## Thursday, January 04, 2007

### Centauro events and small black holes

As a reader has pointed out, sometimes it seems that the physicists have discovered some really strange things. Here is one of them:

It is called a Centauro event. It is so strange that you can't even say whether it is attractive or not. Or at least I can't. Whatever the answer is, the Centauro events have been occasionally observed by the physicists who study the cosmic rays since 1972.

How do the normal collisions of high-energy cosmic rays look like?
• The cosmic particle collides with a particle in the upper atmosphere
• A very small number of partons with high transverse momenta leave the interaction region
• What is left is soft stuff, mainly pions with the 33:33:33 democratic composition of the three differently charged editions
• These particles decay to photons and hadrons, roughly in the 50:50 ratio, and each of the particles has less than 1 GeV of transverse momentum or so: the final photons in the shower have even less

OK, so you can guess what these bizarre Centauro events are going to look like:
• The collision appears in the lower atmosphere, as low as 500 meters above the ground, although you may doubt the testimonies: the initial particle seems to be highly penetrable
• There seem to be many partons with a high transverse momentum
• The produced particles are dominated by hadrons while photons are a minority
• The differently charged pions occur very asymmetrically, suggesting a big violation of the isospin SU(2) symmetry
• The transverse momentum is much more than 1 GeV for the products
Moreover, the lab energy for these events is typically above 500 TeV. Below a certain threshold, the events don't occur. Many people believe that these Centauro events are a result of a sloppy work of the experimenters. Others think about the possibility that they are real.

People have proposed various explanations. In 1979, James Bjorken and Larry McLerran (yes, the Pinocchio Gentleman) used the words "explosive quark matter" to describe what they saw inside the Centauro events. In 1994, Pratt and Zelevinsky proposed that pions were produced in the isospin singlet channel in order to describe the asymmetry. People have proposed to look for Centauros in Brookhaven as well as the LHC, via CASTOR, as early as in 1997. At the LHC, the Centauros should be studied together with strangelets. In fact, a quark-gluon-plasma-strangelet mechanism was proposed to explain the Centauro events, too.

However, we live in the 21st century so we should have a sexier explanation, shouldn't we? ;-) Yes, it is based on
in a braneworld. You can try to go through the papers. There are quite many of them that propose and analyze this truly revolutionary explanation of the horse woman we started with. How do the black holes help to explain the weird features of the Centauro events?

First of all, we need a lot of stuff with a high transverse momentum. A lore - or a fact - is that the more transverse momentum you find, the more new physics occurred in the experiment. Old, QCD-like phenomena tend to have a small transverse momentum. The collisions don't change the direction of the particles much. However, a big transverse momentum means a hard collision, a short-distance core, a powerful explosion, or a new unstable particle that had to live before they voted against it.

The high transverse energy is similar to the good luck that allowed Rutherford to discover the atomic nuclei and Bjorken to discover the quarks in the deep inelastic scattering data. The black holes are maximally explosive and they like to emit energy in transverse directions as much as they do in all other directions.

The story behind these hypothetical black hole events is that a high-energy neutrino with 1,000-10,000 TeV of energy or more collides a 1 GeV hadron and creates a 1 TeV or more energy in the center of mass which is the lower bound for the black hole masses in the most radical but acceptable braneworld scenarios: all black holes deserving the name must be heavier than 1 TeV, otherwise the model would contradict something that we have already observed.

With these numbers, you can calculate the number of expected Centauro-like events. It will be pretty high but roughly 100 times less than what is claimed to be observed. Things don't look perfect but on the other hand, you should appreciate that the theory behind this explanation is both sensible as well as extraordinarily exciting. That's why you should think twice before you discard it.

And that's the memo.