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| Belle II upgrade | |
|---|---|
| Name | Belle II upgrade |
| Location | Tsukuba, Japan |
| Established | 2018 (upgrade phase) |
| Type | Particle physics experiment |
| Affiliation | KEK, SuperKEKB |
Belle II upgrade
The Belle II upgrade refers to the comprehensive set of enhancements to the Belle II experiment and the SuperKEKB accelerator complex aimed at increasing luminosity, improving detector performance, and enabling a broad program of precision measurements and searches for phenomena beyond the Standard Model. The project builds on the legacy of the Belle experiment and the KEKB collider, integrating advances in accelerator physics, detector technology, and computing developed by an international consortium including institutions such as KEK, CERN, SLAC, and numerous university groups.
The upgrade was motivated by limitations observed during the operations of KEKB and the original Belle experiment and by physics priorities articulated in studies like the European Strategy for Particle Physics and the P5 recommendations. The luminosity goal responds to anomalies and open questions from results at the LHC experiments (notably ATLAS and CMS), flavor puzzles highlighted by LHCb and BaBar, and precision tests of CP violation exemplified by measurements at the CKM angles and B meson decays. The upgrade aimed to probe processes sensitive to virtual contributions from hypothetical particles predicted in models such as Supersymmetry, Leptoquark models, and extra-dimension scenarios.
SuperKEKB incorporates the nano-Beam scheme inspired by concepts from LNF and theoretical proposals by Pantaleo Raimondi and colleagues, implementing extremely small beam sizes at the interaction point to raise instantaneous luminosity. Upgrades include new RF cavity systems, higher-current positron source improvements, redesigned final focusing magnets including quadrupole and sextupole elements, enhanced vacuum system components, and upgraded beam instrumentation such as synchrotron radiation monitors and beam position monitor arrays. These changes were coordinated with accelerator physics collaborations involving CERN Accelerator School alumni and experts from DESY and SLAC. The injector complex was refurbished, with damping ring and LINAC improvements to deliver low-emittance beams and increased bunch currents.
The detector suite was extensively modernized relative to Belle. Key subsystems replaced or enhanced include a pixelated vertex detector based on DEPFET and silicon pixel detector technologies, an upgraded central drift chamber with fine cell design, and a new particle-identification system employing the TOP and an improved aerogel Cherenkov counter array, developed with contributions from groups at KEK, University of Tokyo, University of Melbourne, and MIT. The electromagnetic calorimeter was refurbished using refurbished CsI(Tl) crystals and upgraded electronics; the K-long and muon detector system adopted resistive-plate chambers and scintillator modules with wavelength-shifting fibers. Trigger and readout electronics were redesigned to cope with higher background rates documented in studies from B-factory operations.
To handle the anticipated increase in event rates, the data acquisition system was overhauled with scalable front-end electronics, high-throughput optical links, and real-time trigger processors leveraging field-programmable gate arrays and multi-core processors developed in collaboration with computing centers like KEK Computer Research Center and the GridPP and Open Science Grid communities. Offline computing expanded distributed analysis through WLCG-style resources, integrating cloud services and opportunistic cycles from institutions including CERN and national laboratories such as BNL. Software frameworks were modernized to support advanced reconstruction algorithms, machine-learning-based particle identification, and alignment and calibration workflows coordinated via continuous integration systems.
The upgrade targeted an integrated luminosity increase by two orders of magnitude relative to Belle to enable precision measurements of rare processes: lepton-flavor-violating decays (e.g., searches related to tau lepton channels), improved determinations of CKM matrix elements through exclusive and inclusive B meson decays, tests of lepton universality in channels compared with LHCb results, and searches for dark-sector signatures overlapping with programs at NA62, Fermilab experiments, and proposed intensity-frontier facilities. Sensitivity projections used fast and full detector simulations benchmarked against results from Belle and BaBar; expected improvements included finer vertex resolution, enhanced particle identification efficiency, and increased background rejection enabling discovery potential for scenarios suggested by anomalies in B→K(*)ℓ+ℓ− measurements and muon g−2-related models.
Commissioning phases included machine studies at low beam current, cosmic-ray runs for detector alignment with contributions from university groups, and pilot physics runs to validate trigger menus and calibration constants. Early results validated detector performance metrics such as impact-parameter resolution, time-of-flight precision, and Cherenkov angle reconstruction; preliminary publications and conference presentations were delivered at meetings like ICHEP and Lepton Photon Conference. Background mitigation strategies, informed by beam-gas and Touschek scattering studies, were iteratively improved through joint accelerator–detector working groups including experts from KEK, SLAC, and collaborating universities.
The project is managed by an international collaboration of hundreds of physicists, engineers, and technicians from institutions across Asia, Europe, North America, and Australia, with governance structures drawing on models used by ATLAS and CMS. Funding and oversight involve national agencies such as JSPS, MEXT, NSF, and European research programs, supplemented by in-kind contributions from partner laboratories like KEK, BNL, and DESY. The upgrade proceeded through defined milestones from design and prototyping to installation, with commissioning phases beginning in the late 2010s and physics data-taking ramping up thereafter, following timelines coordinated with accelerator upgrades and international review panels.
Category:Particle physics experiments Category:High energy physics