Belle II

Belle II
Name	Belle II
Location	Tsukuba, Japan
Facility	KEK
Collider	SuperKEKB
Type	Particle detector
Operation	2018–present
Collaborators	KEK, CERN, Fermilab, DESY, University of Tokyo, KEK Theory Center, SLAC, Purdue University, University of Melbourne

Belle II Belle II is a high-luminosity flavor physics experiment located at KEK in Tsukuba, designed to exploit the SuperKEKB asymmetric-energy electron–positron collider for precision studies of heavy-flavor decays and rare processes. The experiment extends the legacy of the Belle detector and the BaBar program, targeting measurements complementary to searches at the LHC experiments such as ATLAS and CMS. Belle II integrates advanced detector technologies and an international collaboration spanning universities and laboratories including KEK, CERN, DESY, and Fermilab.

Overview

Belle II operates in the energy region of the Υ(4S) resonance to produce copious pairs of B mesons and also collects data at other bottomonium states such as Υ(5S) and at charmonium thresholds like the J/ψ and ψ(3770). The physics goals include precision tests of the CKM matrix, searches for CP violation beyond the Standard Model, studies of lepton flavor universality in decays such as B → K(*)ℓ+ℓ− and measurements of rare decays sensitive to new physics scenarios including supersymmetry, leptoquarks, and dark sector portals. The experiment complements flavor programs at LHCb and inputs to global fits performed by groups such as the CKMfitter Group and the UTFit Collaboration.

Detector and Accelerator

The detector is installed at the interaction point of SuperKEKB, an upgrade of the KEKB collider utilizing a nano-beam scheme pioneered with input from SLAC and LNF. Key subdetectors include a vertex detector composed of DEPFET-pixel layers and double-sided silicon strip detectors, a central drift chamber influenced by designs at Belle and BABAR, a time-of-propagation counter inspired by work at LHCb and BaBar, an electromagnetic calorimeter using CsI(Tl) crystals with heritage from Belle, and a K-long and muon detector based on resistive plate chambers and scintillators developed with expertise from CERN and DESY. The SuperKEKB accelerator relies on superconducting radio-frequency cavities and final-focus quadrupoles with collaboration from KEK Accelerator Laboratory and technology sharing with SLAC and BNL.

Physics Program

Belle II's program spans measurements of |Vub| and |Vcb| magnitudes in semileptonic B meson decays with theoretical input from Lattice QCD groups at RIKEN and Fermilab/MILC, searches for charged-lepton-flavor-violating decays such as τ → μγ and τ → 3μ with sensitivity competitive with BaBar limits, and spectroscopy of exotic hadrons including X(3872), Z_c(3900), and Y(4260) states discovered at Belle and BESIII. The experiment probes radiative and electroweak penguin processes like b → sγ and b → sℓ+ℓ− to test theoretical frameworks such as Operator Product Expansion and effective theories used in analyses by groups like HFLAV. Belle II contributes to determinations of the CKM angle γ through measurements of B → DK decays and provides inputs to global fits relevant to potential CP violation sources beyond those probed by Kaon experiments such as NA62.

Collaboration and Organization

The collaboration is multinational, including institutions such as KEK, University of Tokyo, Nagoya University, Tohoku University, Oxford University, University of Melbourne, University of Cincinnati, and national laboratories including CERN, DESY, Fermilab, and SLAC. Governance includes an institutional board, spokespersons elected by members, and working groups covering topics like detector performance, software and computing, physics analysis, and machine-detector interface with links to accelerator teams at SuperKEKB and the KEK Accelerator Laboratory. Funding and oversight involve agencies such as MEXT, NSF, DOE, and equivalents in participating countries.

Construction and Commissioning

Construction drew on international contributions: the vertex detector modules from groups at KEK, University of Melbourne, and CERN; the calorimeter refurbishment coordinated with teams from Belle and SLAC; and the time-of-propagation counter developed with expertise from University of Hawaii and DESY. Commissioning phases included a first beam campaign with the BEAST II background detector program to validate shielding and background models, followed by phase runs that incrementally integrated the pixel detector and full data acquisition systems with support from computing centers including KEK Computing Research Center and grid sites at CERN and national grid infrastructures.

Results and Impact

Early results have provided improved measurements of tau lepton properties, searches for dark sector particles such as dark photons motivated by models from Holdom and constraints on invisible decays relevant to axion-like particles and light dark matter scenarios. Belle II's precision flavor measurements feed into global fits by HFLAV, CKMfitter Group, and UTFit Collaboration, informing tensions observed in semileptonic decay ratios reported by LHCb and BaBar. The experiment's detector R&D impacts future projects at facilities like SuperKEKB upgrades, proposals for a Future Circular Collider flavor program, and synergy with Belle legacy analyses and ongoing work at BESIII.

Category:Particle physics experiments