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An internal radiator made of a carbon wire is employed to produce bremsstrahlung-photon beams for meson photoproduction experiments from circulating electrons in a synchrotron at the Research Center for Electron Photon Science, Tohoku University. The horizontal electron-beam size at the radiator location is determined from the decay rate of the resultant photon intensity as a function of the distance between the fixed position of the radiator and the centroid of the electron beam. The horizontal beam size at the radiator location is determined to be 0.74 mm with a statical error less than 0.01 mm for 1.3-GeV circulating electrons. The horizontal correlation between the position and momentum direction of the electron beam is also measured. The horizontal size and correlation are well-reproduced by the design values of the parameters of the synchrotron.

Coherent double neutral-pion photoproduction on the deuteron, γ d → π 0 π 0 d , has been experimentally studied at incident photon energies ranging from 0.75 to 1.15 GeV. The total cross section as a function of the γd center-of-mass energy shows resonance-like behavior, which peaks at approximately 2.47 and 2.63 GeV. The measured angular distribution of deuteron emission is rather flat, which cannot be reproduced by the kinematics of quasi-free π 0 π 0 production with deuteron coalescence. In π 0 d invariant-mass distributions, a clear peak is observed at 2.14 ± 0.01 GeV / c 2 with a width of 0.09 ± 0.01 GeV / c 2 . The spin-parity of this state is restricted to 1 + , 2 + or 3 − from the angular distributions of the two π 0 s. The present work shows strong evidence for the existence of an isovector dibaryon resonance with a mass of 2.14 GeV / c 2 . The 2 + assignment is consistent with the theoretically predicted D 12 state, and also with the energy dependence of the πd partial-wave amplitude P 2 3 for the π ± d → π ± d and π + d → p p reactions.

We measured missing mass spectrum of the ^{12}C(γ,p) reaction for the first time in coincidence with potential decay products from η^{'} bound nuclei. We tagged an (η+p) pair associated with the η^{'}N→ηN process in a nucleus. After applying kinematical selections to reduce backgrounds, no signal events were observed in the bound-state region. An upper limit of the signal cross section in the opening angle cosθ_{lab}^{ηp}

Photoproduction of the $\omega$ meson on the proton has been experimentally studied near the threshold. The total cross sections are determined at incident energies ranging from 1.09 to 1.15 GeV. The 1/2 and 3/2 spin-averaged scattering length $a_{\omega p}$ and effective range $r_{\omega p}$ between the $\omega$ meson and proton are estimated from the shape of the total cross section as a function of the incident photon energy: $a_{\omega p} = \left(-0.97^{+0.16_{\rm stat}}_{-0.16_{\rm stat}}{}^{+0.03_{\rm syst}}_{-0.00_{\rm syst}}\right)+i \left(0.07^{+0.15_{\rm stat}}_{-0.14_{\rm stat}}{}^{+0.17_{\rm syst}}_{-0.09_{\rm syst}}\right)$ fm and $r_{\omega p}=\left(+2.78^{+0.68_{\rm stat}}_{-0.54_{\rm stat}}{}^{+0.11_{\rm syst}}_{-0.13_{\rm syst}}\right)+i\left(-0.01^{+0.46_{\rm stat}}_{-0.50_{\rm stat}}{}^{+0.07_{\rm syst}}_{-0.00_{\rm syst}}\right)$ fm, resulting in a repulsive force. The real and imaginary parts for $a_{\omega p}$ and $r_{\omega p}$ are determined separately for the first time. A small $P$-wave contribution does not affect the obtained values.

The non-strange \(B=2\) system (dibaryon) has been studied using coherent neutral-meson photoproduction from the deuteron, \(\gamma d\rightarrow \pi ^0\pi ^0d\) and \(\gamma d\rightarrow \pi ^0\eta d\), measured with the FOREST detector at the ELPH facility. A rather flat angular distribution of deuteron emission and a peak in the \(\pi ^0d\) invariant-mass distribution give evidence for the 2.15-GeV isovector dibaryon. The excitation function of the total cross section as a function of the \(\gamma d\) center-of-mass energy for the former reaction shows the evidence for the existence of 2.47- and 2.63-GeV isoscalar dibaryons.

Although the ω meson is one of the best established hadrons, its fundamental properties such as the interaction with the nucleon is not known yet due to the difficulties in realizing scattering experiments. Recently, we have performed high-precision photoproduction experiment of the ω meson from the proton near the threshold. The total cross sections are determined at incident energies ranging from 1.08 to 1.15 GeV. The 1/2 and 3/2 spin-averaged scattering length aωN and effective range rωN between the ω meson and nucleon are extracted from the excitation function of the total cross section using a model with the final-state ωN interaction based on the Lippmann-Schwinger equation. The obtained values are aωp=(−0.97−0.16+0.16)+i(0.07−0.14+0.15) fm and rωp=(+2.78−0.54+0.68)+i(−0.01−0.50+0.46) fm, suggesting a strong repulsion force.

The current experimental information about interactions between the \(\eta \) meson and nucleon (N) is not enough to understand the nature of the \(\eta \) meson in a nucleus. We installed a new spectrometer and started an experiment to determine the \(\eta N\) scattering parameters at the Research Center for Electron Photon Science, Tohoku University. We present the current status of the experiment.

A transmittance monitor has been developed for the second tagged-photon beamline at the Research Center for Electron Photon Science, Tohoku University, Japan. In this beamline, an internal radiator is employed to produce the bremsstrahlung photon beam out of circulating electrons in a synchrotron. The transmittance, which is defined as the probability of finding a photon coming to the target position when an electron is detected with a photon-tagging counter, should be determined to deduce cross sections for photo-induced reactions. The developed monitor consists of a telescope of thin plastic scintillators with a positron and electron converter, and a dedicated circuit implemented in a field-programmable gate array chip. The transmittance can be measured with this monitor for high-intensity photon beams corresponding to 20 MHz tagging signals. The measured transmittance is found to be constant with respect to the photon intensity for each photon-tagging channel.

We have developed a charge-to-time converting (QTC) leading-edge discriminator for plastic-scintillator signals, which is packaged in a NIM-standard double-width module. The pulse width of the discriminator output is approximately proportional to the charge integration of input signals up to 40–50 pC. The non-linearity is 20%, 10%, 4%, and 2% at maximum for the input charges below 7 pC, for 7–10 pC, for 10–20 pC, and above 20 pC, respectively, for the minimum input charges larger than 2 pC. The discriminator could be suitable in the measurement of the timing and charge of plastic-scintillator signals together with a multi-hit time-to-digital converter (TDC) module.

A fast beam-profile monitor has been developed for high-energy photon beamlines at the Research Center for Electron Photon Science, Tohoku University. The position of the photon converted into an electron–positron pair in a 0.5 mm-thick aluminum plate is measured with two hodoscopes made of scintillating fibers with cross-sections of 3 × 3 mm 2 . Events in which charged particles are produced upstream are rejected with a charge veto plastic scintillator placed in front of the plate, and pair-production events are identified with a trigger plastic scintillator placed behind the plate. The position is determined by a developed logic module with a field-programmable gate array. The dead time for processing an event is 35 ns, and a high data acquisition efficiency ( ~ 100 % ) can be achieved with this monitor for high-intensity photon beams corresponding to 20 MHz tagging signals.

The B = 2 bound/resonance state (dibaryon) is an interesting object, which can be a molecule consisting of two baryons or a spatially compact hexaquark hadron object. The yd ^ n°n°d reaction has been experimentally investigated at incident energies ranging from 0.58 to 1.15 GeV to study non-strange dibaryons. The angular distributions of deuteron emission in the yd center-of-mass energy cannot be reproduced by quasi-free production of two neutral pions followed by deuteron coalescence. Additionally a 2.14-GeV peak is observed in the n°d invariant mass distribution. These suggest a sequential process such as yd ^ RIS ^ n°RIV ^ n°n°d with an isoscalar dibaryon RIS and an isovector dibaryon RIV. Since the mass of the observed isoscalar dibaryons are close to the sum of the nucleon (N) and nucleon resonance (N*) masses, an S-wave NN* molecule may play a role as a doorway to a dibaryon.

Differential cross sections and photon-beam asymmetries for the γ[over →]p→π^{-}Δ^{++}(1232) reaction have been measured for 0.7

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.

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.

Differential cross sections at $t={t}_{\text{min}}$ and decay asymmetries for the $\ensuremath{\gamma}p\ensuremath{\rightarrow}\ensuremath{\phi}p$ reaction have been measured using linearly polarized photons in the range 1.5 to 2.9 GeV. These cross sections were used to determine the Pomeron strength factor. The cross sections and decay asymmetries are consistently described by the $t$-channel Pomeron and pseudoscalar exchange model in the ${E}_{\ensuremath{\gamma}}$ region above 2.37 GeV. In the lower energy region, an excess over the model prediction is observed in the energy dependence of the differential cross sections at $t={t}_{\text{min}}$. This observation suggests that additional processes or interference effects between Pomeron exchange and other processes appear near the threshold region.

Photo-production of hadrons is experimentally studied using 0.8 – 1.2 GeV photon beams at the Research Center for Electron Photon Science, Tohoku University, Japan. Photons areproduced at the bremsstrahlung process by inserting a carbon fiber into the circulating electrons in the 1.3 GeV electron synchrotron. The energy of each photon is determined from the trajectory of the corresponding recoil electron. The energy resolution and time resolution of the current photontagging system is not enough high for the next-generation physics program. We have developed a new system called TaggerIIs. The angular resolution should be higher than 5° to reject the background particles coming from other points than the carbon fiber. The time resolution is required to be less than 200 ps to identify the bunch corresponding to a produced photon. We have adopted a combined system of a multi wire drift chamber and plastic-scintillator hodoscope for the new system. The wire chamber has a frame with length of 1000 mm along pole shape of a bending magnet and consists of 3 sense wire planes (X, X’, X) of wire length of 116 mm. A shape of the cell is hexagonal andthe spacing of the sense wires is 6 mm. The hodoscope consists of 60 plastic scintillators (EJ212) of 30 mm height and 5mm thickness. The width of each scintillator is designed from 5 mm to 30 mm. A performance of the new system has been tested using positron beams. As a result, weachieved that the detection efficiency of the wire chamber was 99%, the angler resolution was 0.60\ensuremath{\unicode{0x25E6 \textbf{and the timing resolution of the hodoscope was 165.8 ps.

We have measured the cross sections for $\pi^0\pi^\pm$ photoproduction on the nucleon in the deuteron with an electromagnetic calorimeter system, FOREST, at the Research Center for Electron Photon Science, Tohoku University, Japan. The incident photon energy ranges from 570 to 1150 MeV, corresponding to the $\gamma N$ center of mass, $W_{\gamma N}$, from 1400 to 1750 MeV. The baryon resonances coupling to $\rho^+ n$ and $\rho^-p$ are observed as intermediate states together with $\Delta$-Kroll-Ruderman background components. The angular distributions for the produced $\rho^+$ and $\rho^-$ mesons are convex downward and upward at high energies ($W_{\gamma N}\sim 1720$ MeV), respectively.

We develop an event mixing technique with a missing mass consistency (MMC) cut and a pion energy (PE) cut for the measurement of two-pion Bose-Einstein correlations (BEC) in reactions with only two identical pions produced in the final state. A multi mixing mode that allows one event to be mixed with multiple other events is proposed for the purpose of improving the statistics of mixed samples, and compared with the single mixing mode which requires one original event can be mixed only once. Numerical tests using the {\gamma}p -> {\pi}^0 {\pi}^0 p events are used to testify its ability to measure two-pion correlations. To improve the mixing speed, the optimal MMC cut window is also explored via simulations., Comment: Progress of Theoretical and Experimental Physics, 2017

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.

Baryon spectroscopy is important to understand Quantum Chromodynamics at low energies. In this purpose, a series of π 0 and η photoproduction experiments was carried out with an electro-magnetic calorimeter FOREST at Research Center for Electron Photon Science, Tohoku University. The incident tagged bremsstrahlung photon energy ranges from 550 to 1,150 MeV. The differential and total cross sections obtained for π 0 and η photoproduction processes on the proton are consistent with the SAID and MAID calculations. The analysis of π 0 and η photoproduction on the neutron is underway.

A beam profile monitor for 1-GeV region gamma-ray beams has been developed using an X-ray flat panel sensor. In order to increase its sensitivity to gamma-rays, a converter plate was placed on the input window. We tested its sensitivity to input gamma-ray photons with various types of converter plates. The output brightness of the pixels was found to be correlated to the number of electrons and positrons converted from the gamma-ray photons. A measured profile of the 1-GeV region gamma-ray beams obtained with a 1-mm thick lead plate was consistent with the beam profile measured with plastic scintillator hodoscopes. It suggested that the X-ray flat panel sensor was sufficiently applicable to the beam profile monitor for 1-GeV region gamma-ray beams.

Several beam-profile monitors have been developed for high-energy photon beamlines at the Research Center for Electron Photon Science, Tohoku University, Japan. The position of the photon converted into an electron-positron pair in a metal plate is measured in these monitors. In the current monitor, two layers of 16 scintillating-fiber hodoscopes with a cross section of a 3-mm square have been adopted for the position measurement. The effective area is a 48 mm square. Events in which charged particles are produced upstream are rejected with a charge veto plastic scintillator placed in front of the plate, and pair-production events are identified with a trigger plastic scintillator placed behind the plate. The position is determined by a logic circuit with a field-programmable gate array. The dead time for processing an event is several tens of ns, and a high data acquisition efficiency can be achieved with these monitors for high-intensity photon beams (approximately 400 MHz). In the new developing monitor, the scintillating-fiber hodoscopes are planned to be replaced with a multi-anode metal-packaged photomultiplier tube. The secondary emission electrons produced at the photoelectric surface by the electron-positron pair are utilized for the position measurement. In this contribution, the overview of the developed monitor and prospects of the developing monitor will be discussed.

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.

We have developed an event mixing technique to observe Bose-Einstein correlations (BEC) between two identical neutral pions produced in photo-induced reactions in the non-perturbative QCD energy region. It is found that the missing-mass consistency cut and the pion-energy cut are essential for the event mixing method to effectively extract BEC observables. A Monte Carlo (MC) simulation is used to validate these constraints and confirms the efficiency of this method. Our work paves the way for similar BEC studies at lower energies where the multiplicity of emitted bosons is limited.

The total cross sections were measured for coherent double neutral-pion photoproduction on the deuteron at incident energies below 0.9 GeV for the first time. No clear resonance-like behavior is observed in the excitation function for $W_{\gamma d}=2.38$--2.61 GeV, where the $d^*(2380)$ dibaryon resonance observed at COSY is expected to appear. The measured excitation function is consistent with the existing theoretical calculation for this reaction. The upper limit of the total cross section is found to be $0.034$~$\mu$b for the dibaryon resonance at $W_{\gamma d}=2.37$~GeV (90\% confidence level) in the $\gamma d \to \pi^0\pi^0 d$ reaction., Comment: submitted to Physics Letters B

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 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.

Total cross section for the double neutral pion photoproduction off the proton is presented in the incident photon energy range of 0.58 to 1.15 GeV. The data were accumulated in the years 2009–2010, recorded by a 4π EM calorimeter at ELPH, named FOREST. The number of recorded events obtained during this period is 1.6×109 for a hydrogen target. Two neutral pions are reconstructed via detecting their decay products, four gammas (2π 0 → 4γ ). Compared to the previous results obtained by other groups, our data are in good agreement with theirs within error bars.

The exclusive reaction γp→K(+)π(-)Σ(+) was measured for the first time using linearly polarized photons at beam energies from 1.85 to 2.96 GeV. Angular distributions in the rest frame of the K(+)π(-) system were fitted to extract spin-density matrix elements of the K(*0) decay. The measured parity spin asymmetry shows that natural-parity exchange is dominant in this reaction. This result clearly indicates the need for t-channel exchange of the κ(800) scalar meson.

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.

We report the measurement of differential cross sections for $\omega$ and $\eta'$ photoproduction from protons at backward angles ($-1.0 2.4$ GeV. A bump structure in the $\sqrt{s}$ dependence of the differential cross sections for $\eta'$ mesons was observed at $\sqrt{s}\sim$2.35 GeV.

The Laser-Electron-Photon facility at SPring-8 (LEPS) is now taking data for all hadronic reactions at compton-backscattered photon energies between 1.5 GeV and 2.4 GeV. A linearly polarized photon beam is incident on a liquid hydrogen target or nuclear targets. A large dipole spectrometer analyzes charged particles produced at forward angles. First results from SPring-8/LEPS will be presented with special emphasis on the study of photoproduction of φ mesons near threshold. Some prospects for the near future will also be presented and discussed in close connection with photoproduction of hyperons.

The cross sections was measured for coherent double neutral-pion photoproduction on the deuteron using the tagged photon beams from the 0.9 and 1.2 GeV primary electron beams at the Research Center for Electron Photon Science, Tohoku University, Japan. The four-momenta of the four photons from the two $\pi^0$ decays and deuteron are measured with the FOREST detector, and the $\gamma d\to \pi^0\pi^0d$ reaction was identified using 6-constraint kinematic fit (energy and three-momentum conservation, and two $\pi^0$ masses). The total cross section is found to be $18.4\pm 9.2$~nb for the $d^*(2380)$ dibaryon resonance at $W_{\gamma d}=2.37$~GeV, which is a candidate of the isoscalar $\Delta \Delta $ quasi-bound state ${\cal D}_{03}$, in the $\gamma d \to \pi^0\pi^0 d$ reaction. The measured excitation function is consistent with the existing theoretical calculation for this reaction. At high incident energies, $\pi^0d$ invariant-mass distributions show a peak at approximately 2.15 GeV, which can be attributed to a $N\Delta$ quasi-bound state ${\cal D}_{12}$. In this contribution, the properties (mass, width, and decay angular distribution) of this peak is presented.