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We have measured large-angle electron-positron pairs from the reaction γ +Be → Be+e + +e − in the e + e − invariant-mass region of 610 < m < 850 MeV/ c 2 . The phase of the photoproduction amplitude of the ϱ-meson at 4.1 – 6.1 GeV was found to deviate from pure imaginary by 11.8° ± 4.4° which corresponds to a ratio of the real to imaginary ϱ-nucleon amplitude of β = −0.2 ± 0.1.
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We present results of measurements on photoproduction of ρ mesons. Analysis of 106 measured ρ events in a four-dimensional data matrix dσ(A, m, p, t⊥)dΩdm with dimensions 14×20×10×20 yields precise information on nuclear density distributions for ρ production. We obtain for the Woods-Saxon radii R(A)=(1.12±0.02)A13 and, using the vector dominance model, σρN=26.7±2.0 mb and γρ24π=0.57±0.10.
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A systematic study has been made of the reactions pp→pp and pp→pN* in the angular range from θlab=10∘ to θc.m.=90∘ at 3, 4, 5, 6, and 7 GeVc. An orthogonal dispersion magnetic spectrometer detected protons from interactions in hydrogen with momentum transfer (−t) in excess of 0.5 (GeV)2. Well-defined peaks in the missing-mass spectra occurred at average N* masses of 1240±6, 1508±2, and 1683±3 MeV with average full widths of 102±4, 92±3, and 110±4 MeV, respectively. Below 2400 MeV no other significant enhancements were found. The N* production cross sections dσdt near θc.m.=90∘ are in qualitative agreement with the predictions of the statistical model. For each isobar the differential cross section at fixed energy varies as exp(−vv0), where v≡[−tu(t+u)]; v0 varies systematically with energy and tends toward the same value (≈0.4 GeV2) for each isobar at the upper limit of our energy range.
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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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