The polarization of the recoil proton from the reactionγp→π0p has been measured for photon energies between 900–1,350 MeV and pion c.m. angles between 70° and 150°. There are significant deviations from recent analysis.
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Photoproduction of π−-mesons on deuteron has been studied in the first resonance region with an annihilation photon beam with adjustable peak energy (from 250 MeV to 400 MeV). A coincidence detection of both outgoing π−-meson and forward proton has been performed with a set of 9 multiwire proportional chambers (1700 wires) inside the gap of a spectrometer. The momentum of the second proton is computed from three-body kinematics; their distribution is found in excellent agreement with the spectator model, even at the top of the resonance. The differential cross-sections of π− have been measured from 100° to 180° (center of mass); they are in reasonable agreement with conventional multipole calculations and do not indicate an appreciable isotensor term.
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The polarization of the recoil proton from the reactionγp→π0p has been measured for photon energies between 600 and 1,200 MeV and pion c.m. angles between 90° and 150°.
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The inclusive production of D ∗± mesons in photon-photon collisions has been measured by the Aleph experiment at LEP with a beam energy of 45 GeV. The D ∗+ are detected in their decay to D 0 π + with the D 0 observed in three separate decay modes: (1) K − π + , (2) K − π + π 0 and (3) K − π + π − π + , and analagously for the D ∗− modes. A total of 33 events was observed from an integrated luminosity of 73 pb −1 which corresponds to a cross section for Σ( e + e − → e + e − D ∗± X ) of 155 ± 33 ± 21 pb. This result is compatible with both the direct production γγ → c c in the Born approximation and with a more complete calculation which includes both radiative QCD corrections and contributions in which one of the photons is first resolved into its quark and gluon constituents. The shapes of distributions for events containing a D ∗+ are found to be better described by the latter.
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Photoproduction is studied at 2.8 and 4.7 GeV using a linearly polarized monoenergetic photon beam in a hydrogen bubble chamber. We discuss the experimental procedure, the determination of channel cross sections, and the analysis of the channel γp→pπ+π−. A model-independent analysis of the ρ0-decay angular distribution allows us to measure nine independent density-matrix elements. From these we find that the reaction γp→pρ0 proceeds almost completely through natural parity exchange for squared momentum transfers |t|<1 GeV2 and that the ρ production mechanism is consistent with s-channel c.m. helicity conservation for |t|<0.4 GeV2. A cross section for the production of π+π− pairs in the s-channel c.m. helicity-conserving p-wave state is determined. The ρ mass shape is studied as a function of momentum transfer and is found to be inconsistent with a t-independent Ross-Stodolsky factor. Using a t-dependent parametrization of the ρ0 mass shape we derive a phenomenological ρ0 cross section. We compare our phenomenological ρ0 cross section with other experiments and find good agreement for 0.05<|t|<1 GeV2. We discuss the discrepancies in the various determinations of the forward differential cross section. We study models for ρ0 photoproduction and find that the Söding model best describes the data. Using the Söding model we determine a ρ0 cross section. We determine cross sections and nine density-matrix elements for γp→Δ++π−. The parity asymmetry for Δ++ production is incompatible with simple one-pion exchange. We compare Δ++ production with models.
FROM QUOTED TOPOLOGICAL CROSS SECTIONS. 1.44 GEV CROSS SECTION PUBLISHED PREVIOUSLY.
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An experiment has been completed at the Daresbury synchrotron to measure the asymmetry in the photoproduction cross section of neutral pions on hydrogen, for photons polarised normal to and in the production plane. The source of polarised photons was coherent bremsstrahlung of electrons traversing the lattice structure of diamond and the polarisation P of the γ beam was calculated from the measured intensity of the coherent spike. The asymmetry parameter Σ, defined as Σ = ( σ ⊥ − σ |)/( σ ⊥ + σ |) where σ ⊥( σ |) are the cross sections for photons polarised perpendicular (parallel) to the production plane, has been measured over a range of photon energies from 1.2 to 2.8 GeV and over a range of − t (the square of the four-momentum transfer) from 0.13 (GeV/ c ) 2 to 1.4 (GeV/ c ) 2 . A marked energy variation in the value of Σ is found over the energy region 1.6–1.8 GeV.
Axis error includes +- 0.0/0.0 contribution (?////).
Axis error includes +- 0.0/0.0 contribution (?////).
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We have measured the asymmetry of the cross section for γp→π+n from a polarized target at 5 and 16 GeV. The range of four-momentum transfer was 0.02<~−t<~1.0 GeV2. The π+ mesons were produced in a polarized butanol target and detected with the Stanford Linear Accelerator Center 20−GeVc spectrometer. A sizable asymmetry was found at both 5 and 16 GeV, a typical value being -0.6 near −t=0.3 GeV2. A small amount of data on the asymmetry of other photoproduction processes was also obtained.
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The differential cross section has been measured for the reaction γ +p→p+ π o at the Bonn 2.5 GeV electron synchrotron in the energy range from 0.4 to 2.2 GeV for a c.m. angle of 150 degrees. The protons were detected in a magnetic spectrometer system. The excitation curve shows a distinct resonance structure. The total corrections to the counting rate are about 3%. The contribution of the process γ +p→p+2 π was separated. The uncertainty of this separation leads to an error of about 4% in the cross section.
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A mass-dependent asymmetry was observed in the decay angular distribution of a photoproduced K + K − system near the K + K − threshold. The corresponding moments 〈 Y 1 0 〉 have been evaluated. Interpreting the asymmetry as an S-P wave interface due to the states S 993 ∗ (0 + ) and ø 1019 (1 − ) one can compute the moments 〈 Y 1 0 〉 through an amplitude analysis. The theoretical calculation reproduces the experimental results well, if one assumes a real S-wave amplitude for the S 993 ∗ . The data cannot be explained by a non-resonant real S-wave. Other possibilities have been discussed. An estimate of the photoproduction cross section of the S ∗ → K + K − can be given on the basis of the above hypothesis.
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