Belle II experiment

Belle II experiment
Name	Belle II
Location	Tsukuba, Ibaraki Prefecture, Japan
Field	High energy physics, Particle physics
Status	Operating
Detector	Belle II detector
Accelerator	SuperKEKB
Collaboration	Belle II Collaboration
Institution	High Energy Accelerator Research Organization

Belle II experiment is a next-generation particle detector and collider experiment located at the KEK laboratory in Tsukuba, Ibaraki Prefecture, Japan. Built as the successor to Belle and operating with the SuperKEKB asymmetric-energy electron–positron collider, the project aims to perform precision studies of flavor physics, search for physics beyond the Standard Model, and investigate rare decays and CP violation. The collaboration unites institutions across Asia, Europe, and the Americas to exploit unprecedented luminosity and upgraded detector technology.

Overview

Belle II was conceived as the upgrade path following the achievements of Belle and BaBar at the KEK B-factory and PEP-II respectively, focusing on precision measurements in the B meson system, tau lepton physics, and searches for dark sector phenomena. The experiment operates at the Υ(4S) resonance and other center-of-mass energies to produce copious pairs of B and B̄ mesons, enabling tests of the CKM matrix and studies relevant to the Standard Model and its extensions. The project involves contributions from major institutions such as KEK, KEK member groups, University of Tokyo, CERN, DESY, Brookhaven National Laboratory, SLAC National Accelerator Laboratory, and numerous universities.

Detector and Accelerator

The Belle II detector is a multi-layer apparatus comprising a pixel vertex detector, silicon strip detector, central drift chamber, time-of-propagation counter, aerogel ring-imaging Cherenkov counters, electromagnetic calorimeter, and K0L and muon detector, integrated with a superconducting solenoid magnet and advanced readout electronics. Key subsystems were developed in partnership with groups from KEK, Nagoya University, Osaka University, University of Melbourne, University of Cincinnati, University of British Columbia, University of Victoria, University of Hawaii, INFN, CERN, and DESY. The SuperKEKB accelerator, an evolution of KEKB, employs nano-beam focusing and asymmetric beam energies to achieve a target instantaneous luminosity of 8×10^35 cm^−2 s^−1, with accelerator technologies contributed by teams from KEK, SLAC, BINP, IHEP, and CERN.

Physics Program

Belle II’s physics program spans precision determination of CKM matrix elements via measurements of B meson decays such as semileptonic processes, time-dependent CP violation in neutral B systems, and studies of rare decays sensitive to flavor-changing neutral currents and lepton flavor universality. The experiment targets searches for lepton-flavor-violating decays of the tau lepton, probes of dark photons, axion-like particles, and hidden-sector mediators, as well as measurements of hadronic cross sections relevant to the anomalous magnetic moment of the muon. Belle II complements measurements by experiments like LHCb, ATLAS, and CMS by providing clean e+e− collisions at threshold energies and by testing hypotheses from theorists at institutions such as Institute for Advanced Study, CERN Theory Division, and Perimeter Institute.

Data Acquisition and Analysis

Belle II uses a trigger and data acquisition system designed for high-rate operation, employing multi-level triggers, FPGA-based front-end electronics, and a distributed computing model for storage and analysis. The collaboration relies on grid and cloud resources coordinated with KEK, GridKa, TRIUMF, CNAF, NERSC, and national computing centers to process petabytes of raw data and simulated events. Data reduction, event reconstruction, and physics analyses utilize software frameworks developed by groups across the collaboration, incorporating algorithms for track reconstruction, particle identification, calorimetry, and Monte Carlo simulation validated against samples from Belle and testbeams at facilities like DESY and SLAC National Accelerator Laboratory.

Collaboration and Organization

The Belle II Collaboration comprises hundreds of physicists, engineers, and technicians from universities and laboratories including KEK, University of Tokyo, Nagoya University, Tohoku University, Kyoto University, Osaka University, University of Melbourne, Uppsala University, University of Victoria, Brookhaven National Laboratory, TRIUMF, CERN, DESY, INFN, IHEP Beijing, and Institute of High Energy Physics, Chinese Academy of Sciences. Governance is organized via an institutional board, executive board, and spokespersons elected from collaboration members, with working groups focused on detector subsystems, physics analyses, computing, and upgrade plans. Funding and oversight involve agencies such as MEXT (Japan), JST, NSF, DOE, ERC, DFG, INFN, and national ministries supporting participating institutions.

Results and Impact

Since commissioning and early physics runs, Belle II has reported performance metrics for subsystems, precision measurements that refine inputs to global fits of the CKM matrix, and limits on rare and forbidden processes, providing competitive or complementary constraints relative to results from LHCb, ATLAS, CMS, and NA62. Ongoing analyses address tensions in flavor observables, such as anomalies reported by other experiments, and contribute to global efforts like the Particle Data Group and combined fits by theory groups at CERN and University of Cincinnati. Belle II’s technological developments in pixel detectors, particle identification, and high-luminosity accelerator techniques influence future projects including the International Linear Collider, upgrades at LHC, and proposed flavor factories.

Category:Particle physics experiments Category:High energy physics