"In the early 1950s... attention was focused on two new unstable, electrically neutral particles... tau and theta. ...[T]he tau and theta were 'strange'—they carried Gell-Mann's additional charge. They decayed in different ways, and had different parities... [T]he tau and theta had the same mass. ...Chen Ning ('Frank') Yang and , thought it was bizarre for two apparently different particles to have the same mass, and suspected... two faces of the same particle, despite... different parities. ...[They] had to throw overboard ...apparently solid ...assumptions about quantum behaviour: ...[1] it would not be basically altered by left-right mirror reflection... [2] behaviour would not be altered by a mirror that reflected particles as antiparticles and vice-versa... [They] re-examined the evidence for both mirror symmetries, which everyone had assumed ...watertight ...showing that for particle decays this had never been proved conclusively."