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S. Fukui

Publications and source records attributed to S. Fukui.

16 recordsLinked to original sources
arXiv2018

Differential cross section and photon-beam asymmetry for the gamma p -> pi- Delta++(1232) reaction at forward pi- angles for Egamma=1.5-2.95 GeV

Differential cross sections and photon-beam asymmetries for the gamma p -> pi- Delta++(1232) reaction have been measured for 0.7<cos(theta)<1 and Egamma=1.5-2.95 GeV at SPring-8/LEPS. The first-ever high statistics cross section data are obtained in this kinematical region, and the asymmetry data for 1.5<Egamma(GeV)<2.8 are obtained for the first time. This reaction has a unique feature for studying the production mechanisms of a pure $u\bar{u}$ quark pair in the final state from the proton. Although there is no distinct peak structure in the cross sections, a non-negligible excess over the theoretical predictions is observed at Egamma=1.5-1.8 GeV. The asymmetries are found to be negative in most of the present kinematical regions, suggesting the dominance of pi-exchange in the t-channel. The negative asymmetries at forward meson production angles are different from the asymmetries previously measured for the photoproduction reactions which produce a $d\bar{d}$ or an $s\bar{s}$ quark pair in the final state. Advanced theoretical models introducing nucleon resonances and additional unnatural parity exchanges are needed to reproduce the present data.

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arXiv2017

Photoproduction of $\Lambda$ and $\Sigma^{0}$ hyperons off protons with linearly polarized photons at $E_{\gamma} = 1.5-3.0$ GeV

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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arXiv2017

First measurement of coherent $\phi$-meson photoproduction from helium-4 near threshold

The differential cross sections and decay angular distributions for coherent $\phi$-meson photoproduction from helium-4 have been measured for the first time at forward angles with linearly polarized photons in the energy range $E_{\gamma} = \text{1.685-2.385 GeV}$. Thanks to the target with spin-parity $J^{P} = 0^{+}$, unnatural-parity exchanges are prohibited, and thus natural-parity exchanges can be investigated clearly. The decay asymmetry with respect to photon polarization is shown to be very close to the maximal value. This ensures the dominance ($> 94\%$) of natural-parity exchanges in this reaction. To evaluate the contribution from natural-parity exchanges to the forward cross section ($\theta = 0^\circ$) for the $\gamma p \rightarrow \phi p$ reaction near the threshold, the energy dependence of the forward cross section ($\theta = 0^\circ$) for the $\gamma {^{4}\text{He}} \rightarrow \phi {^{4}\text{He}}$ reaction was analyzed. The comparison to $\gamma p \rightarrow \phi p$ data suggests that enhancement of the forward cross section arising from natural-parity exchanges, and/or destructive interference between natural-parity and unnatural-parity exchanges is needed in the $\gamma p \rightarrow \phi p$ reaction near the threshold.

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arXiv2017

Differential cross section and photon beam asymmetry for the gamma p -> pi+ n reaction at forward pi+ angles at Egamma=1.5-2.95 GeV

Differential cross sections and photon beam asymmetries for the gamma p -> pi+ n reaction have been measured for 0.6<cos(theta)<1 and Egamma=1.5-2.95 GeV at SPring-8/LEPS. The cross sections monotonically decrease as the photon beam energy increases for 0.6<cos(theta)<0.9. However, the energy dependence of the cross sections for 0.9<cos(theta)<1 and Egamma=1.5-2.2 GeV (W=1.9-2.2 GeV) is different, which may be due to a nucleon or Delta resonance. The present cross sections agree well with the previous cross sections measured by other groups and show forward peaking, suggesting significant t-channel contributions in this kinematical region. The asymmetries are found to be positive, which can be explained by rho-exchange in the t-channel. Large positive asymmetries in the small |t| region, where the rho-exchange contribution becomes small, could be explained by introducing pi-exchange interference with the s-channel.

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arXiv2016

Interference effect between $\phi$ and $\Lambda(1520)$ production channels in the $\gamma p \rightarrow K^+K^-p$ reaction near threshold

The $\phi$-$\Lambda(1520)$ interference effect in the $\gamma p\to K^+K^-p$ reaction has been measured for the first time in the energy range from 1.673 to 2.173 GeV. The relative phases between $\phi$ and $\Lambda(1520)$ 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 $\phi$ photoproduction. This fact suggests possible exotic structures such as a hidden-strangeness pentaquark state, a new Pomeron exchange and rescattering processes via other hyperon states.

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arXiv2013

Search for $K^-pp$ bound state via $\gamma d \rightarrow K^+ \pi^-X$ reaction at $E_\gamma=1.5-2.4$ GeV

A search for $K^-pp$ bound state (the lightest kaonic nucleus) has been performed using the $\gamma d \rightarrow K^+ \pi^- \rm{X}$ reaction at E$_\gamma$=1.5-2.4 GeV at LEPS/SPring-8. The differential cross section of $K^+ \pi^-$ photo-production off deuterium has been measured for the first time in this energy region, and a bump structure was searched for in the inclusive missing mass spectrum. A statistically significant bump structure was not observed in the region from 2.22 to 2.36 GeV/$c^2$, and the upper limits of the differential cross section for the $K^-pp$ bound state production were determined to be 0.1$-$0.7 $\mu$ b (95$%$ confidence level) for a set of assumed binding energy and width values.

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arXiv2013

Backward-angle photoproduction of $ω$ and $η'$ mesons from protons at $E_γ=1.5-3.0$ GeV

We report the measurement of differential cross sections for $ω$ and $η'$ photoproduction from protons at backward angles ($-1.0<\cosΘ_{C.M}^{X}<-0.8$) using linearly polarized photons at $E_γ=$$1.5-3.0$ GeV. Differential cross sections for $ω$ mesons are larger than the predicted $u$-channel contribution in the energy range $2.0\leq\sqrt{s}\leq2.4$ GeV. The differential cross sections for $ω$ and $η'$ 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 $η'$ mesons was observed at $\sqrt{s}\sim$2.35 GeV.

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arXiv2010

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 $γp \to ϕp$, $γd \to ϕp n$ and $γd \to ϕd$ at forward production angles. A linearly polarized photon beam at $E_γ$=1.6-2.4 GeV was used for the production of $ϕ$ mesons. The natural-parity Pomeron exchange processes remains dominant nearthreshold. The unnatural-parity processes of pseudoscalar exchange is visible in the production from nucleons but is greatly reduced in the coherent production from deuterons. There is no strong $E_γ$-dependence, but 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.

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arXiv2010

Measurement of the incoherent $γd \to ϕp n$ photoproduction near threshold

We report measurements of differential cross sections and decay asymmetries of incoherent $ϕ$-meson photoproduction from the deuteron at forward angles using linearly polarized photons at \Eg=1.5-2.4 GeV. The nuclear transparency ratio for the deuteron shows a large suppression, and is consistent with the A-dependence of the ratio observed in a previous measurement with nuclear targets. The reduction for the deuteron cannot be adequately explained in term of isospin asymmetry. The present results suggest the need of refining our understanding of the $ϕ$-N interaction within a nucleus.

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arXiv2009

Backward-angle photoproduction of eta meson from protons at E_gamma = 1.6 - 2.4 GeV

Differential cross sections for eta photoproduction off protons have been measured at E_gamma = 1.6 - 2.4 GeV in the backward direction. A bump structure has been observed above 2.0 GeV in the total energy. No such bump is observed in cross sections for eta', omega and pi0 photoproductions. It is inferred that this unique structure in eta photoproduction is due to a baryon resonance with a large portion of ss-bar strongly coupled to the etaN channel.

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arXiv2009

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

Photoproduction of $Λ$(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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arXiv2008

Evidence of the Θ^+ in the γd \to K^+K^-pn reaction

The γd \to K^+K^-pn reaction has been studied to search for the evidence of the Θ^+ 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 σ, and the differential cross-section for the γn \to K^- Θ^+ 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 Θ^+ in the γn center-of-mass system. The obtained results support the existence of the Θ^+.

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arXiv2008

Photoproduction of Lambda(1405) and Sigma^{0}(1385) on the proton at E_γ= 1.5-2.4 GeV

Differential cross sections for $γp \to K^+Λ(1405)$ and $γp \to K^+Σ^0(1385)$ reactions have been measured in the photon energy range from 1.5 to 2.4 GeV and the angular range of $0.8<\cos(Θ)<1.0$ for the $K^+$ scattering angle in the center-of-mass system. This data is the first measurement of the $Λ(1405)$ photoproduction cross section. The lineshapes of \LamS measured in $Σ^+π^-$ and $Σ^-π^+$ decay modes were different with each other, indicating a strong interference of the isospin 0 and 1 terms of the $Σπ$ scattering amplitudes. The ratios of \LamS production to \SigS production were measured in two photon energy ranges: near the production threshold ($1.5<E_γ<2.0$ GeV) and far from it ($2.0 <E_γ<2.4$ GeV). The observed ratio decreased in the higher photon energy region, which may suggest different production mechanisms and internal structures for these hyperon resonances.

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arXiv2008

Forward coherent $ϕ$-meson photoproduction from deuterons near threshold

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.

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arXiv2006

Differential cross section and photon beam asymmetry for the gamma n -> K+ Sigma- reaction at Egamma=1.5-2.4 GeV

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 Egamma=1.5-2.4 GeV at 0.6<cosTheta<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 the K*-exchange. A large difference between the asymmetries for the K+Sigma- and K+Sigma0 reactions can not be explained by simple theoretical considerations.

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