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Subject: Re: What is the 'BaBar' project at Stanford Linear Accelerator?

Date: Tue Jun 17 15:57:19 1997
Posted by Suzanne Willis
Position: professor,Northern Illinois University

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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