IN PHYSICS, there is sometimes life after death. The constant
wish of researchers who study the universe at the subatomic
level is for bigger and more powerful machines. When this wish
is granted by the funding agencies, the existing, smaller
machines become redundant. That was the fate of PEP, the
positron-electron project at SLAC, the Stanford Linear
Accelerator Centre, in California. It was overtaken by events,
and turned off in 1990.  In May 1999, it became fully
operational again, having been refitted and redubbed PEP-II. The
refit turned it into a “factory” for producing particles called
B-mesons. Its new mission was to answer a question so profound
that only a physicist would think of asking it: why is the
universe made of matter? This week, its operators reported part
of the possible answer.  Fearful symmetry  The reason why the
question is worth asking is that most fundamental things in the
universe seem to come in symmetrical pairs. In particular, every
type of particle of which matter is composed h