Takashi Nakano

A pair polarimeter for linearly polarized high energy photons

A high quality beam of linearly polarized photons of several GeV will become available with the coherent bremsstrahlung technique at JLab. We have developed a polarimeter which requires about two meters of the beam line, has an analyzing power of 20% and an efficiency of 0.02%. The layout and first results of a polarimeter test on the laser back-scattering photon beam at SPring-8/LEPS are presented.

Numerical Analysis of the Near-field Diffraction Pattern of a Small Aperture

Abstract We show the three-dimensional (3-D) distribution of the time-averaged Poynting vector in the near-field of a small aperture (radius < λ) using Bethes diffraction theory. Our numerical calculation shows that the characteristic of the near-field diffraction depends on the incident angle of the beam on the aperture. Specifically, the near-field diffraction corresponding to the normal incident has a quite different character than that of the oblique incident. We also discuss the variation of the propagation of the time-averaged Poynting vector with the aperture size, the wavelength and the distance from the aperture.

Resonances in QCD

Abstract We report on the EMMI Rapid Reaction Task Force meeting ‘ Resonances in QCD ’, which took place at GSI October 12–14, 2015. A group of 26 people met to discuss the physics of resonances in QCD. The aim of the meeting was defined by the following three key questions: • What is needed to understand the physics of resonances in QCD? • Where does QCD lead us to expect resonances with exotic quantum numbers? • What experimental efforts are required to arrive at a coherent picture? For light mesons and baryons only those with up , down and strange quark content were considered. For heavy–light and heavy–heavy meson systems, those with charm quarks were the focus. This document summarizes the discussions by the participants, which in turn led to the coherent conclusions we present here.

DISCOVERY OF THE STRANGENESS S=+1 PENTAQUARK

The theoretical motivation to look for S=+1 resonances is introduced and searches for such states in the literature are reviewed. The motivation for the recent pentaquark searches is discussed and the experimental details are described for several positive results of the S=+1 resonance with a mass of approximately 1.54 GeV, now called the Θ+. Finally, some new theoretical proposals to explain the recent experimental results are briefly mentioned, followed by a short discussion of future experimental prospects to learn more about pentaquark states.

Charmed baryon spectroscopy experiment at J-PARC

The heavy quark baryon spectroscopy is an essential ways to understand the hadron structure. In the case of the charmed baryon, two light quarks are isolated and form the diquark correlation due to the small color-magnetic interaction between the charm and lighter quarks. The excitation modes from the diquark correlation are reflected to the structure of the excited spectrum and the decay modes of excited states. In addition, by measuring the ratio of the cross section between the ground state Λc + and excited states, the information of the inside structure such as the configuration of the diquark correlation and the spin/isospin of excited states can be obtained [1]. From the systematic study of charmed baryons, the diquark correlation which is expectedly a degree of freedom to describe hadrons can be revealed. We propose a spectroscopy experiment [2] to investigate excited states of charmed baryons at the J-PARC high-momentum beam line. The high-intensity secondary pion beams of more than 10 7 per pulse are used with a momentum of 20 GeV/c. The beam momentum can be measured with the resolution of 0.1% by using the momentum-dispersive optics. The excitation energy, width and production cross section are measured by the missing mass technique. The charmed baryons are produced via the π − + p -> Yc* + + D* − reaction. The decay particles of the D* − meson are detected by the forward magnetic spectrometer system which has a large acceptance and a high-momentum resolution. Due to the forward large acceptance of the spectrometer, the decay measurements of the produced charmed baryons can also be performed for determining the absolute decay branching ratio and the spin/parity of the excited states. The systematic measurement of the excitation energy, production rate and decay properties of charmed baryons are performed at J-PARC.

Results and Prospects from LEPS and LEPS2

The linearly polarized photon beam at SPring‐8 is produced by backward‐Compton scattering of laser photons from 8 GeV electrons. The current LEPS facility studies photoproduction of hadrons in the forward angles, where the high linear polarization plays an essential role to decompose various reaction processes. In this talk, we report on the recent results from LEPS including the latest status of the Θ+ study. A plan to build the second LEPS beamline will be introduced.

φ-MESON PHOTOPRODUCTION AT THRESHOLD AND φ – N SCATTERING LENGTH

We show that the differential cross section dσ/dt of γp → φp reaction at the threshold has a finite value and it is crucial to the mechanism of the φ meson photoproduction and for the models of φN interaction.

DAQ system for LEPS experiment

A data acquisition (DAQ) system for experiments with the Laser Electron Photons at SPring-8 (LEPS) has been developed based on network distributed architecture. The system is designed so as to transfer digitized data with a 1-kHz trigger rate from various front-end electronics. Four local DAQ systems with VME board computers at the front end play a role to provide modularity, scalability, and flexibility. Data from each local DAQ system are sent through fast Ethernet to the data server. With this DAQ system, experiments to measure /spl phi/ photoproduction at LEPS started on May 2000. Signals from scintillation counters, drift chambers, and silicon strip detectors were successfully stored on disk. The performance of the present DAQ system is found to be good enough for the measurement.

Shielding Design Calculations for Laser Electron Photon Beamline of SPring-8

The shielding design of the laser electron photon beamline at SPring-8, which can generate the polarized photons of a few GeV with high intensity, has been performed and now under construction with two separated beamline hutches (enclosures of beam, laser and optical elements or experimental instruments). In addition to the shielding of the synchrotron radiation beamlines, the extremely high energy and high intensity laser electron photon beam brings other shielding problems. They are the issues of high intensity neutrons due to photonuclear reaction and high energy electrons, positrons due to pair creation in the upstream regions.

Time-of-Propagation Counter for the LEPS

We have tested a time-of-propagation (TOP) counter, which is a ring-image Cherenkov counter utilizing the position and propagation time of the Cherenkov light produced by particles moving in a quartz radiator. Position-sensitive photomultipliers (PMTs) are attached to the end of the quartz bar to detect the produced Cherenkov-ring image. The velocity of the charged particle was determined by comparing the time-of-propagation spectra of the Cherenkov light with the results of Monte Carlo simulation. From analysis of the test experiment using the 1.22-GeV/c electron beam at SPring-8, we determined the velocity of the electron beam to be 1.001 with a resolution of 0.002. On the basis of this study, we suggest a full setup for the detector for separating pions and kaons in the momentum region from 1.5 to 2.0 GeV/c for a beam angle of less than 6°, by more than four standard deviations.