Measurements of the differential cross section for the process γ+p→π0+p have been made at eight pion center-of-mass angles in the range 51-135° and for incident photon energies from approximately 600-1200 MeV. The bremsstrahlung photon beam used was obtained from the California Institute of Technology electron synchrotron. Both the recoil proton and one γ ray from the decay of the π0 were detected. The incident photon energy was determined by measuring the laboratory angle and time of flight of the recoil proton. The angular distributions obtained indicate that the third pion-nucleon resonance is predominantly a D(52) resonance excited by a magnetic quadrupole transition. It can also be concluded that any contribution to the π0 photoproduction cross section from a virtual vector-meson exchange process is probably negligible in the region of the second and third pion-nucleon resonances.
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The Λ polarization and the differential cross section for the reaction γ+p→K++Λ have been measured, using the Caltech synchrotron, at 90° in the c.m. system and at laboratory photon energies of 1100, 1200, and 1300 MeV. Protons from the asymmetric decay of the Λ were detected by counters placed above and below the production plane. Kaons were identified by their behavior in a thick range telescope. Polarization results were PΛ=+0.34±0.09 at 1100 MeV, +0.30±0.07 at 1200 MeV, and +0.08±0.07 at 1300 MeV, where PΛ was measured in the p^γ×p^Λ direction. The differential cross section was constant with energy at 0.14±0.01 μb/sr. Although the apparent bump in the polarization at 90° at a total energy of ≈1700 MeV adds support to models which invoke a resonance here, no really new conclusions can be reached.
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The differential cross section for the reaction γ+p→π++n was measured at 32 laboratory photon energies between 589 and 1269 MeV at the Caltech synchrotron. At each energy, data have been obtained at typically 15π+ angles between 6° and 90° in the center-of-mass (c.m.) system. A magnetic spectrometer was used to detect the π+ photoproduced in a liquid-hydrogen target. Two Cerenkov counters were used to reject background of positrons and protons. The data clearly show the presence of a pole in the production amplitude due to one-pion exchange. Moravcsik fits to the angular distributions, including data from another experiment carried out by Thiessen, are presented. Extrapolation of these fits to the pole gives a value for the pion-nucleon coupling constant of 14.2±1.7, which is consistent with the accepted value. The "second" and "third" pion-nucleon resonances are evident as peaks in the total cross section and as changes in the shape of the angular distributions. At the third resonance, there is evidence for both a D52 and an F52 amplitude. The absence of large variations with energy in the 0° and 180° cross sections implies that the second and third resonances are mostly produced from an initial state with helicity 32.
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Cross sections for the photoproduction of neutral pions have been measured at the 1.1-GeV Frascati electron synchrotron for bombarding photon energies k between 400 and 800 MeV and for π0 c.m. angles of θπ*=90∘, 120∘, and 135∘. The main feature of the experiment is good resolution in incident photon energy. The results are in good agreement with the existing theories in the energy range of 450 to 550 MeV. The cross sections exhibit a smooth behavior as a function of energy for k=400−600 MeV. No immediate evidence is found of a contribution of the P11 resonance. An anomaly at the limit of statistical significance appears for k≃700−740 MeV, indicating a possible structure of the so-called second resonance. We attempt to interpret the observed anomaly as a reflection of the sharp opening of the η production channel (η cusp effect).
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The scattering of photons by protons has been measured with a spark-chamber technique using 335-MeV bremsstrahlung. The experimental values obtained at 90° and 135° are compared with those calculated by Contogouris using dispersion relations. The agreement is reasonable except for a persistently low point for 310 MeV at 90°.
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We report the results of the investigation of 18 500 frames of π+p interactions in the Brookhaven 20-in. bubble chamber at an incident energy of 900 MeV. It is found that single-pion production proceeds almost entirely through formation of the N33* isobar. The production mechanism of the N33* is analyzed in terms of its spin density matrix. Comparison is made with Stodolsky and Sakurai's ρ-exchange model and with the absorptive peripheral model.
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The π0 photoproduction cross section has been measured at 180° for photon energies from 220 to 380 MeV, in steps of 20 MeV, by detecting the recoil proton at 0°. The statistical accuracy of the measurements varies between 3 and 7%, depending on the energy. Absolute cross sections have been deduced from a comparison of the measurements with electron-proton scattering. The experimental data are compared with theoretical results calculated from fixed-momentum-transfer dispersion relations. Special attention is paid to the prediction of the multipoles at the first resonance, namely, E1+32, M1+32, and E0+π0 to obtain agreement with experiment.
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We have measured the differential cross section for π−p elastic scattering at 180° in steps of 0.10 GeV/c or less in the region P0=1.6 to 5.3 GeV/c. We detected elastic scattering events, from protons in a liquid H2 target, with a double spectrometer consisting of magnets and scintillation counters in coincidence. The incident π− beam was counted by scintillation counters. The cross section was found to have considerable structure. This may be interpreted as interference between the resonant amplitudes and the nonresonant or background amplitude. Very strong destructive interference occurs around P0=2.15 GeV/c, where the cross section drops almost two orders of magnitude in passing through the N*(2190). Another interesting feature of the data is a large narrow peak in the cross section at P0=5.12 GeV/c, providing firm evidence for the existence of a nucleon resonance with a mass of 3245±10 MeV. This N*(3245) has a full width of less than 35 MeV, which is about 1% of its mass. From this experiment we were able to determine the parity and the quantity χ(J+12) for each N* resonance, where χ is the elasticity and J is the spin of the resonance.
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The reactions π−p→π−p and π−p→π−π0p for 1.7 GeV/c incident π− have been studied, in 3094 and 2244 interactions respectively, identified from 10 106 two-prong events measured in film exposed at the BNL 20 in. hydrogen bubble chamber. The differential elastic-scattering cross-section is found to show a first and second diffraction peak and a first diffraction minimum with indications of a second minimum and onset of a third maximum. The experimental curve has been fitted by a black-dise optical-model formula with radius (0.80±0.03) fm and by a differential cross-section computed from the Dirac equation depending on two ranges, 0.7 fm attractive imaginary and 0.4 fm repulsive. The dominant mode (∼40%) of the π−π0p production is through the two-body channel, π−p→ϱ−p. We find the following cross-sections: σ(π−p→π−p mb, σ(π−p→π−p mb. The differential rhomeson production cross-section shows a diffraction peak having a dependence (dσ/dt)(π−p→ϱ−p)=[(2.5±0.2) exp [(−5.3±0.5)t]] mb/(GeV/c)2, wheret is the squared four0momentum transfer between incoming and outgoing proton in (GeV/c)2, and a second diffraction maximum. It has been fitted by an optical-model formula for a bright ring of radius 0.80 fm and ring thickness 0.25 fm. The cross-section for σ(π−p→π−p was found to be (0.36±0.04) mb. From the inelastic data the Chew-Low dipion scattering cross-section has been computed, using various form factors. A form factor of unity is found to be acceptable.
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Small-angle differential cross sections are presented here for π−+p→π0+n charge-exchange scattering between 2.4 and 3.8 GeV/c. The differential cross section near t=0 displays two minima and one maximum in this momentum interval, reflecting the presence of the N32*(2420), N32*(2850), and N12*(2650) resonances; at larger t values, the cross sections fall off exponentially as a function of t, just as has been previously observed for charge-exchange scattering above 6 GeV/c. The pion-charge-exchange data reported here at 6 and 10 GeV/c extend out to large angles, showing a maximum near t=0, followed by an exponential falloff as e10t, a minimum near −t=0.6 (GeV/c)2, and then a second maximum near −t=1.0 (GeV/c)2. The π−+p→η0+n differential cross section shows a maximum near t=0, followed by an exponential falloff as e4t, much less steep than the π0 slope. These data are compared to our previously published data and to those of the Saclay-Orsay group.
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