N. Matsuoka

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.

Beam polarization asymmetries for the p(gamma, K+) Lambda and p(gamma, K+) Sigma0 reactions at E(gamma) = 1.5 Gev - 2.4 GeV

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<cos((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 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^+.

Projectile break-up and the continuum spectra of the (3He, d) reaction

Abstract Deuteron continuum spectra induced by 70,90 and 110 MeV 3He particles have been measured for a number of target nuclei. Each spectrum shows a broad bump at forward angles, the peak energy of which is about 2 3 of the incident 3He energy and the cross section is nearly proportional to the radius of the target nucleus. The results are compared with projectile break-up calculations reflecting the internal deuteron momentum distribution in 3He. The good fit of the calculations shows that the continuum bump originates mainly from the projectile break-up process, and the mass number dependence of the cross section indicates that such a break-up reaction occurs at the peripheral region of the target nucleus.

Deuteron break-up in the fields of nuclei at 56 MeV

Abstract Proton continuum spectra induced by 56 MeV deuterons have been measured for 14 elements ranging from 12C to 209Bi. Each spectrum shows a prominent bump at forward angles due to the break-up of the incident deuteron. The peak energy of the bump is about half the deuteron energy for light elements and increases slightly for heavier elements. Projectile break-up calculations reproduce well the spectral shapes and the angular dependence of the bump spectra. The characteristics of the bump spectra are determined by the proton momentum distribution in the deuteron. The break-up cross section increases with target mass and is nearly proportional to ( A 1 3 + 0.8) 2 . The target mass dependence indicates that all partial waves smaller than the grazing angular momentum contribute to the deuteron break-up cross section at this energy. The total break-up cross section amounts to 24–35% of the deuteron total reaction cross section.

Matching of a beam line and a spectrometer New beam line project at RCNP

Abstract A magnetic spectrometer combined with an accelerator is a powerful tool for the precise measurements of nuclear-reaction spectra. By applying matching techniques to the beam line connecting an accelerator and the spectrometer, it is possible to compensate the spectrum-line broadening effects caused by the beam momentum spread and reaction kinematics, and thus full capability of high resolving power of a magnetic spectrometer can be extracted. In addition to the usual matching methods, like lateral dispersion matching and focus matching, the importance of angular dispersion matching is discussed for a reaction with finite kinematic factor (K ≠ 0), which is important if the resolution of the scattering angle is needed. Based on the discussed principles, a new beam line called “WS course” is designed in order to realize the beam matching between a cyclotron and a spectrometer with a large dispersion at RCNP, Osaka.

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.

Optical model and folding model potentials for elastic scattering of 56 MeV polarized deuterons

Abstract Differential cross sections σ and all four analyzing powers Ay, Axx, Ayy and Axz have been measured for deuteron elastic scattering from 12C, 16O, 40Ca, 58Ni, 90Zr, 118Sn, 144Sm and 208Pb at 56 MeV. The tensor analyzing power Axz was measured using horizontally polarized deuterons. These data were analyzed in terms of an optical potential containing central, spin-orbit and TR type tensor terms. A complex TR tensor potential was necessary to explain Axz and the quantity X 2 = (2A xx + A yy ) √3 . The optical potentials obtained were compared with exact folding model calculations. The real part of the TR tensor potential is generally smaller than the folding model potential, especially for heavier nuclei. The imaginary TR potential is comparable with the folding model for lighter nuclei and is smaller than the folding model for heavier nuclei. The folding model TR potential overestimates the amplitudes of the Axz and X2 data. Features of the σ, Ay and Ayy data at large angles are discussed in connection with nuclear rainbow scattering.