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Differential cross sections and photon-beam asymmetries for the γ[over â]pâπ^{-}Δ^{++}(1232) reaction have been measured for 0.7
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The Ï-Λ(1520) interference effect in the γpâK^{+}K^{-}p reaction has been measured for the first time in the energy range from 1.673 to 2.173 GeV. The relative phases between Ï and Λ(1520) production amplitudes were obtained in the kinematic region where the two resonances overlap. The measurement results support strong constructive interference when K^{+}K^{-} pairs are observed at forward angles but destructive interference for proton emission at forward angles. Furthermore, the observed interference effect does not account for the sqrt[s]=2.1 GeV bump structure in forward differential cross sections for Ï photoproduction. This fact suggests possible exotic structures such as a hidden-strangeness pentaquark state, a new Pomeron exchange, or rescattering processes via other hyperon states.
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The exclusive reaction γpâK(+)π(-)Σ(+) was measured for the first time using linearly polarized photons at beam energies from 1.85 to 2.96 GeV. Angular distributions in the rest frame of the K(+)π(-) system were fitted to extract spin-density matrix elements of the K(*0) decay. The measured parity spin asymmetry shows that natural-parity exchange is dominant in this reaction. This result clearly indicates the need for t-channel exchange of the κ(800) scalar meson.
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Differential cross sections and photon-beam asymmetries for the gamma(p)-->K{+}Lambda(1520) reaction have been measured with linearly polarized photon beams at energies from the threshold to 2.4 GeV at 0.6
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Photoproduction of Lambda(1520) with liquid hydrogen and deuterium targets was examined at photon energies below 2.4 GeV in the SPring-8 LEPS experiment. For the first time, the differential cross sections were measured at low energies and with a deuterium target. A large asymmetry of the production cross sections from protons and neutrons was observed at backward K+/0 angles. This suggests the importance of the contact term, which coexists with t-channel K exchange under gauge invariance. This interpretation was compatible with the differential cross sections, decay asymmetry, and photon beam asymmetry measured in the production from protons at forward K+ angles.
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We report a measurement of the differential cross section for the gamman-->pi- p process from the CLAS detector at Jefferson Laboratory in Hall B for photon energies between 1.0 and 3.5 GeV and pion center-of-mass (c.m.) angles (thetac.m.) between 50 degrees and 115 degrees. We confirm a previous indication of a broad enhancement around a c.m. energy ([sqrt]s) of 2.1 GeV at thetac.m.=90 degrees in the scaled differential cross section s7dsigma/dt and a rapid falloff in a center-of-mass energy region of about 400 MeV following the enhancement. Our data show an angular dependence of this enhancement as the suggested scaling region is approached for thetac.m. from 70 degrees to 105 degrees.
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The Sigma(1385) resonance, or Sigma;{*}, is well known as part of the standard baryon decuplet with spin J=3/2. Measurements of the reaction gammap-->K;{+}Sigma;{*0} are difficult to extract due to overlap with the nearby Lambda(1405) resonance. However, the reaction gamman-->K;{+}Sigma;{*-} has no overlap with the Lambda(1405) due to its charge. Here we report the first measurement of cross sections and beam asymmetries for photoproduction of the Sigma;{*-} from a deuteron target. The cross sections at forward angles range from 0.4 to 1.2 mub, with a broad maximum near E_{gamma} approximately 1.8 GeV. The beam asymmetries are negative, in contrast with positive values for the gamman-->K;{+}Sigma;{-} reaction.
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Differential cross sections and photon-beam asymmetries have been measured for the gamma n --> K+ Sigma- and gamma p --> K+Sigma0 reactions separately using liquid deuterium and hydrogen targets with incident linearly polarized photon beams of E gamma = 1.5-2.4 GeV at 0.6 < cos ThetacmK< 1. The cross section ratio of sigma K+ Sigma-/sigma K+ Sigma0, expected to be 2 on the basis of the isospin 1/2 exchange, is found to be close to 1. For the K+ Sigma- reaction, large positive asymmetries are observed, indicating the dominance of K* exchange. The large difference between the asymmetries for the K+ Sigma- and K+ Sigma0 reactions cannot be explained by simple theoretical considerations based on Regge model calculations.
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For the first time, the reaction gammad-->DeltanK+ has been analyzed in order to search for the exotic pentaquark baryon Theta+(1540). The data were taken at Jefferson Laboratory, using the Hall-B tagged-photon beam of energy between 0.8 and 3.6 GeV and the CEBAF Large Acceptance Spectrometer (CLAS). No statistically significant structures were observed in the nK+ invariant-mass distribution. The upper limit on the gammad-->DeltaTheta+ integrated cross section has been calculated and found to be between 5 and 25 nb, depending on the production model assumed. The upper limit on the differential cross section is also reported.
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A search for the Theta+ in the reaction gammad --> pK-K+n was completed using the CLAS detector at Jefferson Lab. A study of the same reaction, published earlier, reported the observation of a narrow Theta+ resonance. The present experiment, with more than 30 times the integrated luminosity of our earlier measurement, does not show any evidence for a narrow pentaquark resonance. The angle-integrated upper limit on Theta+ production in the mass range of 1.52-1.56 GeV/c2 for the gammad --> pK-Theta+ reaction is 0.3 nb (95% C.L.). This upper limit depends on assumptions made for the mass and angular distribution of Theta+ production. Using Lambda(1520) production as an empirical measure of rescattering in the deuteron, the cross section upper limit for the elementary gamman --> K-Theta+ reaction is estimated to be a factor of 10 higher, i.e., approximately 3 nb (95% C.L.).
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Photoproduction of a phi meson on protons was studied by means of linearly polarized photons at forward angles in the low-energy region from threshold to Egamma = 2.37 GeV. The differential cross sections at t = -|t|min do not increase smoothly as Egamma increases but show a local maximum at around 2.0 GeV. The angular distributions demonstrate that phi mesons are photoproduced predominantly by helicity-conserving processes, and the local maximum is not likely due to unnatural-parity processes.
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Beam polarization asymmetries for the p(gamma-->,K+)Lambda and p(gamma-->,K+)Sigma(0) reactions are measured for the first time for E(gamma)=1.5-2.4 GeV and 0.6
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The gamman-->K(+)K(-)n reaction on 12C has been studied by measuring both K+ and K- at forward angles. A sharp baryon resonance peak was observed at 1.54+/-0.01 GeV/c(2) with a width smaller than 25 MeV/c(2) and a Gaussian significance of 4.6sigma. The strangeness quantum number (S) of the baryon resonance is +1. It can be interpreted as a molecular meson-baryon resonance or alternatively as an exotic five-quark state (uuddsmacr;) that decays into a K+ and a neutron. The resonance is consistent with the lowest member of an antidecuplet of baryons predicted by the chiral soliton model.
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The excitation and subsequent proton decay of the isovector spin-flip giant monopole resonance (IVSGMR) is studied via the 208Pb(3He,t) reaction at 410 MeV. In the inclusive spectrum (60+/-5)% of the non-energy-weighted sum-rule strength for this 2 variant Planck's over 2h omega resonance was found in the region 29
