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Physics as a science has made incredible progress because of the delicate interplay between theory and experiment. Astonishing predictions based on theories devised to account for known phenomena have been confirmed by experiment. Experiments probing previously unexplored areas often reveal physical effects which are completely unanticipated by theoretical conjecture. The incorporation of the new effects into a theoretical framework then follows. This year Prof. Cronin and I are being honored for a purely experimental discovery, a discovery for which there were no precursive indications, either theoretical or experimental. It is a discovery for which after more than 16 years there is no satisfactory accounting. But showing as it does a lack of chargeconjugation parity symmetry and, correspondingly, a violation of time-reversal invariance, it touches on our understanding of nature at its deepest level. The discovery of failure of CP symmetry was made in the system of K mesons. This observation is especially interesting because it was the study of these same particles that led to the overthrow of parity conservation, the notion that interactions and their mirror-reflected counterparts must be equal. My own interest in K particles started in 1952-53 while I was at Columbia working with Jim Rainwater on µ-mesonic atoms. At that time the strange behavior of the particles newly discovered in cosmic rays (1) was a major topic of conversation in the corridors and over coffee. By strange behavior I am referring to the copious production but slow decay. Protons bombarded by pions would result in the production of A"'s at 10 13 times the rate of their decay back to

We report here some preliminary results from an experiment designed to measure the phase of the CP-nonconserving component in a long-lived neutral K-meson beam. 1 While the results are preliminary in that only a small fraction of the total data have been analyzed, we believe the main conclusion drawn at this time is clear.As concluded in reference 1, the long-lived neutral Kjf meson with a unique lifetime and mass is not a pure eigenstate of CP but is a mixture given by K L°= K2° + £Ki°, where K^ and K 2° are the eigenstates with eigenvalues +1 and-1, respectively, and where Iel = 2xl0~3, A change in this mixture of states occurs when the Kjf beam is passed through a scattering medium. The difference in the K° and K° forward-scattering amplitudes, /(0) and/(0), produces a coherent conversion of the K 2° component to K x° (the reverse process is negligible). The wave becomes ip =K2° + (e +A r )Ki°, where A r is the coherent regeneration amplitude given, at equilibrium, by; k is the wave number; 6 is the mass difference, ra^-ra^, in units of the decay rate of the short-life component K$ 0 ; N r is the number of scattering centers per cm 3 ; and A s is the mean decay length of the #s°.The branching ratio for 277 decay 2 in material is proportional to le +A r \ 2 .Of course e and A r should have equal magnitudes for a maximum interference effect. In solid material, however, A r »e, the interference effects are small, and A r can be separately determined. Neglecting attenuation and interference effects to keep the formulas perspicuous, the coherently regenerated K$° intensity immediately behind a piece of solid material of thickness I (measured in units of A s ) and nuclei density N s is / s = 47T 2 iV s 2 A s 2 |[/ 21 (0)A]x [l-exp(-z'SZ-Z/2)](z6 +£)The experimental procedure was to determine \A y \ for an arbitrary density N r by using Eq. (2b) and measuring I s . The intensity, I a , of 277 decays over a length L was measured with no material present and is proportional to I e I 2 . Finally, diffusely distributed material of a density such that kl~IA r l was placed in the decay volume and the intensity, 1^, of 277 decays in the same length L was measured. In terms of directly determined quantities the phase angle a between A r and e is given bywhere Ip = \A r \ 2 L/A S . The experiment was performed at the Brookhaven National Laboratory AGS. The Kj^° beam, produced by 28-BeV protons in a Be target, is similar to that described in reference 1, except that (a) the beam was at +30° instead of -30° relative to the circulating protons, (b) the detection apparatus was -82 ft from the target, and (c) the collimation system defined a solid angle of 22 /i sr at the target in the accelerator. The vector momenta of the charged secondaries from K° decay were measured in a spectrometer comprised of spark chambers before and after a magnetic field. A plan view, which is a composite of a drawing and spark-chamber photographs of an actual event, is shown in Fig.