Belle II collaboration

Belle II collaboration is a large international scientific collaboration operating the Belle II experiment at the SuperKEKB electron-positron collider in Japan. Its primary mission is to search for new physics beyond the Standard Model by studying the properties of B mesons, charm hadrons, and tau leptons with unprecedented precision. The collaboration involves over 1,000 scientists and engineers from more than 100 institutions across 26 countries, making it a cornerstone of modern high-energy physics research in the flavor sector.

Overview

The collaboration was formed to build and operate the Belle II detector, the successor to the highly successful Belle experiment which ran at the KEKB accelerator from 1999 to 2010. The new experiment is located at the High Energy Accelerator Research Organization (KEK) in Tsukuba, Ibaraki, and began physics data-taking in 2019 following an extensive commissioning period. Its design and research program are directly motivated by the legacy of the Belle experiment and its competitor, the BaBar experiment at the SLAC National Accelerator Laboratory, which together established the phenomenon of CP violation in the B meson system. The operation of the SuperKEKB collider, a major upgrade of KEKB, is a joint endeavor between the accelerator team at KEK and the collaboration, aiming to achieve a world-record instantaneous luminosity to produce vast samples of bottom and charm particles.

Experimental setup

The core of the experimental setup is the Belle II detector, a general-purpose spectrometer surrounding the interaction point of SuperKEKB. It is composed of several nested sub-detectors, including a silicon vertex detector using DEPFET and DSSD technologies for precise tracking, a central drift chamber for momentum measurement and particle identification, and a Cherenkov-based TOP counter and Aerogel RICH for distinguishing between pion, kaon, and proton signatures. The electromagnetic calorimeter utilizes caesium iodide crystals doped with thallium (CsI(Tl)) to measure energy from photons and electrons, while the K_L and muon detector (KLM) uses resistive plate chambers and scintillators to identify muons and neutral kaons. The entire detector is embedded within a 1.5 tesla superconducting solenoid magnet provided by Fermilab.

Physics goals and research program

The primary physics goal is to make precise tests of the Standard Model and uncover discrepancies that could point to new physics, such as supersymmetry or theories involving extra dimensions. A major focus is the detailed study of CP violation and the measurement of the angles and sides of the Unitarity Triangle through decays of B mesons and charmed hadrons. The collaboration also conducts extensive searches for rare or forbidden decays, such as those that could reveal the influence of a Z' boson or leptoquark, and probes for violations of lepton flavor universality. Additional programs include spectroscopy of exotic hadrons like tetraquark and pentaquark states, precision measurements of CKM matrix elements, and studies of quantum entanglement and dark sector particles through invisible decays.

Key results and discoveries

The collaboration has produced several significant results, including the world's most precise measurement of the CKM angle φ₂ (α) using B→ππ decays and stringent new constraints on the branching fraction of the highly suppressed decay B⁰ → K^*0 ν ν̄. It has performed landmark tests of lepton flavor universality in B→D(*) ℓ ν decays, providing important independent checks of anomalies reported by the LHCb experiment at CERN. Furthermore, Belle II has observed the exotic hadron T_cc⁺ and set world-best limits on the mass of the dark photon and the decay τ → μγ. These measurements are crucial for global analyses by theory groups like the UTfit Collaboration and the CKMfitter Group.

Collaboration structure and participating institutions

The collaboration is governed by a Spokesperson and an Executive Board, with scientific work organized into numerous physics and working groups covering areas like software, computing, and detector operations. Major participating institutions include KEK, the University of Tokyo, Tohoku University, the Indian Institutes of Technology, the University of Melbourne, and many leading laboratories and universities from the Americas, Europe, and Asia. Key international partners contributing critical detector components and computing resources include Fermilab, the Max Planck Institute for Physics, the University of Bonn, the University of Cincinnati, and the Budker Institute of Nuclear Physics. Data processing and analysis rely heavily on a distributed computing grid utilizing resources from the Worldwide LHC Computing Grid and other infrastructures.

Category:Particle physics collaborations Category:Experiments in particle physics Category:Research institutes in Japan