# Belle experiment

The Belle experiment is a particle physics experiment conducted by the Belle Collaboration, an international collaboration of more than 400 physicists and engineers investigating CP-violation effects at the High Energy Accelerator Research Organisation (KEK) in Tsukuba, Ibaraki Prefecture, Japan.

The Belle detector, located at the collision point of the ee+ asymmetric-energy collider (KEKB), is a multilayer particle detector. Its large solid angle coverage, vertex location with precision on the order of tens of micrometres (provided by a silicon vertex detector), good pionkaon separation at the momenta range from 100 MeV/c till few GeV/c (provided by a novel Cherenkov detector), and few-percent precision electromagnetic calorimetry (CsI(Tl) scintillating crystals) allow for many other scientific searches apart from CP-violation. Extensive studies of rare decays, searches for exotic particles and precision measurements of B mesons, D mesons, and tau particles have been carried out and have resulted in almost 300 publications in physics journals.

Highlights of the Belle experiment so far include

• the first observation of CP-violation outside of the kaon system (2001)
• observation of: ${\displaystyle B\to K^{*}l^{+}l^{-}}$ and ${\displaystyle b\to sl^{+}l^{-}}$
• measurement of ${\displaystyle \phi _{3}}$ using the ${\displaystyle B\to DK,D\to K_{S}\pi ^{+}\pi ^{-}}$ Dalitz plot
• measurement of the CKM quark mixing matrix elements ${\displaystyle |V_{ub}|}$ and ${\displaystyle |V_{cb}|}$
• observation of direct CP-violation in ${\displaystyle B^{0}\to \pi ^{+}\pi ^{-}}$ and ${\displaystyle B^{0}\to K^{-}\pi ^{+}}$
• observation of ${\displaystyle b\to d}$ transitions
• evidence for ${\displaystyle B\to \tau \nu }$
• observations of a number of new particles including the X(3872)

The Belle experiment operated at the KEKB accelerator, the world's highest luminosity machine. The instantaneous luminosity exceeded 2.11×1034 cm−2·s−1. The integrated luminosity collected at the ϒ(4S) resonance mass is ~710 fb−1 (corresponds to 771 million BB meson pairs). Most data is recorded on the ϒ(4S) resonance, which decays to pairs of B mesons. About 10% of the data is recorded below the ϒ(4S) resonance in order to study backgrounds. In addition, Belle has carried out special runs at the ϒ(5S) resonance to study B
s
mesons
as well as on the ϒ(1S), ϒ(2S) and ϒ(3S) resonances to search for evidence of Dark Matter and the Higgs Boson. The samples of ϒ(1S), ϒ(2S) and ϒ(5S) collected by Belle are the world largest samples available.

The Belle II B-factory, an upgraded facility with two orders of magnitude more luminosity, has been approved in June 2010.[1] The design and construction work is ongoing.