconstruction. We are continuing detailed studies with a longer timescale for completion, but we present here our current findings.”

The conclusions are also clear, but I will leave them to those of you who want to read a paper which might, just might, constitute the first evidence of physics beyond the Standard Model, ever.

That said, if you have read this blog long enough, you know me for a tough sceptic. I of course would be simply delighted if the CDF signal of multi-muons really were a first evidence for new physics; but I have to play the devil’s advocate, and so one word of caution, make it five paragraphs, is mandatory. Of course, despite the evidence is pretty solid from a statistical standpoint, one must lean back and take a breath. We are discussing the possibility that something really spectacular has just lurked out of CDF data. Extraordinary claims require extraordinary evidence, and once statistical evidence is plain, one must delve with systematics. CDF did, and they have not found any significant source which might account for the effect. But investigations should and will continue.

Is CDF sure about the impossibility of explaining this effect away ? No, CDF does not exclude that possibility, although it is my opinion that the collaboration has reviewed the paper with more care and detail than most of the other papers it has published in its illustrious, 25-year-long life. That means nothing in terms of the likelihood that this result is indeed new physics. It just says we are as sure as we can be that we cannot presently explain it with known sources. Also worth mentioning is that CDF is a really disciplined collaboration, which has really been careful with their claims so far. And the present paper is no exception.

Is there a simple New Physics explanation of the observed effect ? No, as far as I understand no existing model of new physics predicted such a signature in advance, although one must acknowledge that a few ideas exist in the literature which might have a connection with the effect, if proven real. However, there is a paper discussing a similar signature, which probably benefitted from knowing the CDF result in advance from an internal source. I will leave this issue to another time and another place.

Can CDF find more evidence in the near future ? Yes, the analysis of electron events may shed more light on the matter, and although electrons are harder to isolate than muons when they have a low energy, the analysis will be carried out.

Can D0 find a similar signal ? Surely. D0 is a similar detector to CDF, and although their charged particle tracking is slightly inferior to CDF’s, their muon system is more extended, and their silicon detector is also at least as good as that of CDF (ok, even slightly better). The problem with D0, I think, is the time it will take to perform such a complicated analysis. One must not forget that before focusing on these anomalous events, CDF produced a lengthy investigation of the correlated cross section, which is the back-bone of the multi-muon analysis, since it demonstrates the understanding of heavy flavors in low-transverse-momentum lepton samples in CDF when particles with large impact parameter are excluded. So, it may take a while to D0 to confirm or disprove the effect CDF is now publishing.

Does the signal hint at other anomalies in different analyses ? That, I am sorry to say, is anybody’s guess. If the multi-muon events are a signal of new physics, then I am sure there is something else to be found, somewhere. The problem is: what is that ? One might be tempted to speculate that data samples collected in past experiments could in principle contain a similar signature: charged tracks with very large impact parameter have been seldom studied at colliders, and tracking algorithms might have purposely discarded those tracks, or could be proven inefficient in their collection. For instance, CDF does collect, with its fantastic SVT trigger, events containing tracks showing a significant impact parameter. However, the efficiency with which the SVT collects those events, if studied as a function of impact parameter, dies out much too soon. Hell, nobody designs a detector aimed at collecting a new physics signature no theorists have thought about!

I imagine hordes of theoretical physicists canceling flights, conferences, and courses today, making room for some serious thinking in their agendas. Good luck!

UPDATE: see the interesting discussion developing at Peter Woit’s site, where he points out a paper by Arkani-Hamed and collaborators which appears quite extraordinarily to have foreseen the above signature of new physics, in a very timely fashion!

UPDATE: there are other bloggers who’ve discussed this. Lubos, Carl, Matti (happy birthday Matti). Others to be added soon…

UPDATE: other excellent, entertaining bloggers have added their own comments to the story: Jester, Chad, theorema egregium. In italian: Marco. In dutch: astroblogs.

UPDATE: John, a fellow collaborator in CDF and one very skilled physicist, explains the result at cosmic variance.