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.
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The total cross sections for interactions of K+ and 1r+ mesons with protons were measured. The following values were obtained: 21.3 ± 4.6 mb and 21 ± 4.3 mb for 4.75 ± 0.15 Bev/c and 3.7 ± 0.1 Bev/c K+ mesons, respectively; the corresponding values for 1r+ mesons of the same momenta were 33.3 ± 1.3mb and 30 ± 1.2mb. Data on the cross sections for inelastic collisions of K+ and 1r+ mesons with various nuclei have also been obtained.
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The energy dependence of the differential cross section for $\pi^+ p$ elastic scattering at a c.m. angle near 174 ° has been measured. The momentum range of incident $\pi^+$ was 2.06-4.70 GeV/c. On this energy dependence one can see a structure, i.e. maxima corresponding to the baryon resonances $\Delta(2420)$ and $\Delta(2840)$. The structure is used for determination of the parities of these resonances.
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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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Differential cross sections have been measured for the reaction π−p→p¯d. At 4 GeV/c the total cross section is 0.58 ± 0.08 μb with an angular distribution consistent with isotropy. At 5 GeV/c a pronounced forward dip occurs. In contradiction to simple baryon-exchange models, the cross section is found to be an order of magnitude smaller than that for the line-reversed reaction pp→π+d. Upper limits for π−d→p¯t were also found.
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Polarization parameters for the π − p → π 0 n charge exchange scattering have been measured at eight beam momenta between 1965 and 4220 MeV/ c using two different experimental set-ups. The angular range covered is −0.90 < cos θ π ∗ < 0.95 at the five momenta of 1965, 2168, 2360, 2566 and 2960 MeV/ c , where θ π ∗ is the emission angle of the π 0 meson in the c.m.s.. For three momenta of 2770, 3490 and 4220 MeV/ c , the measurements cover the forward angles of 0.1 < cos θ π ∗ < 1.0 . The results are compared with the predictions of π N partial wave analyses.
Polarisation measurements from SETUP1. Errors are statistical only.
Polarisation measurements from SETUP2. Errors are statistical only.
Legendre polynomial coefficients for fit to differential cross section data.
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MULTIPLICITIES OF PI0 WAS OBTANED FROM MULTIPLICITIES OF GAMMAS: MULT(PI0)=MULT(GAMMA)/2.
PI- NUCLEON= (PI- P + PI- N)/2.
PI- NUCLEON= (PI- P + PI- N)/2.
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Axis error includes +- 6/6 contribution (THE NUMBER OF NEUTRAL STARS WAS CORRECTED FOR ACCIDENTAL STARS WITHOUT FORMER INTERACTIONS AND FOR STARS PRODUCED BY KL MESONS, THE LAMBDA AND KS PARTICLES YIELD BEING NEGLECTEDTHE MOMENTUM OF NEUTRON IN THE STAR WAS TAKEN OF ONE AND A HALF OF TOTAL MOMENTUM OF CHARGED PARTICLES IN THE STAR).
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In a study of the production mechanism of quasi-two-body final states at the five incident π+ momenta 2.95, 3.2, 3.5, 3.75, and 4.08 GeV/c, approximately 40 000 events with four outgoing charged particles were investigated. The cross sections for the processes π+p→N*++ρ, π+p→N*++ω, π+p→N*++η, and π+p→N*++f have been measured as a function of the pion energy. The differential cross sections and the decay density-matrix elements are discussed in terms of one-meson-exchange models [with absorption (OPEA) and with form factor (OPEW)] and Regge models. For the N*++ρ and the N*++ω reactions, the joint-decay matrix elements are calculated. The formation of N*(2850) in the direct channel is also investigated.
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Measurements of the differential cross section for the reactions π+p→K+Σ+ and π+p→K+Y*+(1385) are reported at 3.5, 3.75, 4.0, 4.25, 4.5, 4.75, 5.0, 6.0, 10.0, and 14.0 GeV/c. Polarization in π+p→K+Σ+ is also reported at 6.0, 10.0, and 14.0 GeV/c. At small |t|, the cross section for π+p→K+Σ+ is well described by an exponential Aebt with slopes in the range b≈8−10 (GeV/c)−2; for |t|>0.5 (GeV/c)2 this slope decreases considerably. The cross section for π+p→K+Y*+(1385) is well described for |t|>0.2 (GeV/c)2 by a single exponential of slope about half that for π+p→K+Σ+; there is no break near |t|>0.5 (GeV/c)2. We observe a dip in this cross section near t=0. The polarization in π+p→K+Σ+ is consistent with zero for |t|<0.4 (GeV/c)2 and becomes large and positive for larger |t|.
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The Track Sensitive Target technique has been used to isolate 5000 events from the reaction π + p→ π + p π 0 π 0 at 4 GeV/ c . Channel cross sections are measured. The data agree with predictions from phase-shift analyses of π + π − data: the effect of the S ∗ is clearly seen. There is strong evidence against any narrow ϵ below 1 GeV/ c 2 .
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The forward cross sections for the reactions π−p→K+Σ− and K−p→π+Σ− have been determined for incident particle momenta in the range of 2.75 to 3.50 GeV/c, and an upper limit was established at 5.00 GeV/c. These measurements show that the double-charge-exchange process π−p→K+Σ− is suppressed by a factor of 1500 at 3 GeV/c relative to the single-exchange reaction π+p→K+Σ+.
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Upper limits are presented for the differential cross section in the reactions π−p→K+Σ− and π−p→K+Y*−(1385) with small momentum transfer from π− to K+.
EXTRAPOLATED TO T=0 ASSUMING SLOPE IS 5 GEV**-2.
ISOTROPIC ANGULAR DISTRIBUTION ASSUMED IN GIVEN T-RANGE.
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INCLUDING DATA FROM PREVIOUS WORK OF THIS GROUP.
Exotic exchange processes observed in K − p and π − p scattering in the neighbourhoood of 4 GeV/ c are analysed in terms of standard models. Some new data on π − p induced processes are presented and used in the analysis. Regge-Regge cut predictions are found to be smaller than the data up to these energies. Baryonium exchange models are shown to be indistindistinguishable from Regge cut models as regards their phenomenological predictions. The double scattering quark model of Białas and Zalewski is compatible with data on the exotic exchange production of Σ and Y ∗ (1385), but fails for Ξ and Ξ ∗ (1530) production. Modifications of this model are discussed.
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FORWARD (SMALL -T) CROSS SECTIONS. THE K- P CROSS SECTIONS ARE COMPUTED FROM THE ACNO DATA AT 4.2 GEV/C, M. MAZZUCATO ET AL., NP B178, 1 (1981).
BACKWARD (SMALL -U) CROSS SECTIONS.
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THE DATA POINTS AT -T = 0.40 AND 0.60 GEV**2 WERE OBTAINED FROM THE PAPER BY J. P. DE BRION ET AL., NP B32, 557 (1971). THESE DATA OF O. GUISAN ET AL., PL 18, 200 (1965) ARE USED TO DETERMINE THE ABSOLUTE NORMALIZATION.
Interference between the I u = 1 2 and I u = 3 2 baryon exchange amplitudes is observed in the reaction π − p → p π − π 0 , with the proton produced forward with cos θ p ∗ >0.8 . The Dalitz plot shows that the reaction is dominated by the quasi two body final states ρ − p( δ exchange) and N ∗0 (1670)π 0 ( N exchange ) , with δ(1238), N ∗ (1520) and higher mass N ∗ 's also produced. The relative phase between the ρ and the N ∗ (1670) production amplitudes is measured to be 135° ± 10° and is compared with the Regge pole signature factor phase predictions.
THE RELATIVE PRODUCTION AMPLITUDE PHASE FROM INTERFERENCE IS 135 +- 10 DEG.
We have measured π±p and pp elastic differential cross sections in the range |cosθc.m.|<0.35 for incident momenta from 2 to 9.7 GeV/c for π−p and pp and from 2 to 6.3 GeV/c for π+p. We find that the fixed-c.m.-angle πp differential cross sections cannot be described as simple functions of s. The data are compared to the energy and angular dependence predicted by the constituent model of Gunion, Brodsky, and Blankenbecler.
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Large-angle π±p elastic-scattering cross sections, measured between 2 and 9 GeV/c in fine intervals of incident momentum and scattering angle, are used to search for cross-section fluctuations occurring for small changes in the center-of-mass energy as suggested by Ericson and Mayer-Kuckuck and by Frautschi. Significant fluctuations are observed.
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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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An experiment using optical spark chambers and a neutron time-of-flight hodoscope has been performed at the Argonne National Laboratory on the reaction π−p→ω0n. The differential cross section and the experimentally accessible density-matrix elements were determined in the momentum transfer interval 0.05≤|t|≤1.0 (GeV/c)2 at each of three incident pion momenta 3.65, 4.50, and 5.50 GeV/c. Our results show the following general features: (1) a dip in the forward differential cross section for |t|≤0.2 (GeV/c)2, (2) a slope at larger momentum transfers which increases as the incident pion momentum increases, and (3) no dips in either dσdt or ρ11+ρ1−1, the natural-parity exchange combination, at |t|=0.6 (GeV/c)2.
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