An unexpected bump has the particle physics world busy tonight; (we’re too sedate to be “aflame”). The bump was discovered at Fermilab by the CDF collaboration: Study of multi-muon events produced in p-pbar collisions at sqrt(s)=1.96 TeV”, hep-ex/0810.5357. In short, they’ve discovered a particle that seems to produce jets of leptons.
CDF found that they have way too many events where there are a lot of muons going in the same direction. This sort of thing is called a jet. Normally jets are associated with the strong force, and consequently, they include hadrons as well as leptons. Getting jets without hadrons is very unusual behavior. This is quite exciting but some of the terminology may be a bit confusing in the original paper linked above. There are two types of particles, leptons which do not experience the strong force, and hadrons that do. Physics experiments can distinguish them because hadrons crash into matter and decay, while leptons do not. Leptons eventually end up as electrons and muons. Of these, the electrons are sufficiently light that they get stripped off leaving only the muons. Photons also get absorbed. What’s left is muons and these are detected in the outer parts of a detector. So “punch through” means hadrons that managed to survive all the matter in the inner part of the detector and survived to the part of the detector where muons are supposed to predominate.
Cosmic ray data is pretty much ignored by particle theorists. When I go to conferences, I make sure to attend these lectures because cosmic rays have much greater energies than accelerators can produce, and consequently they are more likely to see new physics. Very few other theoreticians show up at these lectures. Partly for this reason, I haven’t stressed the cosmic ray data very much. But now that the same unusual behavior is being observed at an accelerator, it is time to revisit the cosmic ray data.
A similar set of events were discovered years ago in high energy cosmic ray experiments. They are called “anti-Centauros” (the Centauros are showers that have too many hadrons and not enough leptons, anti-centauros reverse the proportions). Typically, these experiments use photographic emulsion (film) to detect cosmic rays. The film is layered in between sheets of lead or air gaps. The lead breaks up the hadrons and the resulting showers are detected in the film. These events were called “Centauros”, see the article which discusses them: Are Centauros exotic signals of the QGP? by Ewa Gladysz-Dziadus (2001). A more recent update is Very High Energy Cosmic Rays and Their Interactions, Ralph Engel (2005).
The study of cosmic rays is largely ignored, other than the GZK measurement, but the emulsion cosmic ray researchers are still chasing after Centauros. CASTOR stands for “Centauro and Strange Object Research” and is the name for a calorimeter (to measure energy in particle tracks) at the Large Hadron Collider (LHC).
Cosmic ray experimentalists don’t get no respect. Consequently these observations are difficult to publish and when they are published, they are pretty much ignored. But unusual observations in cosmic rays have been piling up for years. Kopenkin and Fujimoto supposedly explained Centauros in 2006: Exotic models are no longer required to explain the Centauro events, but this hasn’t stopped the observations from lacking explanations.