2b. Has CP violation been observed? guided tour ---> --> ->

Previous to the B factory experiments, what evidence had been seen for CP non-invariance? The hypothesis of CP conservation predicts certain properties for mesons that are electrically neutral and decay by the weak force.  It predicts that such mesons exist in states with CP symmetry - we say they are CP eigenstates.  If CP is conserved, then they can only decay to combinations of particles which as a group have the same CP symmetry as the original meson.  This hypothesis seemed to work pretty well, until 1963, when a decay that violated this rule was observed for particles known as K-long mesons.  The K-long meson is electrically neutral and decays half the time to three other mesons called pions.  Under CP conservation this meant that the K-long should not decay to two pions.  What was found was that it does decay to two pions, at a rate of about two in a thousand.  This caused a lot of excitement, and consternation.  When P (parity) was found to be violated (in 1956), a simple and new conservation rule, for CP, was proposed. No simple rewriting of the rule could fix the observed CP violation.  One would either have to write a new, additional rule, or make the existing ones much more complicated.  We still don't really know which is the way to go, but that's what we hope the B factories will be able to answer.