The Belle experiment was a particle physics experiment conducted by the Belle Collaboration, an international collaboration of more than 400 physicists and engineers, at the High Energy Accelerator Research Organisation ( KEK) in Tsukuba, Ibaraki Prefecture, Japan. The experiment ran from 1999 to 2010. ^[1]

The Belle detector was located at the collision point of the asymmetric-energy electron– positron collider, KEKB. Belle at KEKB together with the BaBar experiment at the PEP-II accelerator at SLAC were known as the B-factories as they collided electrons with positrons at the center-of-momentum energy equal to the mass of the
ϒ
(4S) resonance which decays to pairs of B mesons.

The Belle detector was a hermetic multilayer particle detector with large solid angle coverage, vertex location with precision on the order of tens of micrometres (provided by a silicon vertex detector), good distinction between pions and kaons in the momenta range from 100 MeV/c to few GeV/c (provided by a Cherenkov detector), and a few-percent precision electromagnetic calorimeter (made of CsI( Tl) scintillating crystals).

The Belle II experiment is an upgrade of Belle that was approved in June 2010. ^[2] It is currently being commissioned, ^[3] and is anticipated to start operation in 2018. ^[4] Belle II is located at SuperKEKB (an upgraded KEKB accelerator) which is intended to provide a factor 40 larger integrated luminosity. ^[5]

Results

The experiment was motivated by the search for CP-violation. ^[6] However the experiment also performed extensive studies of rare decays, searches for exotic particles and precision measurements of the properties of D mesons, and tau particles. ^[1] The experiment has resulted in almost 300 publications in physics journals.

Highlights of the Belle experiment include

• an observation of large CP-violation in the neutral B meson system ^[7]
• measurement of the branching fraction of inclusive ${\displaystyle B\to X_{s}\gamma }$ decays ^[8]
• observation of the ${\displaystyle b\to sl^{+}l^{-}}$ transition with ${\displaystyle B\to Kl^{+}l^{-}}$ ^[9] and ${\displaystyle B\to K^{*}l^{+}l^{-}}$ ^[10]
• measurement of ${\displaystyle \phi _{3}}$ using the ${\displaystyle B\to DK,D\to K_{S}\pi ^{+}\pi ^{-}}$ Dalitz plot ^[1]
• measurement of the CKM quark mixing matrix elements ${\displaystyle |V_{ub}|}$ and ${\displaystyle |V_{cb}|}$ ^[1]
• observation of direct CP-violation in ${\displaystyle B^{0}\to \pi ^{+}\pi ^{-}}$ ^[11] and ${\displaystyle B^{0}\to K^{-}\pi ^{+}}$ ^[12]
• observation of ${\displaystyle b\to d}$ transitions ^[13]
• evidence for ${\displaystyle B\to \tau \nu }$ ^[14]
• observations of a number of new particles including the X(3872) ^[15]

Data samples

The KEKB accelerator was the world's highest luminosity machine at the time.^{[ citation needed]} A large fraction of the data was collected at the
ϒ
(4S). The instantaneous luminosity exceeded 2.11×10³⁴ cm⁻²·s⁻¹. The integrated luminosity collected at the
ϒ
(4S) mass was about 710  fb⁻¹ (corresponding to 771 million
B

B
meson pairs). About 10% of the data was recorded below the
ϒ
(4S) resonance in order to study backgrounds. In addition, KEKB 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.^{[ citation needed]}

See also

• B-factory
• 
  B
  –
  B
  oscillation

References

External links

Wikimedia Commons has media related to Belle experiment.

• Official Belle Website
• Belle II Collaboration Website
• Belle II Public Website
• Record for KEK-BF-BELLE Experiment on INSPIRE-HEP

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