The “Gruppenpest” that so many physicists fought against before the war died out during the 1940s. Although some famous physicists made pioneering, fundamental contributions in the theory of infinite-dimensional representations of groups, the community of physicists ignored them.* However, in the late forties, with the discovery of new particles (π mesons and V particles), the need arose to know more about conservation laws. For instance, that a spin-1 boson cannot decay into two photons was known from a short, brilliant paper by Lev Landau and from Eugene P. Wigner's un-published findings, quoted by Jack Steinberger. Using field-theory concepts, particle physicists were eager to establish the list of selection rules for particle decays; for example, C. N. Yang and D. C. Peaslee gave their proof of spin 1 ↛ 2γ. In a long review I wrote in 1951, I carefully referenced all that was known. A complete list of selection rules for angular momentum and parity conservation appeared only later.

Then came the θ – τ puzzle. Its history is recounted excellently by Richard Dalitz in Chapter 30. Apparently this was a typical case of the parity doublet; T. D. Lee and Yang described it in an elegant paper, six months before they proposed parity violation as the explanation. So much has been written on the history of P violation (and some contributions to this conference are devoted to it) that I wish here only to mention some “prehistoric” relevant publications. Pierre Curie, in a fundamental and classic paper (in which he formulated his famous symmetry principles), explained parity conservation and the difference between axial and polar vectors in classical physics.