The Belle II experiment is a particle physics experiment designed to study the properties of B mesons (heavy particles containing a bottom quark) and other particles. Belle II is the successor to the Belle experiment, and commissioned at the SuperKEKB accelerator complex at KEK in Tsukuba, Ibaraki prefecture, Japan. The Belle II detector was "rolled in" (moved into the collision point of SuperKEKB) in April 2017. Belle II started taking data in early 2018. Over its running period, Belle II is expected to collect around 50 times more data than its predecessor, mostly due to a 40-fold increase in an instantaneous luminosity provided by SuperKEKB as compared to the previous KEKB accelerator.

Many interesting analyses of the Belle and BaBar experiments were limited by statistical uncertainties, which was the main motivation to build a new generation of B-factory - Belle II.

The target dataset is 50,000 fb at Belle II compared to 988 fb (with 711 fb at the Υ(4S) energy) at Belle. The dataset of good runs from Belle II before Long shutdown 1 was 424 fb (with 363 fb at the Υ(4S) energy.)

This immense dataset would allow studies of rare physics processes, which were out of reach for the previous e e experiments and improve precision on the already measured physics observables.

The physics program of Belle II includes the studies of the following particles or processes:

• B-meson
• Charm meson
• Bottomonium
• Charmonium
• Tau-lepton
• Dark sector
• Low-multiplicity processes

The majority of the Belle II dataset will be recorded at Upsilon(4S) center-of-mass energy, while a small portion of it will be taken at Upsilon(5S) and as energy scans.

Belle II is a general purpose high-energy particle detector with almost full solid angle coverage. It has a cylindrical shape to cover the e e collisions happening on the central axis of the detector. The detector is asymmetric in beam direction, because the initial energy of the electron beam is larger than the positron beam. Much of the original Belle detector has been upgraded to cope with the higher instantaneous luminosity provided by the SuperKEKB accelerator.

The main components are the following, from the innermost to the outermost systems.:

• Beryllium beam pipe at a minimum radius of 10 mm from the beam
• PiXel Detector (PXD) is composed of two cylindrical layers of depleted field effect transistor (DEPFET) pixels, provide as precise position measurement of the charged particle trajectories. First layer is only 14 mm away from the beam. So far, only two ladders of the second PXD layer have been installed.
• Silicon Vertex Detector (SVD) - 8 layers of silicon strip sensors arranged in cylindrical barrel and an inclined endcap towards electron direction for better coverage. The SVD and PXD detectors sometimes are referred as VerteX Detector (VXD). The total VXD size has been tuned to cover most of the typical ${\displaystyle K_{S}^{0}}$ travel distance.
• Central Drift Chamber (CDC) is a wire chamber central tracking system, which provides the measurement of momentum and charge of the charged particles produced by the collisions as well as particle type identification (PID) via their energy depositions.
• Aerogel ring-imaging Cherenkov detector (aRICH) provides PID information about the charged particle going into the forward direction.
• Time Of Propagation (TOP) counter consisting of quartz bars utilising totally internally reflected Cherenkov photons and measuring the time of propagation, which is used for PID.
• Electromagnetic calorimeter (ECL) a highly segmented array of thallium-doped caesium iodide CsI(Tl) crystals assembled in a projective geometry to measure energies of the neutral final state particles such as photons, ${\displaystyle K_{L}^{0}}$ and neutrons, as well as PID.
• Superconducting solenoid, which provides a 1.5 tesla magnetic field, which bends the trajectories of the final state charged particles to measure their charge and momentum.
• ${\displaystyle K_{L}^{0}}$ and Muon detector (KLM) used to identify ${\displaystyle K_{L}^{0}}$ energy depositions as well as for muon PID.