Lepton Jets and JACEE’s Anti Centauros

The recently announced results by the CDF Collaboration, Study of multi-muon events produced in p-pbar collisions at sqrt(s)=1.96 TeV has been seen before, by the cosmic ray emulsion experiment JACEE. Since few particle physicists follow cosmic ray stuff, I thought I’d describe the JACEE results in more detail.

Some JACEE papers are given at the University of Washington’s Particle Astrophysics Group Webpage. See Excessive production of electron pairs in low multiplicity interactions or Observation of early photon conversions in high-energy cosmic-ray interactions. But my favorite arXiv paper for a long description of emulsion cosmic ray experimental results is Are Centauros exotic signals of the QGP?, Ewa Gladysz-Dziadus, 150pp, (2001).

A “Centauro” is an event with an excess of hadrons relative to electromagnetics. The opposite event, the “anti-Centauro” is an excess of electromagnetics (muons) over hadrons. (For non particle people, I’ll put an explanation of what “electromagnetics” has to do with “muons” below the fold.)

For the particle people, it’s best to just quote the paper. This is section “3.3 JACEE Experiment”, around page 63:

The Japanese-American Cooperative Emulsion Chamber Experiment, JACEE, has flown emulsion chambers with baloons near the top of the atmosphere. Despite of a small area and short time of exposure, as compared to Chacaltaya/Pamir Experiment, a few events of anomalous γ/charged ratio have been observed by JACEE Collaboration. However, these events differ in some essential points from classical Centauros. The anomalies were noticed at incident energies lower than that estimated for “classical” Centauros and unusual γ/charged ratios were observed only in the limited (η − φ) phase space region. Besides that, an excess of photons (anti–Centauro), in contrary to the hadron excess observed in Centauros, was claimed. The examples are:

1.) 4L-II-27 event [74] of incident energy of 80 TeV, yielded 149 charged particles and 120 γ’s. Almost all γ–quanta were produced in a narrow jet in the extreme forward direction. The γ/charged ratio is 2.6 ± 1.1 in the region of pseudorapidity 5.5 ≤ η ≤ 7.5, what is a significant deviation from the expected ratio of ∼ 1. The analysis presented in [74] showed an anomaly at the 5-10% level among 41 studied events with E0 ≥ 40 TeV.

2.) The event, with ΣE = 15.4 TeV, described in [75, 76] was initiated by a singly charged primary. The collision occurred within the detector. Almost all leading particles were γ–quanta. Photons appear to cluster into two groups. The leading cluster consisted of about 32 γ’s with hpT i ≃ 200 MeV and only one accompanying charged particle. A possibly distinct cluster had three times as many photons as charged hadrons (about 54 photons versus 17 charged). This event is one out of a sample of about 70.

3.) The event presented in [77] is a peripheral collision of Fe nucleus (E ≃ 9 TeV/nucleon) in emulsion. There were found 27 γ–quanta with η ≤ 6. As they came from pair conversions at only 0.8 radiation lengths, one can expect that the total number of photons was about 50. At the same time, only 6 charged particles (out of 21 charged tracks detected in the whole angular region) falled in the same kinematical range.

In all these events there was observed a tendency to a group emission of \pi^0 mesons. Such \pi^0 groups, having similar directions and momenta, could be signs of a formation and a subsequent decay of the chiral condensates. It should be mentioned, however, that these events were found in emulsion by scanning for the leading photon showers, so there was a “trigger bias” in favour of a large neutral fraction. It would be interesting to hear something about anti-Centauros from the mountain-top emulsion chambers. Here, there is, however, even much more stronger “trigger bias” in favour of gamma families, and thus the interpretation of data, from this point of view, is a complicated exercise. It is rather difficult to identify anti–Centauros unambigously, with exception of unusual and rare events in which the interaction vertex is close to the top and clearly resolved in the chamber.

How do they detect Muons at CDF?

In order to be seen in the far outer portions of an accelerator detector, a particle must have the following characteristics:
(a) It must be charged, so it leaves a track,
(b) it must also be heavy, so it can penetrate this far without being curved into a little helix by the magnetic field,
(c) it must also not feel the strong force, so it does not collide with the inner part of the detector,
(d) it must also have a long lifetime, otherwise it will decay before it gets that far.

The only particle that can do all four of the above is the muon $\mu^{\pm}$. This is the middle weight of the three leptons. The electron is too light, it misses requirement (b). The tau is heavier but it decays quickly, missing requirement (d). The various mesons and hadrons suffer the strong force and miss requirement (c). And finally, things like neutrinos are not charged, they escape the detector with no trace at all (most of ’em do anyway). So the outer parts of an accelerator detector are designed to measure muons. Here is CDF’s schematic showing the four layers of their detector:

So a short way of describing the situation is that muons are the last man left standing. They are easy to detect for this reason.

Where are the Electrons?

Most of the muons seen in the accelerator are expected to come from the semileptonic decay of hadrons. These are situations where a quark changes its type by giving off a W+ or W-, and the W+/- decays into a neutrino and a charged lepton. The charged lepton can be an electron, a muon, or a tau.

Of the three leptons produced in a semileptonic decay, the tau is likely to decay into a muon and a neutrino and anti-neutrino pair. So only the decay into an electron does not result in a muon hit in the outer section of the detector. Now the new paper from CDF manages to avoid mentioning the word “electon” at all. It would be bizarre to have a process that produced muons without producing electrons and tau as well. So why no mention of electrons?

As you can see from the above chart, the CDF experiment expects electrons and photons to be absorbed in the electromagnetic calorimeter. Their paper does mention “electromagnetic” but only in the context of noting that the muon detectors are outside of this part of the detector: “The central muon detector (CMU) is located around the central electromagnetic and hadronic calorimeters, which have a thickness of 5.5 interaction lengths at normal incidence.”

The regions of the detector where electrons would be detected are also traversed by all the hadrons. Consequently these are difficult regions for track reconstruction. I assume that electrons and the electromagnetics have not been discussed because this simplifies the paper. And I expect that they will be discussed in later papers.

By contrast, in the cosmic ray emulsion data, electrons are easy to identify. So the people working in that area concentrate on electron counting. In either case, electrons or muons, these are leptons and we expect them to be produced in the same process. So the emulsion data for “anti-Centauro” events are compatible with the new “ghost” particles seen by CDF.

1 Comment

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One response to “Lepton Jets and JACEE’s Anti Centauros

  1. carlbrannen

    Also see The Role of the VHE (Very High Energy) Muons in Explanation of Unusual Events Observed in Cosmic Rays by Bogdanov, Petrukhin, and Shalabaeva (2005).

    “Unusual events observed in cosmic-ray experiments, which cannot be explained in frame of modern theories and models, are considered. The peculiarities of VHE ( 100 TeV) muon interactions and their possible contribution to production of various unusual events in cosmic rays are analyzed. Some preliminary results obtained for explanation of unusual events detected in Tien-Shan calorimeter and in the Pamir experiment are discussed.”

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