Since a Princeton group (James Christenson, James Cronin, Val Fitch and Rene Turlay) found the first violation of CP in 1964, high-energy physicists have been eagerly looking for the source of the violation. Of the four classes of theories proposed, at least one and a half classes seem to be ruled out by recent experiments at CERN (by Jean-Marc Gaillard, F. Krienen, W. Galbraith, A. Hussri, M. R. Jane, N. H. Lipman, G. Manning, T. Ratcliffe, P. Day, A. G. Parham, B. T. Payne, A. C. Sherwood, H. Faissner and H. Teithler) and at the Princeton-Pennsylvania Accelerator (by James Cronin, Paul Kunz, Winthrop Risk and Paul Wheeler).
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