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| Belle II vertex detector | |
|---|---|
| Name | Belle II vertex detector |
| Manufacturer | KEK collaboration |
| Introduced | 2018 |
| Type | Vertex detector |
| Used by | Belle II experiment |
| Location | KEK Tsukuba, Ibaraki Prefecture |
Belle II vertex detector is the inner tracking system of the Belle II experiment at the SuperKEKB asymmetric-energy electron–positron collider located at KEK in Tsukuba, Ibaraki Prefecture. It provides precise measurements of decay vertices for studies of CP violation, rare decays, and searches for physics beyond the Standard Model. The detector is central to the Belle II collaboration program and works in concert with the Central Drift Chamber, electromagnetic calorimeter, and KLM muon system.
The vertex detector sits closest to the interaction point inside the Belle II detector and comprises two subsystems optimized for resolution and radiation tolerance. It enables reconstruction of short-lived particles such as B meson, D meson, and tau lepton decays, aiding measurements of mixing and time-dependent CP violation in the B factory environment. The device contributes to flavor physics analyses that test predictions from the Cabibbo–Kobayashi–Maskawa matrix and probe anomalies reported by other experiments like LHCb, ATLAS, and CMS.
The design combines a two-layer pixel detector and a four-layer silicon strip detector arranged in a low-mass, cylindrically symmetric geometry. The inner pixel system uses DEPFET (Depleted P-channel Field Effect Transistor) sensors developed with contributions from groups at MPI Halbleiterlabor, CERN, KEK, INFN, and University of Tokyo. The outer silicon vertex detector uses double-sided silicon microstrip sensors produced by institutes including KEK, TIFR, Princeton University, and Purdue University. Mechanical support structures employ carbon-fiber composites sourced from teams at DESY, University of Bonn, and EPFL. Readout electronics are based on custom ASICs designed in collaboration with NIKHEF, CNRS, and industrial partners. Data acquisition interfaces the vertex detector with the global Belle II trigger and DAQ systems coordinated by the Belle II Computing Group.
Spatial resolution, alignment, and timing are calibrated using tracks from cosmic rays, beam-gas interactions, and well-known resonances such as the J/psi and Upsilon families. Typical impact parameter resolution meets design goals required for separating primary and secondary vertices in B meson decays. Alignment procedures employ algorithms developed at IHEP, University of Birmingham, and Kyoto University and integrate results into the Belle II software framework (Basf2) maintained by the collaboration. Performance validation is cross-checked against control samples from tau lepton pair events, two-photon processes, and radiative Bhabha scattering.
The inner detector must tolerate high integrated doses from luminosity upgrades of SuperKEKB and backgrounds from the Touschek effect and synchrotron radiation. Sensor and electronics radiation hardness were tested at irradiation facilities including TRIUMF, Paul Scherrer Institute, SPring-8, and CERN PS using proton, neutron, and gamma sources. Thermal management employs evaporative cooling with fluorocarbon refrigerants routed through thin titanium or stainless-steel cooling pipes developed by teams at University of Melbourne and KEK. Temperature stabilization is critical for mitigating radiation-induced leakage current and preserving DEPFET gain characteristics, coordinated with environmental monitoring systems designed by RIKEN and University of Melbourne groups.
Integration required mechanical, electrical, and software interfaces with the central drift chamber, vertexing algorithms, and global alignment systems. Services such as power, cooling, and optical fiber routing traverse the Belle II support structure designed by engineers from KEK, Purdue University, and BINP. Firmware for front-end readout synchronizes with the SuperKEKB radio-frequency clock and the global trigger maintained by KEK accelerator teams. Integration tests were conducted in clean-room facilities at KEK and partner institutes including Osaka University, Kyoto University, and University of Melbourne to verify fit, function, and grounding schemes.
Commissioning phases included laboratory bench tests, cosmic-ray runs, and staged installation into the Belle II detector during shutdown periods coordinated with the SuperKEKB schedule. First collisions at design luminosity required iterative tuning of thresholds, masking of noisy channels, and firmware updates from groups at Nagoya University, Seoul National University, and University of Sydney. Operational monitoring uses automated tools in the Belle II control room with alarm systems developed by KEK and partner institutions; routine calibrations are scheduled during dedicated machine studies and low-luminosity runs.
Planned upgrades aim to improve radiation tolerance, reduce material budget, and increase readout speed to cope with future luminosity increases at SuperKEKB. R&D efforts investigate monolithic active pixel sensors (MAPS) and advanced 3D-stacked ASICs with participation from CERN, EPFL, INFN, University of Bonn, and KEK. Cooling innovations explore microchannel cooling and novel refrigerants studied at CERN and PSI. Future modifications will be coordinated within the Belle II collaboration governance and scheduled to coincide with accelerator long shutdowns planned by KEK and international partners.
Category:Particle physics detectors Category:Vertex detectors Category:KEK