MadSci Network: Physics |
The BaBar project is an experiment which will study B mesons in detail, planning to do detailed measurements of their properties. In particular, they expect to be able to measure CP violation (more below) in the B meson system. This involves a neutral B meson becoming its own antiparticle (which is written as B with a bar over the top); hence the name BaBar. The experiment has its own home page, but it is geared primarily to experimental physicists; it can be found at http://heplibw3.slac.stanford.edu/BFROOT/doc/www/bfHome.html. So what is CP violation, and why is it so interesting? Well, first let me tell you a little about parity violation. Until the mid-fifties, physicists thought that the laws of nature should be the same if you looked at everything in a mirror. This is called "parity conservation". (For the nitpickers, a parity transformation actually involves changing the sign of all three axes, and the mirror changes the sign of only one; you can change the other two by a simple rotation.) Then a famous experiment by C.S. Wu and colleagues confirmed a theoretical suggestion that parity might not be conserved in weak interactions - if you reflect a weak interaction in a mirror, you see an impossible interaction (having to do with particles spinning in the wrong direction). However, if you change all the particles in the mirror into their antiparticles (a process called charge conjugation), the mirror now shows a possible interaction again. This combination of charge conjugation (C) and parity violation (P), called CP, was thought to be conserved in all interactions. Well, in the sixties, Fitch and Cronin et al. did an experiment where they looked at neutral K mesons. There are two types of neutral K mesons, one called a K(long) and one called a K(short); they are in different CP states, and the (long) and (short) refer to their lifetimes. The long-lived one decays into three pi mesons, the short-lived one into two (thereby conserving CP). Well, Fitch and Cronin discovered that about one in a thousand K(long) mesons actually decays into two pions, rather than three, violating CP! The neutral K mesons are the only system in which CP violation has been seen, and according to the Standard Model of elementary particles, the only place where one would expect to have seen it, until now. The B mesons are very much like the K mesons (except they are much heavier and much shorter-lived, making them much harder to produce and study). So one expects to see them violating CP about one-thousandth of the time too, but the experiments are much trickier. (The long and short-lived K mesons have lifetimes which differ by about a factor of 100, so one can start out with a mixed beam of them and be sure that one has only K(long)s after a while; the corresponding B mesons, however, are expected to have lifetimes which are virtually identical, so one has to be much cleverer). Exactly how CP violation fits into the theory is still a matter of some speculation, and physicists are eager to have data on a totally different system to help sort this out.
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