Search

The Time-Of-Propagation (TOP) counter is a ring-imaging Cherenkov detector designed to identify the charged hadrons in the barrel region of the Belle II detector. The Belle II experiment collected data delivered by the SuperKEKB accelerator from March 2019 to June 2022. After the first run period (Run1), a long shutdown (LS1) was dedicated to implement several accelerator and detector upgrades. The second collision period (Run2) was started in January 2024. The components of the TOP detector and the performance during the Run1 will be reported and the detector upgrade during LS1 and future upgrades will be described. [ABSTRACT FROM AUTHOR]

Starting from our current impasse at the LHC, of observing an SM-like Higgs boson but nothing beyond, we focus on the General 2HDM (G2HDM), which possesses extra sets of Yukawa couplings as a likely Next New Physics. After expounding its merits, we explore our "Decadal Mission of the New Higgs/Flavor era", reporting on an Academic Summit Project (ASP) in Taiwan that conducts a four-pronged pursuit of G2HDM: CMS and Belle II searches, a lattice study of first-order electroweak phase transition, and phenomenology. The ASP Midterm report is based on ATLAS and CMS searches for c g → t H / t A → t t c ¯ , where H and A are exotic neutral scalar bosons, and now progressing onto a post-Midterm c g → b H + → b t b ¯ search, where H + is the exotic charged Higgs boson, plus a few other searches at the LHC, all with discovery potential. We then discuss a plethora of flavor observables that can be explored by CMS and Belle II, as well as other dedicated experiments. Finally, we elucidate why G2HDM, providing myriad new dynamics, can remain well hidden so far. This brief report summarizes the progress of the ASP of the NSTC of Taiwan. [ABSTRACT FROM AUTHOR]

Flavour physics is one of the essential elements of the Standard Model (SM). Experimental studies of weak decays of B and D mesons at B factories have, together with the discovery of the Higgs boson at LHC, provided the final confirmation of the validity of the SM. The present generation of precision flavour physics experiments is looking for departures from the SM. We discuss studies of anomalies in b hadron decays, and studies of rare decays, which are a very promising method for searching for new physics. This article is part of the theme issue 'The particle-gravity frontier'. [ABSTRACT FROM AUTHOR]

Starting from our current impasse at the LHC, of observing an SM-like Higgs boson but nothing beyond, we focus on the General 2HDM (G2HDM), which possesses extra sets of Yukawa couplings as a likely Next New Physics. After expounding its merits, we explore our “Decadal Mission of the New Higgs/Flavor era”, reporting on an Academic Summit Project (ASP) in Taiwan that conducts a four-pronged pursuit of G2HDM: CMS and Belle II searches, a lattice study of first-order electroweak phase transition, and phenomenology. The ASP Midterm report is based on ATLAS and CMS searches for cg→tH/tA→ttc¯, where H and A are exotic neutral scalar bosons, and now progressing onto a post-Midterm cg→bH+→btb¯ search, where H+ is the exotic charged Higgs boson, plus a few other searches at the LHC, all with discovery potential. We then discuss a plethora of flavor observables that can be explored by CMS and Belle II, as well as other dedicated experiments. Finally, we elucidate why G2HDM, providing myriad new dynamics, can remain well hidden so far. This brief report summarizes the progress of the ASP of the NSTC of Taiwan.

An estimate is given of the simulation with the Monte Carlo (MC) method using the signal to background ratios in invariant mass spectra of pion-kaon system for decays of D0 → K−π+ and → K+π− originated from decays of D*+ → D0π+ and D*− → π− at different momentum/angular bins of pions (when pion is coming to the ARICH acceptance) and kaons (kaon is in ARICH acceptance). The tabulated data for mentioned ratios will be used by Belle II collaboration to study the ARICH performance. Then the results of ARICH efficiencies and misidentification probability extracted based on tabulated ratios, will become important ingredient for future physics analysis. [ABSTRACT FROM AUTHOR]

The iTOP detector is a Cherenkov detector specialized on particle identification at Belle II. The SuperKEKB accelerator collides electrons and positrons with a design luminosity of 6 ⋅ 1 0 35 1 cm 2 s . In order to exploit the high collision rate Belle II has a trigger rate of up to 30 kHz. The iTOP detector uses quartz bars as the source of Cherenkov photons. The photons are reflected inside the bars until they hit photomultipliers installed at one end. The spatial distribution and precise arrival times of the detected photons are used to reconstruct the Cherenkov angle and particle flight time. To achieve a good pion–kaon separation the photon arrival times have to be measured with a resolution of 100 ps. Microchannel plate photomultipliers together with dedicated high-speed electronics for 2.7 GSa/s waveform sampling in 8192 channels are used to achieve this requirement. After four years of operation, the experiment entered its first long shutdown phase in 2022. Aging components of iTOP were exchanged and the detector was prepared for data-taking at increasing luminosities and backgrounds after the long shutdown. In this talk the design of the iTOP detector will be shown and experience and results from operation will be discussed together with an outlook on future running conditions. [ABSTRACT FROM AUTHOR]

The Belle II experiment at the SuperKEKB collider in Tsukuba, Japan, has collected e + e − collision data between 2019 and 2022. After reaching a record-breaking instantaneous luminosity of 4.71 × 1034 cm-2 s-1 and recording a dataset corresponding to 424 fb-1, it completed its first planned long shutdown phase in December 2023. Aside from upgrades of the collider and detector maintenance, the shutdown was used for the installation of the two-layer Pixel VerteX Detector (PXD). As the innermost sub-detector, multiple scattering effects need to be reduced. PXD utilizes the Depleted P-channel Field Effect Transistor (DEPFET) technology, allowing for a material budget of 0.21% X 0 per layer. Each of the tracker's 40 modules consists of an array of 250 × 768 pixels with a pitch ranging from 50 µm × 55 µm for the inner to 85 µm × 55 µm for the outer layer yielding high gain and high signal-to-noise ratio while retaining about 99% hit efficiency. This article discusses the experience of the 4-year operation of the previous single-layer PXD in harsh background conditions as well as commissioning and testing of the fully-populated PXD2 during Long Shutdown 1. [ABSTRACT FROM AUTHOR]

The OBELIX depleted monolithic active CMOS pixel sensor (DMAPS) is currently developed for the upgrade of the vertex detector of the Belle II experiment located at Tsukuba/Japan. The pixel matrix of OBELIX is inherited from the TJ-Monopix2 chip, but the periphery includes additional features to improve performance and allow the integration into a larger detector system. The new features include a trigger unit to process trigger signals, a precision timing module and a possibility to transmit low granularity hit information with low latency to contribute to the Belle II trigger. Additionally, low dropout voltage regulators and an ADC to monitor power consumption and substrate temperature is developed. This paper will focus on the trigger contribution capabilities of the OBELIX chip. [ABSTRACT FROM AUTHOR]

The Belle II experiment searches for beyond-the-standard-model physics using the Belle II detector and SuperKEKB collider. The silicon vertex detector (SVD) is crucial for particle tracking. After the 1.5-year shutdown from June 2022, Run 2 began in January 2024; Run 2 operation shows stable noise levels, high signal-to-noise ratios, and hit efficiency over 99%. To manage higher beam background from increased luminosity, new techniques such as hit-time selection and cluster grouping are being developed. These methods increase the acceptable level of occupancy by distinguishing hits from triggered collisions and other sources. [ABSTRACT FROM AUTHOR]

In the past decade, electroweak penguin decays have provided a number of precision measurements and have become one of the most competitive ways to search for New Physics describing phenomena beyond the Standard Model. An overview of the measurements made at the B factories and hadron colliders is given, and the experimental methods are presented. Experimental measurements required to provide further insight into present indications of New Physics are discussed. [ABSTRACT FROM AUTHOR]

The Belle II experiment at KEK in Japan considers an upgrade for the vertex detector system in line with the accelerator upgrade for higher luminosity at long shutdown 2 planned for 2028. One proposal for the upgrade of the vertex detector called VTX aims to improve background robustness and reduce occupancy using small and fast pixels. VTX accommodates the OBELIX depleted monolithic active CMOS pixel sensor (DMAPS) on all five proposed layers. OBELIX is specifically developed for the VTX application and based on the TJ-Monopix2 chip initially developed to meet the requirements of the outer layers of the ATLAS inner tracker (ITk). This paper will review recent tests of the TJ-Monopix2 chip as well as various design aspects of the OBELIX-1 chip currently under development. [ABSTRACT FROM AUTHOR]

The silicon vertex detector (SVD) is a four-layer double-sided strip detector installed at the heart of the Belle II experiment, taking data at the high-luminosity B -Factory SuperKEKB since 2019. SVD has been operating smoothly and reliably, showing a stable and above-99% hit efficiency, and a large signal-to-noise ratio in all sensors. In June 2022 the data-taking of the Belle II experiment was stopped for the Long Shutdown 1, primarily required to complete the vertex detector (VXD) with the inner two-layer DEPFET detector and to upgrade several components of the accelerator. This article reports on the excellent performance of SVD in terms of the signal-to-noise ratio, the hit position resolution, as well as the hit-time resolution. We briefly describe the challenges and delicate phases of the VXD re-installation and the SVD status for operation starting in early 2024. In SVD layer 3, which is closest to the interaction point, the average occupancy has been less 0.5%, well below the estimated limit for acceptable tracking performance. However, higher machine backgrounds are expected at increased luminosity, and so also increased hit occupancy. To enhance the robustness of offline software in a high-background environment, new algorithms of background suppression using the excellent SVD hit-time information have been developed, which allows a significant reduction of the fake rate, while preserving the tracking efficiency. With the increasing luminosity also the radiation levels are expected to increase, with possible deterioration of the sensor performance. The SVD integrated dose is estimated by the correlation of the SVD occupancy with the dose rate measured by the diamonds of the radiation monitor and beam-abort system. The effects of radiation damage are starting and in good agreement with our expectations. So far, no harmful impact due to the radiation damage on the detector performance has been observed. [ABSTRACT FROM AUTHOR]

Belle II is a new-generation B-factory experiment, dedicated to exploring new physics beyond the standard model of elementary particles in the flavor sector. Belle II started data-taking in April 2018, using a synchronous data acquisition (DAQ) system based on pipelined trigger flow control. The Belle II DAQ system is designed to handle a 30-kHz trigger rate with approximately 1% of dead time, under the assumption of a raw event size of 1 MB. The DAQ system is reliable, and the overall data-taking efficiency reached 84.2% during the run period of January 2020–June 2020. The current readout system cannot be operated in the terms of ten years from the viewpoint of DAQ maintainability; meanwhile, the readout system is obstructing high-speed data transmission. A solution involving a peripheral component interconnect (PCI)-express-based readout module with high data throughput of up to 100 Gb/s was adopted to upgrade the Belle II DAQ system. We particularly focused on the design of firmware and software based on this new generation of readout board, called PCIe40, with an Altera Arria 10 field-programmable gate array chip. The 48-Gb transceiver (GBT) serial links, PCI-express hard IP-based direct memory access (DMA) architecture, interface of timing and trigger distribution system, and slow control system were designed to integrate with the current Belle II DAQ system. This article describes the performances accomplished during the data readout and slow control tests conducted using a test bench and a demonstration performed using on-site front-end electronics, specifically involving Belle II TOP and KLM subdetectors. [ABSTRACT FROM AUTHOR]

The Belle II experiment located at KEK, Japan takes data from asymmetric e + e − collision provided by the SuperKEKB accelerator. The Silicon Vertex Detector (SVD), which is part of the Belle II Vertex Detector (VXD), has been operating smoothly and reliably since the start of data taking in March 2019. In this article, we report on the performance of the SVD in terms of the large signal-to-noise ratio, the good hit position resolution as well as the good hit-time resolution. New algorithms based on hit-time information are under development to improve robustness of tracking performance within the anticipated high background environment. The Background situation of the SVD has been constantly monitored and no degradation in performance is observed so far. To investigate the SVD performance at high luminosity runs in the future, simulation as well as an irradiation campaign are launched and their results are summarized. During the first long shutdown of the Belle II experiment, which starts from June 2022, the VXD has been refurbished with a new two-layer DEPFET pixel detector located inside the SVD. All the delicate phases of the disassembly, re-assembly and installation of the new VXD have been successfully completed. The new VXD commissioning phase began in Sept 2023 to get ready for beam operation starting in early 2024. [ABSTRACT FROM AUTHOR]

The Belle II experiment is designed to collect 50 times more data than its predecessor. For a smooth collection of high-quality data, a robust and automated data transport and processing pipeline has been established. We describe the basic software components employed by the high level trigger. It performs a reconstruction of all events using the same algorithms as offline, classifies the events according to physics criteria, and provides monitoring information. The improved system described in this paper has been deployed successfully since 2019. [ABSTRACT FROM AUTHOR]

The Belle II experiment at the High Energy Accelerator Research Organization (KEK) in Japan started gathering data in Phase II in April 2018 to unravel new physics beyond the standard model by precisely measuring charge conjugation parity symmetry (CP) violation and rare weak decays of heavy quarks and leptons. It was performed at the SuperKEKB electron–positron collider mainly running at the Υ (4S) resonance energy with the goal to reach the maximum instantaneous luminosity of 8 × 1035 cm−2 s−1. A new algorithm is needed to operate the Belle II calorimeter trigger system stably in the much higher luminosity and beam background environment of SuperKEKB compared with the KEKB collider. In order to develop an appropriate algorithm, a detailed simulation study of the Belle II calorimeter trigger system is crucial. In this article, we report the results of the simulation of the electromagnetic calorimeter (ECL) trigger using physics and beam background Monte Carlo (MC) events and compare them with the ECL trigger performance in the Phase II operation. The simulation package is developed with the Belle II Geant4-based analysis framework called Basf2. [ABSTRACT FROM AUTHOR]

While the current Belle II data acquisition (DAQ) system has proven to be reliable in the first collisions delivered since May 2018, an upgrade to increase the bandwidth of the readout system is foreseen to be deployed over the next few years for higher target luminosity SuperKEKB running. Currently, the front-end electronics (FEE) of individual subdetectors are connected to the online event building system via a unified optical link called Belle2Link, offering unidirectional data readout as well as bidirectional slow control access. The FEE data are received by high-speed link boards (HSLB) and common pipelined platform for electronics readout (COPPER) boards collecting data from up to four links, forwarding to readout PCs (ROPC), which further aggregate the data and forward the data to the event builder. It is planned to keep the Belle2Link and subsystems FEE as they are, but to replace the HSLB/COPPER (including the back-end side of Belle2link) and ROPC with an upgraded high-density system with increased throughput and processing capabilities, to enable the operation at higher rates. This article presents a solution for this upgrade based on the xTCA-for-Physics (MicroTCA)-compatible concentration preprocessing and fan-out plus (CPPF+) board featuring significant data concentration and processing power, providing up to 40-Gb Ethernet output bandwidth. Belle2Link back-end firmware and subevent building are implemented in the CPPF+ board to deliver equivalent functionality as the COPPER/ROPC. The MicroTCA platform provides scalability, reconfigurability, and reliable operations, even for a future possible increase in bandwidth and potential FEE upgrades. A fully functional demonstration readout system has been constructed and verified with the time-of-propagation (TOP) detector front-end test bench set up at KEK in terms of functionality as well as performance. Results show that this solution fulfills the requirements of the DAQ upgrade and works stably and reliably in long-term tests. [ABSTRACT FROM AUTHOR]

The Belle II barrel region is instrumented with the Time of Propagation (TOP) particle identification system. Due to its mechanical design, the individual TOP modules do not overlap, leaving a gap of around 2 cm between them in the azimuthal direction. This leads to a 6%–9% drop in acceptance, depending on the track's momentum. We propose a solution to remedy these gaps by instrumenting them with fast silicon detectors to directly measure the time-of-flight of traversing particles. We present here a simulation study discussing the performance requirements and the possible sensor technologies, and we demonstrate that such a project could be realized with novel, fast monolithic CMOS sensors, or alternatively AC-LGAD sensors, both of which are expected to reach MIP timing resolutions of down to 50 ps or better. [ABSTRACT FROM AUTHOR]

Flavour physics is one of the essential elements of the Standard Model (SM). Experimental studies of weak decays of [Formula: see text] and [Formula: see text] mesons at [Formula: see text] factories have, together with the discovery of the Higgs boson at LHC, provided the final confirmation of the validity of the SM. The present generation of precision flavour physics experiments is looking for departures from the SM. We discuss studies of anomalies in [Formula: see text] hadron decays, and studies of rare decays, which are a very promising method for searching for new physics. This article is part of the theme issue 'The particle-gravity frontier'.

The Belle II experiment comes with a substantial upgrade of the Belle detector and will operate at the SuperKEKB energy-asymmetric e + e − collider with energies tuned to ϒ (4 S) resonance s = 10. 588 GeV. The accelerator has successfully completed the first phase of commissioning in 2016 and the first electron–positron collisions in Belle II took place in April 2018. Belle II features a newly designed silicon vertex detector based on DEPFET pixel and double-sided strip layers. Currently, a subset of the vertex detector is installed (Phase 2 of the experiment). Installation of the full detector (Phase 3) will be completed by the end of 2018. This paper describes the Phase 2 arrangement of the Belle II silicon vertex detector, with focus on the interconnection of detectors and their integration with the software framework of Belle II. Alignment issues are discussed based on detector simulations and first acquired data. [ABSTRACT FROM AUTHOR]

Abstract The Belle II central drift chamber (CDC) is crucial in the momenta measurements of charged particles. We present methods for calibration and alignment of the CDC. Using cosmic ray data acquired with and without a magnetic field, the CDC was calibrated and aligned, and its performance was evaluated. The position resolution is better than 120 μ m, depending on the layer and incident angles of the tracks. The alignment of the CDC improves the resolution of track parameters and reduces their systematic shifts. The transverse momentum (P T) resolution for charged tracks is 0. 127 P T ⊕ 0. 321 (%), P T is in GeV/ c. [ABSTRACT FROM AUTHOR]

The Belle II experiment at SuperKEKB e + / e − collider, Japan, is aiming at exploring flavor physics in high intensity frontier. The target instantaneous luminosity of SuperKEKB is at 8 × 1 0 3 5 s − 1 cm − 2 , which is 40 times higher than that at KEKB. The renovated Belle II detectors around the interaction point are expected to collect 5 0 ab − 1 data within 7 years. This gives high data statistics for precision measurements and probing new physics in rare decay studies. The Belle II experiment has successfully recorded the first collision events at SuperKEKB in April 2018, and is ready for physics data taking. This paper will cover the current status of Belle II and highlight its prospects on flavor physics. [ABSTRACT FROM AUTHOR]

The Belle II experiment integrates two dedicated particle identification (PID) subdetectors for pion/kaon separation. The Aerogel Ring Image Cherenkov (ARICH) detector is a forward region PID detector based on two layers of aerogel read out by hybrid avalanche photo detectors. The Time of Propagation (TOP) detector is a novel Cherenkov barrel PID system built for the Belle II detector upgrade based on quartz radiator bars read out by Micro-Channel Plate PMTs. The readout electronics of the TOP system are built around a switched capacitor array waveform sampling ASIC operating at 2.7 GSa/s. Acquired waveforms are processed in real time in the front-end electronics, extracting the individual timing of detected photons to better than 100 ps. This contribution presents the concept of the Belle II particle identification systems with a focus on the current status of commissioning, calibration and operation of the Belle II TOP detector. [ABSTRACT FROM AUTHOR]

Zusammenfassung Das Standardmodell der Teilchenphysik kann trotz unbestreitbarer Erfolge wichtige Beobachtungen nicht erklären. Dazu zählt das Verschwinden der Antimaterie im Universum. Deshalb wird intensiv nach „Neuer Physik" gesucht. Parallel zu den Experimenten am LHC, der die höchsten Kollisionsenergien erzielt, hat nun eine neue Super‐B‐Factory in Japan mit dem Belle‐II‐Detektor seinen Betrieb aufgenommen. Dort sollen mit höchster Präzision die Unterschiede zwischen B‐Mesonen und Ihren Antiteilchen untersucht werden. Für Belle II wurde unter deutscher Federführung ein neuartiger Pixel‐Vertexdetektor entwickelt, der für die genaue Vermessung der B‐Meson‐Zerfälle essentiell ist. [ABSTRACT FROM AUTHOR]

The Belle II experiment is a high luminosity electron and positron collider experiment at SuperKEKB in Japan. The Time-Of-Propagation counter is a detector for particle identification in the barrel region. Micro-channel-plate photomultipliers are used as photodetectors and measure photon timing with a resolution of 30 ps; this leads to excellent particle identification performance. One of the issues is the photocathode lifetime of the photomultiplier tubes. Monitoring tools have been developed to measure the tube gain, accumulated output charge density and quantum efficiency during physics data taking. The gain was seen to decrease by about 10%. Although the accumulated output charge density is small compared to the tube expected lifetime, a quantum efficiency degradation was found larger than expected for 5% of the tubes after threshold efficiency calibration. Several causes for this degradation are being investigated. They can possibly be related to tube production and noise from the read-out system. [ABSTRACT FROM AUTHOR]

In the forward end-cap of the Belle II spectrometer, particle identification is provided by a proximity focusing RICH detector with an aerogel radiator (ARICH). The ARICH's primary function is to effectively distinguish between pions and kaons in the momentum range of 0. 5 GeV / c to about 4 GeV / c , as well as to contribute to identification of low-momentum leptons. Since its operation began, Belle II has collected over 420 fb − 1 of data. Based on this large data sample, studies of several effects that impact the performance of the ARICH detector were carried out. In this paper, we present a comparison of the observed Cherenkov ring image and detector particle identification performance in the measured data and detector simulation. Furthermore, we highlight recent efforts aimed at enhancing the ARICH's performance by taking into account the effects of particle decay in flight and scattering in materials before the detector, as well as by refining the probability density function used for particle identification likelihood evaluation. [ABSTRACT FROM AUTHOR]

Abstract We describe the Belle II detector at the SuperKEKB electron–positron accelerator. SuperKEKB operates at the energy of the ϒ (4 S) resonance where pairs of B mesons are produced in a coherent quantum mechanical state with no additional particles. Belle II, the first Super B factory detector, aims to achieve performance comparable to the original Belle and BaBar B factory experiments, which first measured the large CP violating effects in the B meson system, with much higher luminosity collisions and larger beam-induced backgrounds. [ABSTRACT FROM AUTHOR]

The ongoing upgrade of the asymmetric electron–positron collider SuperKEKB at the KEK laboratory, Japan aims at a 40-fold increase of the peak luminosity to 8 × 10 35 cm −2 s −1 . At the same time the complex Belle II detector is being significantly upgraded to be able to cope with the higher background level and trigger rates and to improve overall performance. The goal of the experiment is to explore physics beyond the standard model with a target integrated luminosity of 50 ab −1 in the next decade. The new vertex detector (VXD), comprising two layers of DEPFET pixel detectors (PXD) surrounded by 4 layers of double sided silicon strip detectors (SVD), is indispensable for vertex determination as well as for reconstruction of low momentum tracks that do not reach the central drift chamber (CDC). Within the confined VXD volume the front-end electronics of the two detectors will dissipate about 1 kW of heat. The VXD cooling system has been designed to remove this heat with the constraint to minimize extra dead material in the physics acceptance region. Taking into account additional heat intake from the environment the cooling system must have a cooling capacity of 2–3 kW. To achieve this goal evaporative two-phase CO 2 cooling in combination with forced N 2 flow is used in the VXD cooling system. In order to verify and optimize the cooling concept and to demonstrate that acceptable operation conditions for the VXD system as well as the surrounding CDC can be obtained, a full size VXD thermal mock-up has been built at DESY. Various thermal and mechanical tests carried out with this mock-up are reported. [ABSTRACT FROM AUTHOR]