Schin Date

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.

Emc Effect and Hadron Production Off Nuclei at Large Transverse Momentum

We have investigated how the defonnation of the nuclear structure function per nucleon, which is known as the EMC effect, reveals itself in hard scatterings off nuclei. Our conclusion is that, in high PT hadron production reaction, the EMC effect as~ 0(10%) effect in nucleon structure function in nuclei cannot make the single hard scattering scheme survive and can have small influence upon the results within multiple hard scattering scheme. We also discuss the anomalous average PT increase observed in high energy heavy ion collisions.

A Study of Collision Frequency and Particle Density in Ultrarelativistic Nuclear Collisions by an Event Generator URASiMA

Local collision frequencies of produced hadrons in ultrarelativistic nucleus-nucleus collisions are studied by an event generator URASiMA. From these frequencies, local number densities of produced hadrons are derived. It turns out that thermalized high density local system is achievable in CERN-SPS energy region although its size is not always large. Time evolution and nuclear mass number dependence of these quantities, frequency and density, are also investigated.

HARPO, prototype of a gamma-ray polarimeter: Results of a polarised photon beam test between 1.7 and 74 MeV

Access to the photon polarisation in the 1-100 MeV energy range is a challenge for the next generation of space telescopes. The current telescopes in space are almost blind in this energy range, mainly due to the degradation of the angular resolution of e+e- pair and due to elastic scattering in the matter. Pair-conversion detector technologies as gaseous detectors are a promising alternative to the technologies based on tungsten-converter/thin-sensitive-layer stacks such as COS-B/EGRET/Fermi-LAT, firstly to improve the single-photon angular resolution and secondly for the polarisation information. The use of a time projection chamber (TPC) as a target and a tracking detector will improve by up to one order of magnitude the single-photon angular resolution (0.5° @ 100 MeV) with respect to the Fermi-LAT (5° @ 100 MeV), and by up to a factor of three with respect to what can be expected for silicon detectors (1.0-1.5° @ 100 MeV). With such a good angular resolution, a TPC can close the sensitivity gap at the level of 10^-6 MeV/cm².s) between 3 and 300 MeV despite having e lower sensitive mass. Furthermore, this good single-track angular resolution allows us to measure the linear polarisation fraction. The HARPO (Hermetic ARgon POlarimeter) detector prototype that we built is a high pressure (0.5-4 bar) low pile-up and low-diffusion gas detector. We will present the results of its high-statistics characterisation in the 1.7-74 MeV fully-polarised and non-polarised gamma-ray beam provided by the BL01 line at NewSUBARU. The excellent value of the polarisation asymmetry dilution factor that we measured opens the possibility of having a polarimeter in space working in the MeV-GeV energy range. In conclusion, we will present the design of a balloon-flight prototype ST3G (Self-Triggered Time projection chamber as a Gamma-ray Telescope) which is being developed. We will discuss its expected performance.

RF noise and longitudinal emittance in an electron storage ring

Dispersions in phase and energy of beam bunches in an electron storage ring are systematically derived by solving difference equations when RF phase noises are significant. Criteria on RF noises in the SPring‐8 storage ring are obtained. An effect of the discrete revolution on the natural emittance is shown.

Favorable Conditions of Θ+ Formation in γD Reaction

The first evidence for the pentaquark hadron Θ+, discovered by the LEPS collaboration at SPring-8,1) was subsequently confirmed in some other experiments. However, many other experiments failed to find the Θ+ signal. For surveys see Ref. 2). The situation became particularly dramatic after the recent publication of the high statistics results of the CLAS collaboration.3),4) Thus, the first experiment is designed to search for the Θ+ signal in γD → pK−nK+ in direct γn interactions at relatively low photon energy, Eγ = 1.7 − 3.5 GeV. Within the experimental significance, no Θ+ signal was observed. The CLAS null result for a given finite Θ+ decay width means large off-shell suppression in Θ+NK vertices of the amplitudes. The K∗-exchange amplitude may be additionally suppressed by the small value of gΘNK∗ coupling constant because it is not related directly the Θ+ decay width and therefore remains unconstraint. Therefore, the best way to check whether the Θ+ exists or not is to study the KN → Θ+ fusion reaction with a quasi-free kaon and a nucleon in the initial state. In this case the gΘNK coupling is fixed (for given ΓΘ), and there is no ambiguity with the off-shell form factor because all hadrons are almost on the mass shell. This situation may be realized in the reaction γD → Λ∗Θ+ → pK−nK+ (Λ∗ ≡ Λ(1520)) with the Θ+ showing up as a peak in the nK+ invariant mass distribution. There are several conditions which can enhance this effect.5) First, the pK− invariant mass must be close to the mass of Λ∗. In this case, the total amplitude is the coherent sum of two amplitudes with charged and neutral kaon exchange shown in Fig. 1. The dominance of the K∗ meson exchange in Λ∗ photoproduction6) results

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

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.

The Polarized-gamma p ---> K+ Lambda and polarized-gamma p ---> K+ Sigma0 reactions at forward angles with photon energies from 1.5-GeV 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.

Laser electron photon facility at SPring-8

Publisher Summary The laser electron photon (LEP) beam with the maximum energy of 2.4 GeV has been successfully produced at a newly developed beamline, BL33LEP at SPring-8. The beam intensity will be increased up to 1 x 107/ sec after the beam commissioning. The detector system has been constructed and the tests are underway. A detector system, optimized to carry out the photoproduction of Φ meson near threshold, is nearly completed. The maximum energy of the LEP is determined by the electron energy and the laser wavelength. The Laser Electron Photon facility at SPring-8 produces the maximum photon energy of 2.4 GeV with 350 nm Ar laser. Another feature is high polarization of photons. When the electron energy is high, LEPs are emitted in a narrow cone in the electron beam direction. The position and polarization of the laser can be monitored on both sides of the interaction region; at the Laser Hutch and the end of the beamline.