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Register a new userPhotoproduction of $Lambda$ and $Sigma^{0}$ hyperons off protons with linearly polarized photons at $E_{gamma} = 1.5-3.0$ GeV
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We report the measurement of the $gamma p rightarrow K^{+}Lambda$ and $gamma p rightarrow K^{+}Sigma^{0}$ reactions at SPring-8. The differential cross sections and photon-beam asymmetries are measured at forward $K^{+}$ production angles using linearly polarized tagged-photon beams in the range of $E_{gamma}=1.5$--3.0 GeV. With increasing photon energy, the cross sections for both $gamma p rightarrow K^{+}Lambda$ and $gamma p rightarrow K^{+}Sigma^{0}$ reactions decrease slowly. Distinct narrow structures in the production cross section have not been found at $E_{gamma}=1.5$--3.0 GeV. The forward peaking in the angular distributions of cross sections, a characteristic feature of $t$-channel exchange, is observed for the production of $Lambda$ in the whole observed energy range. A lack of similar feature for $Sigma^{0}$ production reflects a less dominant role of $t$-channel contribution in this channel. The photon-beam asymmetries remain positive for both reactions, suggesting the dominance of $K^{*}$ exchange in the $t$ channel. These asymmetries increase gradually with the photon energy, and have a maximum value of +0.6 for both reactions. Comparison with theoretical predictions based on the Regge trajectory in the $t$ channel and the contributions of nucleon resonances indicates the major role of $t$-channel contributions as well as non-negligible effects of nucleon resonances in accounting for the reaction mechanism of hyperon photoproduction in this photon energy regime.
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We report the measurement of differential cross sections for $omega$ and $eta$ photoproduction from protons at backward angles ($-1.0<cosTheta_{C.M}^{X}<-0.8$) using linearly polarized photons at $E_{gamma}=$$1.5-3.0$ GeV. Differential cross sections for $omega$ mesons are larger than the predicted $u$-channel contribution in the energy range $2.0leqsqrt{s}leq2.4$ GeV. The differential cross sections for $omega$ and $eta$ mesons become closer to the predicted $u$-channel contribution at $sqrt{s}>2.4$ GeV. A bump structure in the $sqrt{s}$ dependence of the differential cross sections for $eta$ mesons was observed at $sqrt{s}sim$2.35 GeV.
Differential cross sections at $t=t_{text{min}}$ and decay asymmetries for the $gamma prightarrowphi p$ reaction have been measured using linearly polarized photons in the range 1.5 to 2.9 GeV. These cross sections were used to determine the Pomeron strength factor. The cross sections and decay asymmetries are consistently described by the $t$-channel Pomeron and pseudoscalar exchange model in the $E_{gamma}$ region above 2.37 GeV. In the lower energy region, an excess over the model prediction is observed in the energy dependence of the differential cross sections at $t=t_{text{min}}$. This observation suggests that additional processes or interference effects between Pomeron exchange and other processes appear near the threshold region.
Differential cross sections for $gamma p to K^+Lambda(1405)$ and $gamma p to K^+Sigma^0(1385)$ reactions have been measured in the photon energy range from 1.5 to 2.4 GeV and the angular range of $0.8<cos(Theta)<1.0$ for the $K^+$ scattering angle in the center-of-mass system. This data is the first measurement of the $Lambda(1405)$ photoproduction cross section. The lineshapes of LamS measured in $Sigma^+pi^-$ and $Sigma^-pi^+$ decay modes were different with each other, indicating a strong interference of the isospin 0 and 1 terms of the $Sigmapi$ scattering amplitudes. The ratios of LamS production to SigS production were measured in two photon energy ranges: near the production threshold ($1.5<E_gamma<2.0$ GeV) and far from it ($2.0 <E_gamma<2.4$ GeV). The observed ratio decreased in the higher photon energy region, which may suggest different production mechanisms and internal structures for these hyperon resonances.
The quasifree $overrightarrow{gamma} dtopi^0n(p)$ photon beam asymmetry, $Sigma$, has been measured at photon energies, $E_gamma$, from 390 to 610 MeV, corresponding to center of mass energy from 1.271 to 1.424 GeV, for the first time. The data were collected in the A2 hall of the MAMI electron beam facility with the Crystal Ball and TAPS calorimeters covering pion center-of-mass angles from 49 to 148$^circ$. In this kinematic region, polarization observables are sensitive to contributions from the $Delta (1232)$ and $N(1440)$ resonances. The extracted values of $Sigma$ have been compared to predictions based on partial-wave analyses (PWAs) of the existing pion photoproduction database. Our comparison includes the SAID, MAID, and Bonn-Gatchina analyses; while a revised SAID fit, including the new $Sigma$ measurements, has also been performed. In addition, isospin symmetry is examined as a way to predict $pi^0n$ photoproduction observables, based on fits to published data in the channels $pi^0p$, $pi^+n$, and $pi^-p$.
Total cross sections, angular distributions, and invariant-mass distributions have been measured for the photoproduction of $pi^0pi^0$ pairs off free protons and off nucleons bound in the deuteron. The experiments were performed at the MAMI accelerator facility in Mainz using the Glasgow photon tagging spectrometer and the Crystal Ball/TAPS detector. The accelerator delivered electron beams of 1508 and 1557~MeV, which produced bremsstrahlung in thin radiator foils. The tagged photon beam covered energies up to 1400~MeV. The data from the free proton target are in good agreement with previous measurements and were only used to test the analysis procedures. The results for differential cross sections (angular distributions and invariant-mass distributions) for free and quasi-free protons are almost identical in shape, but differ in absolute magnitude up to 15%. Thus, moderate final-state interaction effects are present. The data for quasi-free neutrons are similar to the proton data in the second resonance region (final state invariant masses up to $approx$1550~MeV), where both reactions are dominated by the $N(1520)3/2^-rightarrow Delta(1232)3/2^+pi$ decay. At higher energies, angular and invariant-mass distributions are different. A simple analysis of the shapes of the invariant-mass distributions in the third resonance region is consistent with strong contributions of an $N^{star}rightarrow Nsigma$ decay for the proton, while the reaction is dominated by a sequential decay via a $Deltapi$ intermediate state for the neutron. The data are compared to predictions from the Two-Pion-MAID model and the Bonn-Gatchina coupled channel analysis.
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