VOLUME 36, NUMBER 19 PHYSICAL REVIEW LETTERS 10 MAY 1976 2 For example: F. W. Bttsser et al., Phys. Lett. 46B, 471 (1973); M. Banner^ al., Phys. Lett. 44B, 537 (1973); B. Aliper etal., Phys. Lett. 44B, 521, 527 (1973). 3 For example: D. C. Carey et al., Phys. Rev. Lett. 33, 327 (1974); J. W. Cronin et al., Phys. Rev. D LI, 3105 (1975); J. A. Appel et al., Phys. Rev. Lett. 33, 719 (1974). 4 The detector and other apparatus were used in a previous experiment on ir~p charge exchange and are described in A. V. Barnes et al., SLAC Report No. SLAC-179, 1974 (unpublished), Vol. I, p. 1; R. John- son, Lawrence Berkeley Laboratory Report No. LBL- Multiparticle-final-state reactions form the major part of the total cross section at high en- ergies. The general case is difficult to treat both experimentally and theoretically because of the high multiplicity. Inclusive channels of the form a + b-*c +X, however, may be described in a fair- ly simple way because of the sum over the unob- served states X. There has been considerable experimental and theoretical activity in the study of kinematic distributions of inclusive channels for various choices of particles a, b, and c. 1 If any of these three particles has spin, then polar- ization effects are possible which furnish infor- mation sensitive to interference between various amplitudes contributing to the reaction. It is known that as the energy increases polarization effects in elastic scattering become very small. 2 Few measurements of high-energy inclusive po- larization effects have been made. 3 This Letter reports the first observation of substantial polar- 4610, 1975 (unpublished). 5 Because of the steep falloff of the photon spectra with increasing p ± , we find that our neglect of the pho- ton pairs with lower p ± causes an error of less than 2% in the measured invariant cross sections. 6 The contamination in the p and 7r + samples from mis- identified particles is ^0.5%. In the 7r~ sample the con- tamination is % 0.1%. 7 A. H. Mueller, Phys. Rev. D 2, 2963 (1970). 8 J. Erwin et al., Phys. Rev. Lett. 33, 1352 (1974). 9 This extrapolation is consistent with the measure- ments in Refs. 2 and 3. 10 B. L. Combridge, Phys. Rev. D10, 3849 (1974). ization effects in inclusive production at 300 GeV. The polarization was observed in the channel p + B e - A ° + X . A reaction of this type, where par- ticle c is a A 0 hyperon, is particularly well suit- ed to polarization measurements because the A 0 serves as its own spin analyzer through the de- cay A°-»p +7T". 4 Figure 1 shows the apparatus. The 300-GeV protons were deflected vertically (positive angles upwards) with a magnet 150 m upstream of the A 0 production target, and then restored to the target with magnets 5 m upstream, to obtain production angles between 0 and 9.5 mrad in a vertical plane. The neutral beam was defined by a fixed collima- tor with its axis in the horizontal plane. The col- limator was 5.3 m long, compared to the decay length for 150-GeV/c A°'s of 10.4 m. A vertical magnetic field (the sweeping magnet) of 21 kG was applied to the collimator. A circular tung- sten aperture 4 mm in diameter at 3.2 m defined A 0 Hyperon Polarization in Inclusive Production by 300-GeV Protons on Beryllium G. Bunce, R. Handler, R. March, P. Martin, L. Pondrom, and M. Sheaff Physics Department,* University of Wisconsin, Madison, Wisconsin 53706 and K. Heller, O. Overseth, and P. Skubic Physics Department,| University of Michigan, Ann Arbor, Michigan 48104 and T. Devlin, B. Edelman, R. Edwards, J. Norem, L. Schachinger, and P. Yamin Physics Department,t Rutgers University, New Brunswick, New Jersey 08903 (Received 1 December 1975) A 0 polarization has been observed in p + Be— A 0 + anything at 300 GeV. A total of 1.2 xiO 6 A 0 decays were recorded at fixed lab angles between 0 and 9.5 mrad, covering a range of kinematic variables 0.3 ^x =£ 0.7 and 0^p ± ^ 1.5 GeV/c. The observed polarization was consistent with parity conservation and increased monotonically with increasing p ± , in- dependently of x, reaching P A = 0.28 ±0.08 at 1.5 GeV/c. 1113