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SuperKEKB is an electron–positron asymmetric-energy double-ring collider, which was built in Japan. It has been operated to explore new phenomena in B-meson decays. Hence, extremely higher luminosity is required. A collision scheme of low emittance with a large Piwinski angle called a “nano-beam scheme” has been adopted to achieve higher luminosity by squeezing the vertical beta function at the interaction point to be smaller than the bunch length. A “crab waist collision scheme” proposed by P. Raimondi et al. has also been adopted to improve the luminosity performance. The article presents an overview of the operation of the nano-beam and crab waist collision schemes at SuperKEKB.

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.

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.

A timing system provides high-precision signals to allow the controls over a variety of hardware and software components in the accelerator complex. This is guaranteed by the radio frequency (RF) and trigger signal synchronization for subsystems such as klystrons, pulsed magnets, and beam monitors. The main trigger signal should be distributed throughout the facility and repeated at the beam repetition rate. This trigger signal is usually generated by the same phase of an AC power line to follow the source of the fluctuation of an electrical grid and reduce the unwanted variation of the beam quality. To fulfill the needs of the multi-accelerator facility at KEK, apart from the normal trigger synchronization and bucket selection injection control, a beam operation scheme called the pulse-to-pulse modulation is utilized; hence, the complexity of the timing system increases. Uncertainty in the system and a trigger signal delivery error caused by a drastic AC power line drift are observed. Further, the effort to establish a reliable timing system at KEK and several solutions to improve the system robustness are presented.

An electromagnetic calorimeter complex, FOREST, has been constructed for meson photoproduction experiments at the Research Center for Electron Photon Science, Tohoku University. It consists of three types of calorimeters, which are made of pure cesium-iodide crystals, lead scintillating-fiber modules, and lead glass Cherenkov counters. Each calorimeter is equipped with a plastic scintillator hodoscope to identify charged particles. The design and performance of FOREST are described. The energy responses of test calorimeters have been investigated by using 100–800 MeV positron beams. The energy resolutions of the three calorimeters are found to be approximately 3%, 7%, and 5% for 1-GeV photons, respectively. A cryogenic hydrogen/deuterium target system fitted to the FOREST experiments and a newly developed data acquisition system are also presented.

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.

A new field-programmable gate array (FPGA)-based time-to-digital converter (TDC) with a wide dynamic range greater than 20 ms has been developed to monitor the timing of various pulsed devices in the trigger timing distribution system of the KEKB injector linac for the Super KEK B-factory project. The pulsed devices are driven by feeding regular as well as any irregular (or event-based) timing pulses. The timing pulses are distributed to these pulsed devices along the linac beam line with fiber-optic links on the basis of the parameters to be set pulse-by-pulse in the event-based timing and control system within 20 ms. For monitoring the timing as precisely as possible, a 16-ch FPGA-based TDC has been developed on a Xilinx Spartan-6 FPGA equipped on VME board with a resolution of 1 ns. The resolution was achieved by applying a multisampling technique, and the accuracies were 2.6 ns (rms) and less than 1 ns (rms) within the dynamic ranges of 20 ms and 7.5 ms, respectively. The various nonlinear effects were improved by implementing a high-precision external clock with a built-in temperature-compensated crystal oscillator.

The design of a 100 km circular e⁺e⁻ collider with extremely high luminosity is an important component of the global Future Circular Collider (FCC) study hosted by CERN. FCC-ee is being designed to serve as Z, W, H and top factory, covering beam energies from 45.6 to 175 GeV. For the injectors, the Z-operation is the most challenging mode, due to the high total charge and low equilibrium emittance in the collider at this energy. Thus, fulfilling the Z-mode will also meet the demands for all other modes of FCC-ee. This goal can be achieved by using a 6 GeV NC linac with an S-band RF frequency of 2.856 GHz and a repetition rate of 100 Hz. This linac will accelerate two bunches per RF pulse, each with a charge of 6.5 nC. Positrons will be generated by sending 4.46 GeV e⁻ onto a hybrid target so that the e⁺ created can still be accelerated to 1.54 GeV in the remaining part of the same linac. The emittance of the e⁺ beam will then shrink to the nm level in a 1.54 GeV damping ring. After damping, the e⁺ will be reinjected into the linac and accelerated to 6 GeV. The e⁻ and e⁺ will then be accelerated alternately to 45.6 GeV in the booster, before they are injected into the collider., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

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.

International audience; The hybrid positron source made of a crystal-radiator, as a source of channeling radiation, and an amorphous converter is briefly described. Optimization of the heating effects and thermal shocks led to a granular converter made of small tungsten spheres. Simulation results with application to two linear collider projects CLIC and ILC are presented. A test operated with this kind of source at KEK is described and the results gathered last year, reported. Plans for the future are briefly described.

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

The development of terahertz (THz) technology and science relevant to accelerator-based sources has been rapid, based on laser and semiconductor physics. To assure further progress of this scientific field, extremely intense radiation, involving gigawatt-class peak power, is needed. In addition to discussing the prospects for typical accelerator-based THz sources, such as coherent synchrotron radiation (CSR) and free electron lasers (FELs), novel, advanced THz sources employing isochronous-ring and bunched-FEL approaches are proposed.

A test accelerator as coherent terahertz (THz) source project (t-ACTS) has been under development at Tohoku University, in which generation of intense coherent THz radiation from the very short electron bunch less than 100 fs will progress. Such short bunch will be utilized to generate broad-band coherent radiation from bending magnets in a ring of which isochronous optics is adapted to preserve the short bunch length. In addition, narrow-band coherent THz radiation from an undulator has been considered. We describe design of the test accelerator and expected performance of undulator radiation.

The second GeV bremsstrahlung photon beamline has been constructed for meson photo-production experiments at Research Center for Electron Photon Science, Tohoku University, where a 1200 MeV electron synchrotron called STretcher Booster (STB) has been in operation primarily for Quark Nuclear Physics. An internal radiator is employed to produce the bremsstrahlung photon beam out of circulating electrons in the STB ring. A photon tagging system is placed inside one of the bending magnets of the ring. The tagged energy of the photon beam ranges from 740 to 1150 MeV for 1200 MeV operation of the STB synchrotron. The details of the tagging system and the characteristics of the photon beam are described.

The total cross sections for the π+π− photoproduction on the deuteron were measured in an energy range of 0.8 to 1.1 GeV. The obtained total cross section for the quasi-free π+π− photoproduction on the deuteron was about 60 % of those on the free proton. The cross section for Δ++Δ− photoproduction was derived from the non-quasi-free π+π− photoproduction events. It was smaller than the previous data.

Differential and total cross sections of the D(γ, η)pn reaction have been measured for photon energies between 600 and 1150 MeV. The data provide the information of the γn → ηn reaction above Eγ ≥ 800 MeV. The total cross section shows a narrow resonance structure peaked at Eγ = 1020 MeV. The angular distribution coefficients together with the total cross section are compared with the calculation of the isobar model by using the impulse approximation. Although the spin and parity of the resonance cannot be determined unambiguously, a negative parity assignment gives better fit to the data.

A beam test has been performed on a prototype electromagnetic calorimeter of bismuth silicate BSO crystals for the first time using electrons of 0.5–3.0 GeV. The calorimeter consists of nine BSO crystals, 22 mm × 22 mm × 180 mm (15.6 radiation lengths), arranged in a 3 × 3 array. Each crystal is coupled to a small photomultiplier tube of 20 mm in diameter. The obtained energy resolution is σ E / E = ( 0.023 ± 0.003 ) / E ( GeV ) ⊕ ( 0.017 ± 0.002 ) for incident electrons of the energy E in GeV. The position information for an electron injected onto the center crystal of the calorimeter array is given with energy deposit signals from nine BSO crystals. The RMS position resolution σ x is parameterized as σ x = [ ( 3.2 ± 0.1 ) / E ( GeV ) ⊕ ( 1.3 ± 0.1 ) ] mm at the center of the crystal. Misidentification probability of pions to electrons is found to be ∼ 10 - 3 for the pion momentum ranging from 1.0 to 3.0 GeV/c.

The γ n → K 0 Λ process plays an important role in the study of strangeness production by the electromagnetic interaction. We have investigated the quasi-free production reaction on a 12 C target in the threshold region ( E γ = 0.8 – 1.1 GeV ) at the Laboratory of Nuclear Science, Tohoku University. Preliminary results are presented.

The Pulse-to-Pulse Modulation of Injector Linac at KEK is developed for efficient injections. It automatically switches the beam mode with the real-time process. We integrated this system in 2009 to simultaneously perform the top-up filling into more than one rings with only one injector. The system is based on the Event Timing System. Recently, we upgraded the configuration of Event Timing System to satisfy the complicated requirements of SuperKEKB and developed the test procedure to understand stability of real-time process. The failure of real-time process at Main Trigger Station is intentionally caused when we shorten the allowed computing time as the stress test. The reasonable dependence between the failure rate and the allowed computing time is determined. It indicates we can evaluate the failure rate in case of normal operation at Main Trigger Station.

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 photoproduction process of neutral kaons on a liquid deuterium target is investigated near the threshold region, ${E}_{\ensuremath{\gamma}}=0.8\text{\ensuremath{-}}1.1$ GeV. ${K}^{0}$ events are reconstructed from positive and negative pions, and differential cross sections are derived. Experimental momentum spectra are compared with those calculated in the spectator model using a realistic deuteron wave function. Elementary amplitudes as given by recent isobar models and a simple phenomenological model are used to study the effect of the new data on the angular behavior of the elementary cross section. The data favor a backward-peaked angular distribution of the elementary $n(\ensuremath{\gamma},{K}^{0})\ensuremath{\Lambda}$ process, which provides additional constraints on current models of kaon photoproduction. The present study demonstrates that the $n(\ensuremath{\gamma},{K}^{0})\ensuremath{\Lambda}$ reaction can provide key information on the mechanism of the photoproduction of strangeness.

A resonance-like structure has been observed in the total cross section for the γd → ηnp reaction at E γ ∼ 1.02 GeV. The experiment was conducted at Laboratory of Nuclear Science, Tohoku University, by utilizing a tagged photon beam provided with an internal radiator in a 1.2 GeV electron synchrotron. A solid deuterium target was employed as well as a solid hydrogen target to extract information on the process γn → ηn. A clear bump has been observed in the total cross section for the γn’ → ηn reactio where n’ means a neutron in Fermi motion. A simple analysis gives the position of about 1670 MeV and the width of less than 40 MeV for the bump, respectively, taking into account the Fermi motion. There is no bump structure observed, in contrast, in the γp→ ηp process at the same energy region. The U-spin conservation may naturally account for this phenomenon, if the bump is an indication of a hidden-strangeness pentaquark baryon N 5(1670) in the anti-decuplet state.

\ensuremath{\eta} mesons emitted from the photoproduction off protons were measured for $0.70\ensuremath{\leqslant}{E}_{\ensuremath{\gamma}}\ensuremath{\leqslant}1.15$ GeV. The $\ensuremath{\gamma}p\ensuremath{\rightarrow}\ensuremath{\eta}p$ reaction process was separated from the $\ensuremath{\pi}\ensuremath{\eta}p$ final state by the momentum difference of \ensuremath{\eta} mesons for ${E}_{\ensuremath{\gamma}}\ensuremath{\geqslant}930$ MeV. The angular distributions obtained and the excitation function of the $\ensuremath{\gamma}p\ensuremath{\rightarrow}\ensuremath{\eta}p$ reaction are essentially in good agreement with previously reported ones and are well explained by \ensuremath{\eta}-MAID calculations. For the $\ensuremath{\gamma}p\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\eta}N$ reaction, the cross section increases monotonically with increasing photon energy, and it almost reaches the value of the $\ensuremath{\gamma}p\ensuremath{\rightarrow}\ensuremath{\eta}p$ reaction at ${E}_{\ensuremath{\gamma}}~1.15$ GeV. The momentum distributions are compared with kinematic calculations. It is suggested that the sequential decay process through the ${P}_{33}$(1232) resonance [$\ensuremath{\gamma}p\ensuremath{\rightarrow}\ensuremath{\eta}{P}_{33}(1232)\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\eta}N$] contributes to the $\ensuremath{\gamma}p\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\eta}N$ reaction, in addition to the sequential process $\ensuremath{\gamma}p\ensuremath{\rightarrow}\ensuremath{\pi}{S}_{11}(1535)\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\eta}N$ and/or \ensuremath{\pi} and \ensuremath{\eta} emissions without intermediate nucleon resonances.

The $\eta$-meson photoproduction cross sections have been measured on the C and Cu targets for the photon energies between 600 and 1100 MeV to investigate the behavior of the $S_{11}$(1535) resonance in a nucleus. The excitation functions of the cross section as well as the angular and momentum distributions of $\eta$-mesons are in quantitative agreement with the Quantum Molecular Dynamics (QMD) model calculations in which the $\eta$-meson emission processes other than the $S_{11}$(1535) resonance are also incorporated as proposed in the $\eta$-MAID model. It is shown that the excitation of the $D_{15}$(1675) resonance might play an important role for $E_{\gamma}>900$ MeV., Comment: 16 pages 4 figures, submitted to Phys. Lett. B

The beam from thermionic RF gun is quite suitable for the bunch compression by means of velocity bunching. Since the thermionic gun is very stable in general, we anticipate very short electron pulses of the bunch length around 50 fs can be provided satisfactorily. The t-ACTS program has been developed toward updating terahertz science and technology in which intense coherent synchrotron radiation (CSR) from very short electron bunches will be employed. Careful numerical simulation showed possibility of the bunch length of less than 100 fs with a bunch charge of 20 pC, which will provide sufficiently large form factor for production of CSR in the THz wavelength region. The train of short bunches will be stored in an accumulator ring. The peak brilliance of THz radiation emitted from one bunch is expected to be 5×10 6 photons/mrad2/0.1%bandwidth for each bending magnet of the ring.

The π + π − photoproductions on the proton and deuteron have been studied in a photon energy range of 0.8–1.1 GeV at the Laboratory of Nuclear Science, Tohoku University. Charged pions and protons were detected using Neutral Kaon Spectrometer. We obtained the cross sections for the p ( γ , p π + π − ) and d ( γ , p π + π − ) n . The quasi-free process with a neutron spectator was extracted by the neutron momentum cut of p n > 0.3 GeV / c . The cross section for the Δ + + Δ − production was deduced in the non-quasi-free process of the γ d → p n π + π − . It was 13.4 ± 0.4 μb at E γ = 0.82 GeV .