Belle & Belle II

Particle physics experiments Belle & Belle II

In the big bang, matter and antimatter should have been created in equal amounts. But why is the universe today filled almost only with matter? The Belle II experiment at the SuperKEKB accelerator in Japan aims to solve this great mystery of particle physics. DESY too is involved in the ambitious experiment, which started operation in 2018, as well as in the data analysis of its predecessor, Belle.

Physicists attribute the different behaviour of matter and antimatter to the violation of the so-called CP symmetry, which states that the laws of physics should not change if a particle is interchanged with its antiparticle and the signs of all spatial coordinates are flipped. Scientists have already identified processes that break this CP symmetry. However, the extent of the observed CP violation is not sufficient to explain the actual excess of matter in the universe. According to the researchers, this excess is brought about by “new physics” that goes beyond the Standard Model of particle physics.

There are many ideas and theories about what this new physics could be – ranging from extra dimensions to supersymmetry. One way to find out which of those theories actually corresponds to reality is to measure the difference in behaviour of matter and antimatter extremely accurately. To this end, the Japanese accelerator laboratory KEK in Tsukuba is upgrading its KEKB ring accelerator, which it operated from 1999 to 2010, to a “super B factory” – an electron–positron collider that produces a large amount of particles called B mesons. The bases for these B factories were established, among others, at the DORIS storage ring at DESY, where researchers observed the conversion of a B meson into its antiparticle, an anti-B meson, for the first time in 1987 using the ARGUS detector

Particle production at the B factory

The three-kilometre-long upgraded accelerator SuperKEKB will deliver a 40-fold higher collision rate than its predecessor and thus produce about 1000 pairs of B mesons per second. Their decays will be measured with high precision using the Belle II detector. Together with seven German universities and the Max Planck Institute for Physics in Munich, DESY is building a central component of the large device, the pixel vertex detector, which records the particle decays right at the collision point. The DEPFET technology, which was originally designed for the future International Linear Collider ILC, will be used for this purpose for the first time. In addition, DESY will participate in the data acquisition and analysis of Belle II.

The Belle II collaboration consists of about 600 scientists from 94 institutes in 23 countries, with German physicists constituting the second largest group after their colleagues from Japan. Within Germany, Belle II is thus the third largest particle physics project after the ATLAS and CMS experiments at the LHC accelerator at CERN in Geneva. SuperKEKB has accelerated its first particles in 2018, and the Belle II detector started the research programme in 2019. The record collision rate at SuperKEKB will allow unique experiments that are complementary to those at the LHC, where B mesons are examined using the LHCb detector.

Data analysis at DESY

In 2012, DESY also joined the Belle collaboration, which evaluates the data of the predecessor experiment and hunts for evidence of new physics. DESY provides storage space for the huge amount of data recorded from 1999 to 2010, as well as Grid infrastructure and computer resources for data analysis as part of its National Analysis Facility (NAF). The Belle data will provide valuable insights for the future data acquisition and analysis at Belle II.

Collaboration with Japanese particle physics institutes has a long tradition at DESY. More than 40 years ago already, Japanese scientists participated in the DASP experiment at the DORIS storage ring, followed by the JADE („JApan-Deutschland-England“) experiment at the PETRA accelerator. HERA saw major Japanese participation in the ZEUS and HERMES experiments. DESY is also cooperating with Japanese institutions within the framework of future projects, such as the detector development for the ILC. With DESY’s participation in Belle and Belle II, these joint research activities have been extended for the first time to an experiment in Japan.

Facts and figures


  • Experiment for measuring electron–positron collisions at the KEKB accelerator of the KEK high-energy accelerator research organization in Tsukuba, Japan
  • Operation: from 1999 to 2010
  • Number of generated B meson pairs: nearly 800 million
  • Data analysis: ongoing

Belle II

  • Successor of Belle at the upgraded accelerator, SuperKEKB
  • 7.5 m long, 7 m high
  • Start of research operation: 2016
  • Expected number of generated B meson pairs: ca. 40 billion