Belle II experiment

Belle II experiment
Name	Belle II experiment
Caption	The Belle II detector at the SuperKEKB accelerator.
Collaboration	Belle II collaboration
Accelerator	SuperKEKB
Location	KEK (Tsukuba, Ibaraki)
Energy	7 GeV (electrons) / 4 GeV (positrons)
Type	Particle physics experiment
Runtime	2018–present
Predecessor	Belle experiment

Belle II experiment. It is a major international particle physics experiment located at the KEK laboratory in Tsukuba, Ibaraki, Japan. The experiment is designed to study the properties of B mesons and other particles produced in high-energy collisions at the SuperKEKB particle accelerator. As the successor to the highly successful Belle experiment, its primary mission is to investigate the subtle differences between matter and antimatter, known as CP violation, and to search for signs of physics beyond the Standard Model.

Overview

The Belle II experiment represents a significant upgrade and continuation of the research program initiated by its predecessor at the KEK B-Factory. It operates at the intensity frontier, colliding intense beams of electrons and positrons at the Υ(4S) resonance energy to produce copious amounts of B meson-anti-B meson pairs. The international Belle II collaboration involves over 1,000 scientists and engineers from more than 100 institutions across 26 countries. The experiment began its physics data-taking run in 2019 following an extensive commissioning phase, aiming to collect a dataset approximately 50 times larger than that of the original Belle experiment.

Physics goals

The core physics program is focused on making ultra-precise measurements of CP violation in the B meson system, building upon the Nobel Prize-winning work of Makoto Kobayashi and Toshihide Maskawa. Key measurements include refining the parameters of the Cabibbo–Kobayashi–Maskawa matrix and searching for rare B meson decays that are highly suppressed in the Standard Model. The experiment also conducts detailed studies of charm, tau, and quarkonium physics. A major goal is to identify anomalies that could indicate the presence of new particles or forces, such as those predicted by theories of supersymmetry or dark matter.

Detector design

The Belle II detector is a sophisticated, hermetic spectrometer surrounding the interaction point of SuperKEKB. Its innermost component is a two-layer silicon pixel detector based on DEPFET technology, providing precise vertex reconstruction. This is surrounded by a four-layer double-sided silicon strip detector and a central drift chamber for tracking charged particles and measuring their momentum. Particle identification is achieved by a time-of-propagation counter in the barrel and an aerogel ring-imaging Cherenkov detector in the forward endcap. An electromagnetic calorimeter composed of caesium iodide crystals detects photons and electrons, while the outermost layers consist of resistive plate chambers and scintillators for identifying muons and K_L mesons.

Accelerator and SuperKEKB

The experiment is powered by the SuperKEKB accelerator, a major upgrade of the original KEKB collider. SuperKEKB employs the innovative "nano-beam" scheme, where the colliding electron and positron beams are squeezed to unprecedented vertical sizes at the interaction point to dramatically increase the luminosity. The electron beam, with an energy of 7 GeV, circulates in the high-energy ring, while the 4 GeV positron beam circulates in the low-energy ring. This asymmetric design provides a boost to the center-of-mass system, crucial for time-dependent CP violation measurements. Achieving its design luminosity would make SuperKEKB the world's highest-luminosity collider.

Data collection and results

Following initial commissioning, the experiment recorded its first collisions in 2018 and began full physics operations in 2019. The collaboration has already published numerous results based on early data, including measurements of B meson lifetimes, studies of charmonium states, and searches for rare decays like B → K ν ν̄. A landmark early analysis was the observation of the Ω_c^0 baryon decay. The experiment continues to accumulate data toward its ultimate goal of an integrated luminosity of 50 ab⁻¹, which will enable searches for extremely rare processes with sensitivities to potential new physics at energy scales far beyond the direct reach of the Large Hadron Collider.

Collaboration and institutions

The Belle II collaboration is a global partnership coordinated by the KEK institute. Major contributing nations include Germany, the United States, India, Russia, China, Italy, France, and Australia. Key participating institutions include the University of Hawaii, the Max Planck Institute for Physics, Tata Institute of Fundamental Research, Budker Institute of Nuclear Physics, and the University of Melbourne. The collaboration is organized into working groups covering detector subsystems, physics analysis, computing, and accelerator operations, with project leadership provided by spokespersons elected from the member institutions.

Category:Particle physics experiments Category:KEK Category:B physics