A graphite-plate spark chamber has been used to analyze the polarization of protons recoiling from π−−p scattering. The observations were made at 90° (c.m. system) pion scattering angle for seven incident pion energies between 500 and 940 Mev, at 120° or 135° for five energies in this interval, and also at 75° for 500 Mev only. The results are compared with predictions of several models used to explain the maxima in the π−−p scattering cross section. Qualitative arguments show that the energy intervals between these maxima are not completely dominated by neighboring single-state resonances. Phase shifts found to be large in scattering also seem to be large in polarization.
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A total of 24 360 events having two charged particles in the final state from π−+p interactions at an incident π− momentum of 2.7 GeVc have been analyzed. The final states π−π+n and π−π0p are found to be dominated by rho-meson production, and in addition, significant N*(1238) production is seen. The partial cross sections for the dominant resonant channels are σ=(pρ−)=(1.3±0.2) mb, σ(nρ0)=(2.3±0.2) mb, and σ[π−N*+(→pπ0)]=(0.5±0.2) mb. The production of the ρ− and ρ0 and the decay of the ρ− agree very well with the predictions of an absorption-modified one-pion-exchange model. The production angular distributions of the ρ0 and ρ− follow an exponential of the form Ae+Bt. The results from a least-squares fit give B(ρ−)=9.32±0.08 (GeVc)−2, B(ρ0)=10.26±0.06 (GeVc)−2. A similar analysis for the elastic-scattering events gave B(el)=7.77±0.05 (GeVc)−2. The ρ0 decay distributions are asymmetric and they have been analyzed using a simple model which includes S−P-wave interference. No clear evidence is seen for a T=0, J=0 resonance at a mass near that of the ρ. The N*(1238) resonance production is found to be in agreement with the ρ-exchange model of Stodolsky and Sakurai. Indication of other resonance production with small cross section is seen, such as A1 and A2 production in the multiple missing neutral events. The masses and widths of the ρ0 and ρ− as a function of the four-momentum transfer squared to the nucleon have been determined.
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An analysis of π−p two-prong interactions at 4.16 GeV/c is presented. The total two-prong cross section is 19.11±0.40 mb, based on 33 672 events. The elastic-scattering differential cross section shows an exponential behavior, Kexp(−AΔ2). With A=7.36±0.14 GeV−2, the "absorption parameters" are derived as C+=0.846±0.017 and γ+=0.040±0.001. The final-state π−π0p exhibits a strong ρ−, and the π−π+n a strong ρ0 and f0. The partial cross sections for the dominant resonant channels pρ−, π−Δ+(1236) (→pπ0), ρ0n, and f0n are 0.59±0.03, 0.17±0.01, 1.15±0.05, and 0.53±0.06 mb, respectively. The ρ− production and decay angular distributions do not agree with the predictions of the absorption-modified one-pion-exchange model. However, an inclusion of the contribution from ω exchange adequately accounts for the discrepancy. The ρ0 asymmetry is interpreted as a result of an interference of the resonant P wave and isospin-zero S wave, and the corresponding spin-density matrix elements are obtained. In the final state π−p+neutrals, a clear peak for the η meson and some evidence for the ω meson are seen.
Axis error includes +- 0.0/0.0 contribution (?////EVENT NORMALIZATION).
The polarization parameter in elastic π−p scattering has been measured, at the Berkeley 184-in. synchrocyclotron, with the use of a polarized proton target. At 318-, 337-, and 390-MeV incident pion kinetic energy, the angular range from 70° to 180° in the center-of-mass system was covered. At 229 MeV, polarization measurements were made in the angular range 150° to 180°. Phase-shift analyses, using these and other published data, were made at the two lowest energies.
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This paper presents differential cross sections for backward π−p elastic scattering in the angular region −0.55≥cosθc.m.≥−0.98 for five incident momenta: 2.38, 2.50, 2.65, 2.80, and 3.00 GeV/c. The structure of the angular distribution undergoes a marked change over this momentum interval. A pronounced dip near 180°, which is seen for momenta ≲2.50 GeV/c, becomes a sharp peak at 2.80 and 3.00 GeV/c. A minimum in the cross section at cosθc.m.≃−0.7 is indicated at all momenta. In addition, a dip in the differential cross section appears at cosθc.m.≃−0.92 at 3.00 GeV/c. A good fit to the data from 2.1 to 3.0 GeV/c is obtained with a direct-channel resonance model.
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Differential cross sections are presented for pion-proton elastic scattering in the angular range −0.6≳cosθc.m.≳−0.98 at 15 incident π+ momenta from 2.18−5.25 GeVc. The angular distributions rise steeply near 180° at all momenta. For laboratory momenta ≳2.75 GeVc they show a minimum at u≈−0.17 (GeVc)2 and a broad maximum near u≈−0.6 (GeVc)2. When the data are plotted versus s, for fixed u, a strong signal from the Δ(2420) resonance is observed. The data are compared with a direct-channel resonance model and with a Regge model which considers the exchange of the Nα, Nγ, and Δδ Regge trajectories. The qualitative success of both the direct-channel resonance model and the Regge model lends support to the concept of duality.
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Production of B(1235) and ρ(1710) mesons is observed in the four-pion decay modes π+π+π−π0 in 16−GeVc π±p collisions. Decay distributions and branching fractions into various modes are presented. Absence of the two-pion mode π−π0 for the ρ(1710) is noted.
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Data from the reactions π−p→π−π−π+p and π+p→π+π+π−p have been obtained at 16 GeV/c in approximately equal samples from the BNL 80-in. hydrogen bubble chamber. We have studied both reactions for resonance production and compared our results with predictions of the one-pion-exchange model (OPEM) calculated by Wolf. The four-body longitudinal phase-space (LPS) plot suggested by Van Hove was used to analyze the data from both reactions. The events were separated according to distinct regions of the LPS plot in order to study the following classes of quasi-two-body final states: (1) diffractively produced three-pion states, (2) diffractively produced ππp states, (3) double-resonance production such as ρ0Δ++, (4) single-resonance production such as π−π−Δ++. The Van Hove analysis provided a much cleaner separation of resonances from background effects than the usual techniques. We observe production of the A1 and A2 mesons as well as the N*(1470) and N*(1700) isobars in the π+π−p system for both reactions. There is strong formation of the doubly resonant states ρ0Δ++ and f0Δ++ and the general features of the nondiffractive events are all in good agreement with one-pion exchange. Detailed comparisons between the data and the OPEM in each LPS plot region show that Van Hove analysis provides a sensitive test for the OPE model.
DEDUCED FROM PRONG CROSS SECTIONS NORMALIZED USING THE DATA OF K. J. FOLEY ET AL., PRL 19, 330 (1967).
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A quasi-two-body model based on one-particle exchange and diffraction dissociation has been fitted to data from π−p interactions at 3.9 and 11.9 GeV/c in which a nucleon and 3-6 pions are present in the final state. It is used to estimate partial cross sections for the contributing interaction mechanisms and the dominant resonances which are produced at these energies. The energy dependence of the cross sections is examined and found to be consistent with expected behavior, and reactions are compared and found to agree with simple factorization.
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