Results on the following π−p reactions involving a hyperon are studied at 4.5 and 6.0 GeV/c from a high-statistics bubble-chamber experiment. (1) π−p→(Λ, Σ0)K0: Differential cross sections and hyperon polarizations are presented. Comparison with the line-reversed reactions K¯N→(Λ, Σ0)π indicates the failure of the predictions of K*(890) and K*(1420) exchange degeneracy. Effective trajectories for these two reactions are compared. Shrinkage is observed in K¯N→Λπ and not in π−p→ΛK0. (2) π−p→(Λ, Σ0)K*(890)0: Differential cross sections, hyperon polarizations, and K*(890)0 density-matrix elements are determined. ΛK*(890)0 decay correlations are found to impose strong constraints on the scattering amplitudes. The data indicate that both natural- and unnatural-parity exchanges contribute large, but opposite, Λ polarizations. This behavior cannot be explained by a simple exchange model utilizing K and the exchange-degenerate K*(890) and K*(1420) only. Additional trajectories or absorption effects are required to obtain the observed Λ-polarization effects. Comparison of ΛK*(890)0 and Σ0K*(890)0 indicates the greater importance of unnatural-parity exchange in the former reaction. We observe no evidence for deviations from isospin predictions in ΛK*(890)0 production where K*(890)0→K+π− and KS0π0. (3) π−p→ΛK*(1420)0 and ΛK*(1300)0: K*(1420)0 density-matrix elements satisfying positivity constraints are determined allowing for s-wave interference effects. Evidence of the existence of a narrow K*(1300)0→Kππ with a dominant K+ρ− decay mode is observed in the 4.5- and 6-GeV/c data. (4) Σ(1385), Λ(1405), Λ(1520) production: Differential cross sections for the quasi-two-body reactions π−p→Y0K0, where Y0 is Λ(1405), Λ(1520), or Σ(1385)0, are presented and found to have a very similar flat slope in the forward direction. Data for forward K+ scattering in the reaction π−p→Σ(1385)−K+ are presented and discussed. It is argued that this forward peak cannot be explained by kinematic reflection or an s-channel effect and therefore must be due to either two-particle exchange or a single exotic exchange in the t channel.
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FIT FOR FORWARD CROSS SECTION AND SLOPE.
Cross sections and density-matrix elements for π−p→ϕn have been measured for - −t≲1.5 GeV2 at 3, 4, 5, and 6 GeV/c, using the Argonne effective-mass spectrometer to observe the decay ϕ(1019)→K+K−. This is the first observation of the reaction in this energy range. The remarkably flat differential cross section at 4 GeV/c and the strong energy dependence suggest a production mechanism not normally seen at these energies. Data on K−p→ϕΛ and K−p→ϕΣ0 from the same experiment are also presented.
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We have observed the D(1285), E(1420) and δ(975) mesons produced in 12 and 15 GeV/ c π − p interactions at the CERN Omega Spectrometer. Production cross sections and decay branching ratios are presented. Analysis of the decay D(1285) → δ (975) π favours a spin-parity assignment of 1 + .
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CORRECTED FOR DECAY MODES OTHER THAN <ETA PI+ PI-> AND FOR THE UNOBSERVED PARTS OF THE T-DISTRIBUTION.
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We have measured the differential cross sections for the associated-production reactions π−p→K0Λ0 and π−p→K0Σ0 at 3, 4, 5, and 6 GeV/c, with a total of over 40 000 events. We find that both reactions have exponential forward peaks for −t≲0.4 (GeV/c)2, with no indication of forward-direction flattening or turnover; the slopes of the forward peaks show little if any variation with momentum; and the two cross sections are equal within experimental error from −t=1.2 (GeV/c)2 out to at least −t=2.0 (GeV/c)2.
INTERCEPT AND SLOPE FROM FIT TO D(SIG)/DT (PRESUMABLY FOR -TP < ABOUT 0.4 GEV**2).
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The backward angular distributions obtained in an experiment at the Zero Gradient Synchrotron of Argonne National Laboratory were used to systematically study the energy dependence of the 180° differential cross section for π+p elastic scattering in the center-of-mass energy region from 2159 to 3487 MeV. At each of 38 incident pion momenta between 2.0 and 6.0 GeV/c, a focusing spectrometer and scintillation counter hodoscopes were used to obtain differential cross sections for typically five pion scattering angles from 141° to 173° in the laboratory. Values for dσdΩ at 180° were then obtained by extrapolation. A resonance model and an interference model were used to perform fits to the energy dependence of dσdΩ (180°). Both models led to good fits to our data and yielded values for the masses, widths, parities, and the product of spin and elasticity for the Δ(2200), Δ(2420), Δ(2850), and Δ(3230) resonances. Our data confirm the existence of the Δ(3230) and require the negative-parity Δ(2200).
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Angular distributions for π+p→π+p were measured for 13 incident-pion momenta from 4.4 to 6.0 GeV/c and for −t less than ∼0.1 (GeV/c)2. This experiment was performed at the Zero Gradient Synchrotron of Argonne National Laboratory, where a focusing magnetic spectrometer and a scintillation-counter hodoscope were used. In fitting the angular distributions the strong-interaction contribution was parameterized by an exponential form exp(bt); the Coulomb interference was also included. The resulting values of the slope parameter for |t|<∼0.1 (GeV/c)2 are presented for each incident beam momentum.
ENLARGED GRAPHS OF FIGURES SUPPLIED BY J. A. POIRIER.
SLOPE IS FROM FITTING EXP(SLOPE*T) TO FORWARD DIFFERENTIAL CROSS SECTION FOR -T < 0.1 GEV**2 APPROX AFTER ALLOWING FOR COULOMB INTERACTION.
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