DECAY M EASU RE M EN TS I N EM ULS ION ON PARTI CLES the nucleus may have been sufficient to cause (PvXy. )observed tO he OPPOStte tO (PZXPv)at production. This would have tended to mask any effect that might have been present. ACKNOW'LED GMENTSWe wish to express our appreciation and indebtedness: To Dr. Joseph J. Murray and his co11eagues for providing the separated E-meson beams, and to Dr. Edward J. Lofgren and the Bevatron crew for their cooperation and aid in the exposures. participation in the design and assembly of the equipment for magnetic analysis of beams, and for their help in various phases of the analyses.To all the scanners involved in this program and particula, rly to Miss Ernestine Beleal, Alan 8etz, Mrs. Marilyn O'I ollin, Mrs. Penny Vedder, and Mrs. Hester Yee for considerable assistance with the measurements and calculations.To i4Irs. Penny Vedder for much of the programming and aid with the IBM 650 computations.To James Hodges for constructing, and participating in the design of, the automated microscopes, and to Thomas Taussig for designing the associated electronics.A beam of Xs mesons was produced by passing a beam of 1.1-Bev/c negative pions through a liquid hydrogen target and accepting the neutral reaction products in the forward direction after allowing the X& component to decay. The resultant beam was observed in a 30-in. propane bubble chamber 6tted with lead and iron plates. About 200 regenerated IC1 mesons were identi6ed by their characteristic Q value and decay rate. All three types of regeneration were observed: by transmission in the plates, by nuclear di8raction, and by interaction with single nucleons. The detection of the erst two types of regeneration constitutes strong evidence for the correctness of the Gell-Mann and Pais particle mixture theory. Comparison of the transmission and diffraction regeneration e8ect, using the method of M. L. Good, gives the E~-Xg mass difference B. Two important corrections must be applied to Good's formula: One originates from the nuclear scattering of the transmission component, the other from the multiplicity of scatterings in a thick plate.The independence from nuclear parameters, which was an advantageous property of Good's formula, is no longer rigorously valid; but due to the sharp dependence of the transmission intensity upon the mass difference, the nuclear properties of Eo and Xo, as derived from E'+ and X data, still allow a measurement, of 8. We find that 5 is 0.84 s, ss+'ss in units of k/rip where rr is the Er mean lifetime. With 90% confidence level, the diiference is between 0.44 and 1.2 k/rr. The probability that the transmission peak we observe is due to a statistical fluctuation is one in a million.

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