S. C. Wang

Evidence for a narrow S = +1 baryon resonance in photoproduction from the neutron

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.

The γ→p→K+Λ and γ→p→K+Σ0 reactions at forward angles with photon energies from 1.5 to 2.4 GeV

Differential cross sections and photon beam asymmetries for the gamma p rightarrow K+ Lambda and gamma p rightarrow K+ Sigma0 reactions have been measured in the photon energy range from 1.5 GeV to 2.4 GeV and in the angular range from Theta_{cm} = 0 to 60 of the K+ scattering angle in the center of mass system at the SPring-8/LEPS facility. The photon beam asymmetries for both the reactions have been found to be positive and to increase with the photon energy. The measured differential cross sections agree with the data measured by the CLAS collaboration at cosTheta_{cm} 0.9. In the K+Lambda reaction, the resonance-like structure found in the CLAS and SAPHIR data at W=1.96 GeV is confirmed. The differential cross sections at forward angles suggest a strong K-exchange contribution in the t-channel for the K+Lambda reaction, but not for the K+Sigma0 reaction.

Near-Threshold Diffractive phi (variant)-Meson Photoproduction from the Proton

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 E{sub {gamma}}=2.37 GeV. The differential cross sections at t=- vertical bar t vertical bar{sub min} do not increase smoothly as E{sub {gamma}} 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.

Evidence for the Θ+ in the γd→K+K-pn reaction by detecting K+K- pairs

The gamma d to K^+K^-pn reaction has been studied to search for the evidence of the Theta^+ by detecting K^+K^- pairs at forward angles. The Fermi-motion corrected nK^+ invariant mass distribution shows a narrow peak at 1.524 +- 0.002 + 0.003 GeV/c^2. The statistical significance of the peak calculated from a shape analysis is 5.1 sigma, and the differential cross-section for the gamma n to K^- Theta^+ reaction is estimated to be 12 +- 2 nb/sr in the photon energy range from 2.0 GeV to 2.4 GeV in the LEPS angular range by assuming the isotropic production of the Theta^+ in the gamma n center-of-mass system. The obtained results support the existence of the Theta^+.

Near-threshold Λ(1520) production by the γ→p→K +Λ(1520) reaction at forward K+ angles

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 or=5/2 or by a new reaction process, for example, an interference effect with the phi photoproduction having a similar bump structure in the cross sections.

Measurement of spin-density matrix elements for φ-meson photoproduction from protons and deuterons near threshold

The LEPS/SPring-8 experiment made a comprehensive measurement of the spin-density matrix elements for gammap->phip, gammad->phipn, and gammad->phid at forward production angles. A linearly polarized photon beam at E{sub g}amma=1.6-2.4 GeV was used for the production of phi mesons. The natural-parity Pomeron exchange processes remain dominant near threshold. The unnatural-parity processes of pseudoscalar exchange is visible in the production from the nucleons but is greatly reduced in the coherent production from deuterons. There is no strong E{sub g}amma dependence, but there is some dependence on momentum transfer. A small but finite value of the spin-density matrix elements reflecting helicity-nonconserving amplitudes in the t-channel is observed.

Cross sections and beam asymmetry for K;{+}Sigma;{*-} photoproduction from the deuteron at E_{gamma}=1.5-2.4 GeV.

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.

Near-threshold photoproduction of Lambda(1520) from protons and deuterons

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.

Forward coherent ϕ-meson photoproduction from deuterons near threshold

Abstract Differential cross sections and decay asymmetries for coherent ϕ -meson photoproduction from deuterons were measured for the first time at forward angles using linearly polarized photons at E γ = 1.5 – 2.4 GeV . This reaction offers a unique way to directly access natural-parity pomeron dynamics and gluon exchange at low energies. The cross sections at zero degrees increase with increasing photon energy. The decay asymmetries demonstrate a complete dominance of natural-parity exchange processes, showing that isovector unnatural-parity π -meson exchange is small. Nevertheless the deduced cross sections of ϕ -mesons from nucleons contributed by isoscalar t -channel exchange processes are not well described by the conventional pomeron model.

Backward-angle photoproduction of π0 mesons on the proton at Eγ=1.5–2.4 GeV

Abstract Differential cross sections and photon beam asymmetries for π 0 photoproduction have been measured at E γ = 1.5 – 2.4 xa0GeV and at the π 0 scattering angles, − 1 cos Θ c . m . − 0.6 . The energy-dependence of differential cross sections for u -channel π 0 production has been determined. An enhancement at backward angles is found above E γ = 1.9 xa0GeV . This is likely to be due to u -channel diagrams. Photon beam asymmetries have been obtained for the first time at backward angles, showing a strong angular dependence at E γ > 2.0 xa0GeV .