Search

A search for the K − p p bound state (the lightest kaonic nucleus) has been performed using the γ d → K + π − X reaction at E γ = 1.5 – 2.4 GeV at LEPS/SPring-8. The differential cross section of the K + π − photo-production off deuterium ( d 2 σ / d cos θ K + l a b / d cos θ π − l a b ) has been measured for the first time in this energy region. A peak structure was searched for in the inclusive missing mass spectrum. A statistically significant peak structure was not observed in the region from 2.22 to 2.36 GeV/c2. The upper limits of the differential cross section of the K − p p bound state production were determined to be (0.17–0.55), (0.55–1.7) and (1.1–2.9) μb at 95% confidence level with the assumed widths of 20 MeV, 60 MeV and 100 MeV.

The $\ensuremath{\phi}\text{\ensuremath{-}}\mathrm{\ensuremath{\Lambda}}(1520)$ interference effect in the $\ensuremath{\gamma}p\ensuremath{\rightarrow}{K}^{+}{K}^{\ensuremath{-}}p$ reaction has been measured for the first time in the energy range from 1.673 to 2.173 GeV. The relative phases between $\ensuremath{\phi}$ and $\mathrm{\ensuremath{\Lambda}}(1520)$ production amplitudes were obtained in the kinematic region where the two resonances overlap. The measurement results support strong constructive interference when ${K}^{+}{K}^{\ensuremath{-}}$ 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 $\sqrt{s}=2.1\text{ }\text{ }\mathrm{GeV}$ bump structure in forward differential cross sections for $\ensuremath{\phi}$ 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.

The ϕ-Λ(1520) interference effect in the γp→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 ϕ 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 ϕ 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.

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 E γ = 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.

Differential cross sections and decay asymmetries for coherent $\phi$-meson photoproduction from deuterons were measured for the first time at forward angles using linearly polarized photons at $E_{\gamma}$= 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 $\pi$-meson exchange is small. Nevertheless the deduced cross sections of $\phi$-mesons from nucleons contributed by isoscalar t-channel exchange processes are not well described by the conventional Pomeron model., Comment: 14 pages, 6 figures. Final published version

Differential cross sections and photon beam asymmetries for π 0 photoproduction have been measured at E γ = 1.5 – 2.4 GeV 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 GeV . 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 GeV .

Photon beam asymmetries for the γ → p → K + Λ and γ → p → K + Σ 0 reactions have been measured for the photon energy range from 1.5 GeV to 2.4 GeV and for the angular range of 0 ° Θ cm K + 60 ° at the SPring-8/LEPS facility. The observed asymmetries are found to be positive and increase with increasing the photon energy. Present theoretical calculations do not reproduce the data. To extend our knowledge on the K + photoproduction mechanism including the effect of nucleon resonances, further theoretical development is required to fit both the cross section and asymmetry data.

The photo-production of ϕ mesons from Li, C, Al, and Cu at forward angles has been measured at E γ = 1.5 – 2.4 GeV . The number of events for incoherent ϕ photo-production is found to have a target mass number dependence of A 0.72 ± 0.07 in the kinematical region of | t | ⩽ 0.6 GeV 2 / c 2 . The total cross section of the ϕ –nucleon interaction, σ ϕ N , has been estimated as 35 −11 +17 mb using the A -dependence of the ϕ photo-production yield and a Glauber-type multiple scattering theory. This value is much larger than σ ϕ N in free space, suggesting that the ϕ properties might change in the nuclear medium.

The γ A → π 0 π 0 X reactions is one of the tools to effectively investigate partial restoration of chiral symmetry. An experiment was performed at the LEPS/SPring-8 facility using various nuclear targets. In this experiment, a 2π calorimeter consisting of 252 Lead/SCIFI modules was used to detect the photons coming from the π 0 π 0 decay. A preliminary analysis shows that π 0 π 0 X events are observed.

We have constructed a Time Projection Chamber (TPC) as the 4π detector for hadron photoproduction experiments at SPring-8. A feature of this TPC is to detect low momentum hyperons as close as 16 mm from the target center. We have studied basic performances of the TPC such as the resolution of dE / dx measurement and the spatial resolution with cosmic rays. The resolution of dE / dx measurement is 16% for minimum ionizing particles. The spatial resolution in pad plane is 280 μm and that along drift direction is 620 μm. The spatial resolution at the distance of 16 mm from the target is obtained to be 400 μm and 880 μm in pad plane and along drift direction respectively.

The GeV photon beam at SPring-8 is produced by backward-Compton scattering of laser photons from 8 GeV electrons. Polarization of the photon beam is ∼100% at the maximum energy with fully polarized laser photons. We report the status of the new facility and the prospect of hadron physics study with this high quality beam. Preliminary results from the first physics run are presented.

シャワー拡散層付エマルションチェンバーによるエネルギー決定法について報告する． この方法は，1995年の日ロ共同気球実験(RUNJOB-program, RUssia-Nippon JOint Balloon-program) で観測されたPeV陽子のエネルギー推定方法を基に開発され，1997年のRUNJOB実験より導入した．シャワー拡散層はカロリメーター層の下部に設置され，カロリメーター層で発達した電磁カスケードシャワーはシャワー拡散層で幾何学的に広がり，電子数密度が減少する．普通，電磁カスケードシャワーによるエネルギー決定の場合，シャワー最大発達点を捉えていないとそのエネルギーは信頼できない．しかし，本論文で報告する方法により，シャワー極大を捉えていない場合でもエネルギー決定が可能となった．約４ｃｍの厚さのシャワー拡散層があれば陽子の場合，６c.u.の鉛厚で数１０TeVまで，９c.u.あれば１００TeV近くまでシャワーエネルギーをσ～０．２以内の精度で決定できる．又，鉄核の場合，６c.u.の鉛厚で１００TeVまでのシャワーエネルギーをやはりσ～０．２以内の精度で決定できる．したがって，これまでシャワー極大を捉えていないためデータとして採用できないような高エネルギーシャワーについてもエネルギー決定が行え，特に高エネルギー領域の統計量を増やす事ができる．本論文では，主にシャワー拡散層付エマルションチェンバー内でのカスケードシャワーシミュレーション方法およびエネルギー決定精度について議論する．また実際にRUNJOB実験データに対して適用した例についても報告する．, We report a new energy determination method using the emulsion calorimeter with diffuser module. The idea of diffuser method is based on the energy estimation for a PeV proton observed by the first RUNJOB(RUssia-Nippon JOint Balloon-program) experiment in 1995 and we have used a new type of emulsion chamber with diffuser module since 1997. The diffuser module was set below calorimeter, and the electromagnetic cascade shower develops in the calorimeter, and spreads geometcally in the diffuser. In general, it is very hard to determine the energy unless we detect the shower maximum, but it might be possible to determine the shower energy with help of the diffuser module. We find the energy resolution is better than σ～0.2 for proton with several tens TeV and for iron with 100TeV if the path length of calorimeter and the diffuser are 6c.u. and 4cm respectively. With the thickness of calorimeter of 9c.u. with 4cm diffuser, we can estimate the energy up to nearly 100TeV for proton with the resolution of better than σ～0.2. In this report, we disscus the energy resolution in the present method based on the simulation calculation of cascade shower in the calorimeter with diffuser module. We show several events observed by RUNJOB experiment in 1997 and alloy the present method for the energy determination., 資料番号: SA0200107000

We report experimental results obtained by the emulsion chambers on board of the long duration balloon. We have been carrying out the trans-Siberian-continental balloon flight since 1995, and the results from 1995 to 1996 experiments are presented here. Total exposure of these two years amounts to 231.5 m 2 h at the average altitude of ∼32 km. The energy range covers 10–500 TeV for proton-primary, 3–70 TeV/n for helium-primary, and 1–5 TeV/n for Fe-group ( Z =26–28), though statistics of heavy components is not yet enough. Our preliminary data show that the spectra of the proton and the helium have nearly the same power indices ∼2.80, while those of heavier ones become gradually harder as the mass gets heavier, for instance the index is ∼2.70 for CNO-group and ∼2.55 for Fe-group. It is remarkable that a very high energy proton with multi-PeV is detected in 1995 experiment, and the estimated flux of this event coincides with a simple extrapolation from the energy spectrum with the power index 2.8 observed in the range 10–500 TeV. It indicates that there is no spectral break at around 100 TeV, in contrast to the maximum energy predicted by the current shock-wave acceleration model. This evidence requires some modification on the acceleration and/or propagation mechanism. Also we present all-particle spectrum and the average primary mass in the energy range 20–1000 TeV/particle. Our preliminary data show no drastic change in mass composition over the wide energy range, at least up to 1 PeV/particle, though the statistics is not yet enough to confirm it concretely. The flight performance and the procedure of the analysis, particularly the energy determination methods and the detection efficiency calculation are also given.

We report experimental results obtained by using a wide-gap type emulsion chamber flown in the first Japanese-Russo joint balloon project, called RUNJOB ( RU ssia- N ippon JO int B alloon-program). Two balloons were launched from Kamchatka in July 1995, and both were recovered successfully near the Volga River. The exposure time was 130 hours for the first flight and 168 hours for the second. The mean ceiling altitude, in both flights, was 32 km corresponding to 10 g/cm 2 . Total area of the emulsion chamber was 0.8 m 2 , and the thickness 0.385 and 2.28 collision m.f.p.'s for vertically incident proton- and iron-primaries, respectively. We detected 381 showers using Fuji-#200-type X-ray film; of these 174 showers were due to atmospheric secondary γ-rays, and the rest 207 came from nuclear components. The energy range covers 20∼200 TeV for proton-primary, 3∼30 TeV/nucleon for helium-primary, and 0.7∼5 TeV/nucleon for iron-primary. We give the energy spectra for various elements (proton, helium, …, iron) as well as the all-particle spectrum and the average mass of the cosmic-ray primaries.

The latest results of studies of the gamma-ray spectra recorded together with charged particles in the Russian-Nippon Joint Balloon (RUNJOB) experiment are presented. A comparison of the experimental spectra with the results of the calculations based on assumptions of different intensities of the gamma-ray spec- tra demonstrates that the previously published intensities of primary cosmic rays measured in the RUNJOB experiments are underestimated.

The LEPS/SPring-8 experiment made a comprehensive measurement of the spin-density matrix elements for $\gamma p \to \phi p$, $\gamma d \to \phi p n$ and $\gamma d \to \phi d$ at forward production angles. A linearly polarized photon beam at $E_{\gamma}$=1.6-2.4 GeV was used for the production of $\phi$ 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 $E_{\gamma}$-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., Comment: 25 pages, 14 figures

Differential cross sections and photon-beam asymmetries for the $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}}p\ensuremath{\rightarrow}{K}^{+}\ensuremath{\Lambda}(1520)$ reaction have been measured with linearly polarized photon beams at energies from the threshold to 2.4 GeV at $0.6l\mathrm{cos} {\ensuremath{\theta}}_{\mathrm{c}.\mathrm{m}.}^{K}l1$. A new bump structure was found at $W\ensuremath{\simeq}2.11\text{ }\text{ }\mathrm{GeV}$ in the cross sections. The bump is not well reproduced by theoretical calculations introducing a nucleon resonance with $J\ensuremath{\le}\frac{3}{2}$. This result suggests that the bump might be produced by a nucleon resonance possibly with $J\ensuremath{\ge}\frac{5}{2}$ or by a new reaction process, for example, an interference effect with the $\ensuremath{\phi}$ photoproduction having a similar bump structure in the cross sections.

Differential cross sections for {eta} photoproduction from protons have been measured at E{sub {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 {eta}{sup '},{omega}, and {pi}{sup 0} photoproductions. It is inferred that this unique structure in {eta} photoproduction is due to a baryon resonance with a large ss component that is strongly coupled to the {eta}N channel.

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{sup +/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{sup +} 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 0.6cos(theta){c.m.}{K}1. A new bump structure was found at W approximately 2.11 GeV in the cross sections. The bump is not well reproduced by theoretical calculations introducing a nucleon resonance with Jor=3/2. This result suggests that the bump might be produced by a nucleon resonance possibly with Jor=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.

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.

The {gamma}d{yields}K{sup +}K{sup -}pn reaction has been studied to search for the evidence of the {theta}{sup +} by detecting K{sup +}K{sup -} pairs at forward angles. The Fermi-motion-corrected nK{sup +} invariant mass distribution shows a narrow peak at 1.524{+-}0.002+0.003 GeV/c{sup 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{yields}K{sup -}{theta}{sup +} reaction is estimated to be 12{+-}2 nb/sr in the photon energy range from 2.0 to 2.4 GeV in the LEPS angular range by assuming the isotropic production of the {theta}{sup +} in the {gamma}n center-of-mass system. The obtained results support the existence of the {theta}{sup +}.

Differential cross sections for {gamma}p{yields}K{sup +}{lambda}(1405) and {gamma}p{yields}K{sup +}{sigma}{sup 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

Cross sections for the {gamma}p{yields}K{sup +}{lambda} reaction were measured at backward angles using linearly polarized photons in the range 1.50 to 2.37 GeV. In addition, the beam asymmetry for this reaction was measured for the first time at backward angles. The {lambda} was detected at forward angles in the LEPS spectrometer via its decay to p{pi}{sup -} and the K{sup +} was inferred using the technique of missing mass. These measurements, corresponding to kaons at far backward angles in the center-of-mass frame, complement similar CLAS data at other angles. Comparison with theoretical models shows that the reactions in these kinematics provide further opportunities to investigate the reaction mechanisms of hadron dynamics.

Differential cross sections and photon-beam asymmetries have been measured for the {gamma}(vector sign)n{yields}K{sup +}{sigma}{sup -} and {gamma}(vector sign)p{yields}K{sup +}{sigma}{sup 0} reactions separately using liquid deuterium and hydrogen targets with incident linearly polarized photon beams of E{sub {gamma}}=1.5-2.4 GeV at 0.6

Differential cross sections and photon-beam asymmetries for the {gamma}-vectorp{yields}K{sup +}{lambda} and {gamma}-vectorp{yields}K{sup +}{sigma}{sup 0} reactions have been measured in the photon energy range from 1.5 to 2.4 GeV and in the angular range from {theta}{sub c.m.}=0 deg. to 60 deg. of the K{sup +} 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 cos{theta}{sub c.m.} 0.9. In the K{sup +}{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{sup +}{lambda} reaction, but not for the K{sup +}{sigma}{sup 0} reaction.

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 0.6cos ThetacmK1. 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.

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.

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 0.6cos((theta(c.m.)(K+))1.0 by using linearly polarized photons at the Laser-Electron-Photon facility at SPring-8 (LEPS). The observed asymmetries are positive and gradually increase with rising photon energy. The data are not consistent with theoretical predictions based on tree-level effective-Lagrangian approaches. Including the new results in the development of the models is, therefore, crucial for understanding the reaction mechanism and to test the presence of baryon resonances which are predicted in quark models but are thus far undiscovered.

The $\ensuremath{\gamma}n\ensuremath{\rightarrow}{K}^{+}{K}^{\ensuremath{-}}n$ reaction on $^{12}\mathrm{C}$ has been studied by measuring both ${K}^{+}$ and ${K}^{\ensuremath{-}}$ at forward angles. A sharp baryon resonance peak was observed at $1.54\ifmmode\pm\else\textpm\fi{}0.01\text{ }\text{ }\mathrm{G}\mathrm{e}\mathrm{V}/{c}^{2}$ with a width smaller than $25\text{ }\text{ }\mathrm{M}\mathrm{e}\mathrm{V}/{c}^{2}$ and a Gaussian significance of $4.6\ensuremath{\sigma}$. 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 ($uudd\overline{s}$) 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.