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Prestressed concrete sleepers are generally designed taking into account the influence of the dynamic wheel load. In Japan, the dynamic wheel load factor of 2.0 has been typically used for the serviceability limit state since the 1950s. However, there are few examples that have proved its validity. In this study, field measurement tests and three-dimensional numerical analysis were conducted for prestressed concrete sleepers laid on a straight section of a railway track with continuously welded rails. According to the results of the field tests, the measured dynamic wheel load factor was less than the conventionally used dynamic wheel load factor of 2.0, and the bending moments exceeding the full prestress condition, i.e. the compressive stress on all sections, were generated in some cases. Furthermore, even with the prestressed concrete sleepers of the same type laid continuously, the bending moment was increased more than three times due to the support conditions of the prestressed concrete sleepers and the rail roughness. Results of the numerical analyses also revealed that the bending moment was increased more than two times because of the hanging (unsupported) sleepers, and the tensile stress of the prestressed concrete sleeper exceeded 3 N/mm2 when the rail roughness was approximately 2 mm or more. Based on this investigation, the validity of the dynamic wheel load factor of 2.0 was proved.

We present a search for the direct production of a light pseudoscalar a decaying into two photons with the Belle II detector at the SuperKEKB collider. We search for the process e+e-→γa, a→γγ in the mass range 0.2

Theories beyond the standard model often predict the existence of an additional neutral boson, the Z′. Using data collected by the Belle II experiment during 2018 at the SuperKEKB collider, we perform the first searches for the invisible decay of a Z′ in the process e+e-→μ+μ-Z′ and of a lepton-flavor-violating Z′ in e+e-→e±μZ′. We do not find any excess of events and set 90% credibility level upper limits on the cross sections of these processes. We translate the former, in the framework of an Lμ-Lτ theory, into upper limits on the Z′ coupling constant at the level of 5×10-2-1 for MZ′≤6 GeV/c2.

When a train passes along prestressed concrete (PC) bridges, crack opening reduces the girder stiffness, providing important indicators for the practical evaluation of bridge performance. However, this phenomenon is difficult to detect in the free vibrations induced by trains or via hammer tests as these vibrations have very small amplitudes. Moreover, although the bridge under a passing train vibrates with large amplitude, the vibrational state is forced, and the girder stiffness induced by crack opening occurs only in the downward displacement. To estimate this complex but valuable system, this study establishes a time-varying autoregressive with eXogenous (TV-ARX) model and a hierarchical Bayesian estimation (Bayesian TV-ARX) method. The exogenous variables in the TV-ARX model are the modal-transformed moving loads, which constitute the main excitation frequency of the running train. The variation of the autoregressive (AR) coefficient includes the modal characteristic as a random walk process. When a train passes, the Bayesian TV-ARX estimates the instantaneous amplitude-dependent drop of the bridge frequency from the displacement response. After formulating the Bayesian TV-ARX, the effects of the vehicle–bridge interaction (VBI), train speed, and track irregularity on the accuracy of the instantaneous stiffness decrease estimated via the Bayesian TV-ARX were verified using VBI simulations of a nonlinear beam, in which the bending stiffness decreased only during a downward displacement. Even when the VBI effects overlapped, the model accurately estimated the reduced bridge stiffness due to crack opening. Next, the Bayesian TV-ARX was applied to two PC bridges on a real high-speed railway. On one of the bridges, the estimated stiffness decreased by ~ 12% when the bridge was downward-displaced by crack opening during the train passage. Such results on a real bridge under operation have not been previously reported. Four months later, the amplitude-dependent decrease in the bridge frequency was amplified by crack propagation. These results are important evidences of a nonlinear and nonstationary system. Besides solving real problems, the proposed method is expected to significantly contribute to condition-based maintenance and structure-health monitoring of PC bridges. In particular, it enables early detection of damage and deterioration in addition to bridge performance evaluation over time.

Resonance of railway bridges is a matter of major concern in high-speed railways because it reduces riding comfort, damages the attached structures, and decreases the strength of girder bodies. Although this abnormal behavior can be inspected using in situ displacement measurements taken from the ground, its investigation is tedious and, consequently, is very expensive. In this study, authors developed a novel drive-by system for high-speed railways to detect resonant bridges; the difference between two track irregularities at the same position using devices mounted on the first and last vehicles of a train was measured. This is based on the idea that the response of the last vehicle passing through the resonant bridge is attributed to the superposition of track irregularities and abnormal bridge displacement because of resonance mechanisms. However, the response of the first vehicle primarily arises from track irregularities since the resonance phenomenon is not yet sufficiently excited. The Resonance Detection Index (RDI), defined as the difference between the track irregularities measured by the devices mounted on the first and last vehicles, is used to emphasize the component excited by vehicle length to propose a methodology for detecting resonant bridges. Numerical simulations clarify that bridge span and track irregularity have greater effects on the RDI than either measurement noise or positioning error. Track irregularity may cause the RDI to vary by ± 2 mm depending on vehicle length wave component of the track irregularity. Consequently, this study demonstrates that resonant bridges with spans between 20 and 60 m can be detected using the proposed RDI. Finally, the proposed method was applied to track irregularity measurements using actual high-speed trains operating in Japan. More than ten bridges, out of more than 800 bridges, were detected as resonant bridges according to RDI. In situ displacement measurements were performed for three of these bridges, demonstrating the presence of resonance phenomenon and thereby validating the proposed method.

Simple bridge displacement measurement using a video camera is effective in realizing the efficient management of numerous railway structures via condition-based maintenance. Although non-marker image measurement is significantly influenced by the measuring environment, its practical applicability considering the displacement measurement accuracy of non-marker images and the influence of various environments is not completely understood. In this study, the accuracy of non-marker image displacement measurement and the influence of illuminance are confirmed using a model bridge, and the accuracy and applicable range are discussed. Moreover, field tests on two bridges—a steel and a concrete bridge—on low-speed and high-speed railways confirm the accuracy and practical application of non-marker image measurement in a real environment. The displacement was observed to be measured with an accuracy of ~1/30 pixel (error of ~0.4 mm at 20 m position) in the daytime with sufficient brightness. Moreover, the settings for subset positions and post-processing methods to ensure accuracy in non-marker image measurement on concrete bridges with low surface contrast are discussed.

Indications of lepton flavor universality violation in the decays \(B \to D^{(*)}\tau\nu\) have triggered substantial interest and could be a hint for new physics. Useful observables to probe new physics in these decays are the ratios of the branching fractions, \(R(D^{(*)}) \equiv \mathcal{B}(\bar{B} \to D^{(*)}\tau\bar{\nu}_{\tau})/\mathcal{B}(\bar{B} \to D^{(*)} \ell \bar{\nu}_{\ell})\), where \(\ell = e\) or \(\mu\), and the polarizations of the \(\tau\) lepton and \(D^*\) meson. This article summarizes analyses on those observables in the Belle experiment at the KEKB \(e^+ e^-\) collider based on the full data set recorded by the Belle detector at the \(\Upsilon(4S)\) resonance containing 772 million \(B\bar{B}\) pairs. The analyses utilize different methods to reconstruct or tag the accompanying \(B\) meson other than the \(B\) of the semitauonic decay: semileptonic tagging for the simultaneous measurement of \(R(D)\) and \(R(D^*)\), and hadronic and inclusive ones for the measurement of the polarizations. In terms of the lepton flavor universality test, the ratio of the branching fractions of the purely leptonic decays, \(\mathcal{B}(B^- \to \tau^- \bar{\nu}_{\tau})/\mathcal{B}(B^- \to \mu^- \bar{\nu}_{\mu})\), is also interesting because its theoretical uncertainty is small, though the \(B^- \to \mu^- \bar{\nu}_{\mu}\) decay has not been discovered yet. Belle searched for this decay, and the result is also overviewed in this article.

A micro-channel-plate photomultiplier tube (MCP-PMT) is an excellent photon sensor which has a time resolution better than 50 ps. However the lifetime of the photocathode is a major problem of using it under a high background. We developed a square shape multi-anode MCP-PMT for the Belle II TOP counter, and succeeded in extending its lifetime significantly by more than one order of magnitude. It was achieved by adopting an atomic layer deposition coating on the MCP and by applying some processes in the production to reduce the residual gas on the MCP. Due to a dead time after the micro-channel fires, degradation of the MCP-PMT performance under a high background is another concern. It was examined for four MCP-PMTs of different MCP resistances. It was found that the gain and efficiency drops at a background hit rate above 1 MHz/anode, and the transit time spread degrades above ∼200 kHz/anode independently of the MCP resistance.

The dependence of the quantum efficiency on the angle and polarization of the incident photon needs to be formulated for a precise description of the response of photomultiplier tubes. A simplified one-step model of photoelectron emission was derived from Spicer's three-step model, and it enabled the formulation of the dependence of the quantum efficiency in the visible range for thin multi-alkali (NaKSbCs) photocathodes. The expression of the quantum efficiency was proved by a measurement of the photocurrent for linearly polarized light at various incident angles. Meanwhile, the measurement revealed the complex refractive indices and thicknesses both of the stratified photocathode and antireflection coating. It is indicated that the angular dependence of the quantum efficiency is dictated by the optical properties of the photocathode, which are discussed in detail on the basis of the obtained parameters.

The front-end electronics system for the Belle II Time-Of-Propagation (TOP) counter was fully installed in May 2016. The detector is a novel particle identification device for the barrel region, where Cherenkov ring images are reconstructed with precise timing information of each photon. The readout electronics need to have excellent timing performance for single photon detection. To exploit the benefit of high luminosity, the electronics must also be able to cope with a high input trigger rate (30 kHz) and have buffer memory which is deep enough to wait for a trigger decision ( > 5 μ s ). The TOP electronics has switching capacitor arrays to sample waveforms with 2.7 GSample/s, which allows a timing resolution of 50 ps for a single photon signal. The programmable logic and the processing system are the effective implementation to meet these requirements, where flexible optimization and step-by-step development of readout logic are possible. The system has been successfully operated in the first accelerator commissioning runs with beam collisions in 2018, where the typical trigger rate was 500 Hz . Operation with a 20 kHz trigger rate was also tested.

Monitoring the conditions or damages of bridges under train passages demands a high-accuracy modal-characteristic identification method that separates the apparent fluctuations caused by vehicle–bridge dynamic interaction (VBI) effects from other fluctuations. This study proposes a novel method based on a time-varying autoregressive model, which is solved using a hierarchical Bayesian estimation approach. The VBI effect is estimated from the displacement response of the railway bridges as temporal fluctuations of the natural frequency and modal damping ratio. The exogenous variable is the train load, expressed as an external force. Numerical experiments verified the higher accuracy of the proposed method than the existing method. The influences of train speed and rail irregularity on the VBI effects are clarified by the application of the proposed method to various VBI simulations. The proposed method was applied to the measured resonance responses of actual bridges and succeeded in empirically demonstrating the decreased natural frequency and the increased modal damping ratio under train passage. Additionally, using the proposed method, modal characteristics variation due to VBI effect calculated using VBI model simulation was verified by comparing with those estimated from the measured results.

European design standards have established an upper limit on the deck acceleration of the high-speed railway bridges, however the influence of local vibrations of the deck members is rarely considered when modelling the vibrational responses of bridges. To evaluate how the inclusion of local deck vibrations might influence predictions of the maximum acceleration, detailed measurements were taken from a steel-concrete composite box-girder bridge on the Italian high-speed railway, and a numerical model of the system was developed. Deck vibrations were measured during high-speed train passages at the maximum train speed of 374 km/h, and compared against a numerical model of the vehicle-bridge system. This analysis revealed that the maximum deck acceleration is 1.3 times greater than the acceleration of the bridge girders, because of the sixth- and seventh-order resonance between the deck’s local vibration modes and the structure with a train running at high speeds over 300 km/h. Moreover, when considering local deck vibrations in the numerical model, we found that the interaction between transient local rail deformations and the vehicle travelling on the rails can increase the acceleration of the deck through resonance.

A square shape micro-channel-plate photomultiplier tube (MCP-PMT) was developed for the Belle II TOP counter, where the count rate of the MCP-PMT is as high as several MHz/PMT due to the accelerator beam backgrounds. A major problem of the MCP-PMT in such an environment is depression of the quantum efficiency along with the integrated amount of output charge. We succeeded in diminishing the depression significantly by introducing an atomic layer deposition technique and some other processes in the production of the MCP-PMT.

The time-of-propagation counter for the Belle II experiment is a new particle identification device using ring imaging Cherenkov technique. In order to detect each Cherenkov photon with a timing precision of 30–40 ps in a 1.5 T magnetic field, a micro-channel plate photomultiplier tube is a suitable device for the TOP counter. By introducing an atomic layer deposition technique on the micro-channel plate surface, the tube lifetime was improved by a factor of 3–10 relative to more conventional devices. A total of 530 tubes have been produced. To ensure appropriate tube performance, the quantum efficiency, gain and transit time spread have been measured for all units. The results from each measurement are discussed. Results from a beamtest with a 2 GeV/c positron beam are also reported and demonstrate the good tube performance.

A square shape multi-anode micro-channel-plate photomultiplier tube (MCP-PMT) was developed for the Belle II TOP counter. It has a 23×23 mm2 multi-alkali photocathode with a peak quantum efficiency (QE) of 28% around a wavelength of 360 nm. It has a 2 × 10 6 gain and is capable of detecting a single photon with a transit time spread (TTS) of 40 ps or less. The QE, gain and TTS were measured for each MCP-PMT to check the quality in the mass production of 550 MCP-PMTs. The lifetime of the MCP-PMT was extended to 10 C/cm2 by adapting the atomic layer deposition technique for the MCP coating. Other properties of the MCP-PMT are also discussed: the angle and polarization dependence of the QE, and the effects of charge sharing and cross-talk. A prototype TOP counter was tested with a 2 GeV/c positron beam. The distribution of the time of propagation was consistent with the calculated expectation which included those properties of the MCP-PMT.

The Belle II experiment operating at SuperKEKB of the luminosity frontier accelerator will perform high precision flavor physics measurements. An efficient particle identification is essential for Belle II to extend its reach toward new physics, and it is realized by the TOP counter. The TOP counter is a novel ring imaging Cherenkov detector, which measures the time of propagation (TOP) of the internally reflected Cherenkov photons. The TOP counter mainly consists of a quartz optics, micro-channel-plate photomultiplier tubes and front-end electronics. After the long R&D of more than a decade, we succeeded in producing those components, assembling the 16 TOP counter modules and finally installing them in 2016. That is one of the biggest milestones of the Belle II construction. This article reviews the TOP counter in terms of the production of each component and the construction.

Evaluation of critical responses of a bridge including resonances in high-speed (HS) railway is a dominant issue in order to confirm the structural safety of bridge and the stability of ballast on bridge deck. In numerical way for critical assessment, it is important to make a sophisticated numerical model and considering the variation of actual properties of structural members. This study proposed a train-track-bridge interaction model focusing on Sesia viaduct, and calibrated the model properties as matching these modal characteristics and dynamic responses to measured ones. By the numerical and experimental comparison, calibrated model could reproduce accelerations up to 30 Hz both of on the time and frequency domains accurately. Especially, good agreements on sleeper accelerations can indicate the high reproducibility of interaction force, which is one of key factors of train-track-bridge interaction simulation. Numerical computation by calibrated model clarified that the deck acceleration up to 30Hz increases by seventh and eighth resonances between passing vehicle length and structural modes, not only global third bending or third torsional but also high order local deck modes. In addition, high performance computing technique based on super computer in RTRI was adopted for parametric analysis in order to investigate the requirements to realize highly accurate estimation for the maximum deck acceleration by the numerical way. As the results of parametric study focusing on design assumption, calculation modal order and train/bridge interaction, it was clarified that the design model causes overestimation by twice maximum acceleration on the deck, and the amplification of excitation forces caused by passage on deflected rails, which is considered in high-order modes and train/bridge interaction, is a key factor to achieve accurate deck acceleration evaluation.

Evaluation of resonances phenomenon in high-speed railway bridges during train passage is a dominant issue in order to confirm the structural safety of the bridge and the stability of the ballast on bridge deck. The Sesia viaduct, located in the Turin-Milan high-speed rail line, is one of the most investigated high-speed railway bridges. The maximum accelerations and the resonance train speed were predicted while the bridge vibration modes and the effect of adjacent spans on these were estimated through some detailed experimental and numerical investigations. Nevertheless, some important points remain unclear, such as the actual resonance speed and the influence of deck local vibrations. To verify and clarify these issues, experimental dynamic analyzes based on acceleration measurements of the Sesia viaduct under ETR1000 train passages with speed up to 374km/h were conducted in this study. In the measurements, accelerometers were installed not only on steel box girder but also on concrete deck slab in order to identify local vibration modes of deck members so as to estimate their effect on the evaluation of the accelerations. Comparing the maximum accelerations up to 15Hz with various train speeds, the resonance speed corresponding to the first bending mode of the Sesia viaduct was identified. Furthermore, an analysis based on the identification of the natural frequencies clarified that the high-order resonances between passing train and deck local vibration modes have the largest impact on the maximum acceleration up to 30Hz which is the limit used for the verification of ballast stability in the Eurocode.

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 $^\circ$ , by more than four standard deviations.

A novel RICH detector called TOP counter has been developed for particle identification in Belle II. It measures the time of propagation (TOP) of Cherenkov photons traveling in the quartz radiator with micro-channel-plate photomultiplier tubes (MCP-PMTs) with a precision of 50 ps. A prototype TOP counter was tested with the 1.2 GeV/ c electron beam in the Laser Electron Photon beam line at SPring-8 (LEPS). The TOP distribution was obtained as expected after taking into account the dependence of the MCP-PMT quantum efficiency on light incidence angle and polarization. In this paper, the key features of the TOP counter design and the beam test results are presented.