Project Belle II Experiment: Objectives, Working and Current Status

Source: The Hindu
GS III: Science and Technology

Original publication: 13 April 2017
Last updated                : July 2026

Overview

1. News in Brief
2. About Belle II

Why in the News?

The High Energy Accelerator Research Organisation (KEK) completed the much-awaited ‘rolling-in’ of the Belle-II experiment in Tsukuba, Japan.

In 2026, the Belle-II experiment crossed several major scientific and operational milestones.

News in Brief

• This experiment is designed to study violations of the Standard Model of particle physics.
• A grand collaboration of 700 scientists from 23 countries, Belle-II has significant Indian participation both on experimental and theoretical sides.
• Belle II is a particle physics experiment located at the KEK research facility in Tsukuba, Japan.
• It operates at SuperKEKB, one of the world’s most powerful electron–positron colliders.
• The experiment studies B mesons, D mesons, tau leptons and other short-lived particles.
• Its main objective is to search for physics beyond the Standard Model.
• In March 2026, SuperKEKB achieved a world-record peak luminosity of nearly 5.2 × 10³⁴ cm⁻²s⁻¹.
• In May 2026, Belle II accumulated the world’s largest Υ(4S) dataset.
• Belle II had collected about 864 fb⁻¹ of data by the end of its latest run.
• Its long-term target is to collect 50 ab⁻¹ of integrated luminosity.
• Indian scientists have contributed to the experiment’s silicon vertex detector, computing infrastructure and scientific analysis.

What Is Project Belle II?

• Project Belle II, commonly called the Belle II experiment, is an international scientific experiment designed to investigate the fundamental structure of matter and the forces governing subatomic particles.
• It is operated at the High Energy Accelerator Research Organization, known as KEK, in Tsukuba, Japan.
• Belle II is installed around the collision point of SuperKEKB. The accelerator brings together beams of electrons and positrons travelling in opposite directions.
• When these particles collide, they produce several other particles, including pairs of B mesons.
• The Belle II detector records the particles created during these collisions. Scientists then study their movement, energy, lifetime and decay patterns.

The experiment is described as a B-factory experiment because it produces very large numbers of B mesons for detailed scientific study.

What Is SuperKEKB?

• SuperKEKB is an electron–positron collider located at the KEK laboratory in Japan.
• An electron is a negatively charged elementary particle, while a positron is its positively charged antimatter counterpart.
• SuperKEKB accelerates electrons and positrons to very high energies and makes them collide.

The collider consists of two storage rings:

• • • The High Energy Ring carries electrons.
    • The Low Energy Ring carries positrons.
• The electron and positron beams are made extremely narrow at the collision point through a technique called the nano-beam scheme. This increases the probability of collisions and allows the experiment to produce a very large number of B mesons and other particles.
• SuperKEKB is called a luminosity-frontier accelerator. Instead of merely achieving the highest collision energy, it seeks to generate an exceptionally large number of collisions.

What Is Luminosity in Particle Physics?

• Luminosity measures the number of potential particle collisions produced by an accelerator within a given area and time.
• Higher luminosity means more collisions. More collisions produce more particle-decay events for scientists to examine.
• This is particularly important for Belle II because many of the processes it studies are extremely rare. Some particle decays may occur only once in millions or billions of events.

In March 2026, SuperKEKB achieved an instantaneous luminosity of about 5.2 × 10³⁴ cm⁻²s⁻¹. Belle II had accumulated approximately 864 fb⁻¹ of integrated luminosity by the end of its latest run period.

What Are B Mesons?

• B mesons are short-lived subatomic particles containing a bottom quark, also known as a beauty quark, combined with another quark.
• They exist only for a very short period before decaying into lighter particles. However, their decay patterns provide valuable information about the fundamental laws of physics.

B mesons are important because:

• They help scientists study matter–antimatter differences.
• Their rare decays may reveal the presence of unknown particles.
• They can be used to test the accuracy of the Standard Model.
• They provide information about quark mixing and CP violation.
• Their decay channels may offer indirect evidence of dark-sector particles.

At SuperKEKB, electron–positron collisions are commonly produced at an energy corresponding to the Υ(4S) resonance. This state decays almost immediately into a B meson and an anti-B meson.

What Is the Belle II Detector?

• The Belle II detector is a large, highly sensitive scientific instrument surrounding the collision point of SuperKEKB.
• It does not directly photograph subatomic particles. Instead, its different layers measure the tracks, energy, charge and identity of particles produced during collisions.

The major components of the detector include:

• Pixel Detector: Located closest to the collision point, it provides highly precise information about where particles are produced and where they decay.
• Silicon Vertex Detector: It tracks charged particles close to the interaction point and helps reconstruct the decay positions of short-lived particles.
• Central Drift Chamber: It measures the trajectories and momentum of charged particles.
• Time-of-Propagation Detector: It helps distinguish between different types of charged particles, especially pions and kaons.
• Aerogel Ring-Imaging Cherenkov Detector: It identifies charged particles using the light produced when they travel through a medium.
• Electromagnetic Calorimeter: It measures the energy of electrons and photons.
• K-Long and Muon Detector: It identifies muons and long-lived neutral kaons.

Together, these components enable scientists to reconstruct complex particle-decay events.

What Are the Main Objectives of Belle II?

Belle II has several interconnected scientific objectives.

Study CP violation

• CP violation refers to differences in the behaviour of matter and antimatter.
• The known level of CP violation under the Standard Model is insufficient to explain why the observable universe is dominated by matter even though matter and antimatter are believed to have been created in nearly equal amounts after the Big Bang.
• Belle II studies CP violation in B mesons and other particles to search for additional sources of matter–antimatter asymmetry.

Test the Standard Model

• The Standard Model explains the known elementary particles and three of the four fundamental forces.
• However, it does not adequately explain:
  • Dark matter
  • Dark energy
  • Gravity
  • Neutrino masses
  • The dominance of matter over antimatter
  • The full pattern of particle masses

Belle II conducts precise measurements to test whether experimental observations match Standard Model predictions.

Study rare particle decays

• Some particle decays occur extremely rarely. Unknown particles may indirectly influence these decay processes, even when the new particles cannot be produced or detected directly.
• Belle II therefore studies rare decays of:
  • B mesons
  • D mesons
  • Tau leptons
  • Bottomonium and charmonium states

Search for dark-sector particles

Belle II searches for possible particles connected with dark matter, including:

• Dark photons
• Axion-like particles
• Invisible particles
• Long-lived particles
• Other weakly interacting particles

Test lepton flavour universality

• According to the Standard Model, the fundamental interactions of electrons, muons and tau leptons should be similar after differences in their masses are considered.
• Belle II compares particle decays involving different types of leptons. Significant deviations could indicate new physics.

Study exotic hadrons

• The experiment investigates unusual combinations of quarks that do not fit easily into the traditional classification of mesons and baryons.
• These include tetraquarks, pentaquarks and other exotic hadronic states.

Belle and Belle II: What Is the Difference?

• Belle II is the upgraded successor to the original Belle experiment.
• The original Belle experiment operated at the KEKB accelerator and made several important contributions to particle physics, especially the study of CP violation in B mesons.
• Its results, together with those of the BaBar experiment in the United States, helped confirm the mechanism of CP violation described by Japanese physicists Makoto Kobayashi and Toshihide Maskawa. Their work was recognised with the 2008 Nobel Prize in Physics.

Belle II has several improvements over Belle:

• SuperKEKB provides a much higher collision rate.
• Belle II contains more advanced tracking and particle-identification systems.
• Its vertex detector provides greater positional accuracy.
• Its computing system can process much larger quantities of data.
• It is designed to study rarer particle decays with better precision.
• It aims to collect about 50 times the integrated luminosity of Belle.

Timeline of the Belle II Experiment

2010: The Belle II Technical Design Report outlined the proposed detector and scientific programme.

2016: SuperKEKB began its initial accelerator commissioning.

2017: The Belle II detector was rolled into its position around the SuperKEKB interaction point.

2018: Initial collision data were recorded without the complete vertex detector.

2019: Full physics data collection began with the main detector systems operational.

2020: SuperKEKB achieved a world record in instantaneous luminosity, later improving the record several times.

2022: The first major data-taking phase ended, and the experiment entered its first long shutdown.

2023: A new and more complete vertex detector was installed and commissioned.

2024: Belle II began Run 2 and resumed collision-data collection following the long shutdown.

2026: SuperKEKB achieved a peak luminosity of approximately 5.2 × 10³⁴ cm⁻²s⁻¹. Belle II surpassed the original Belle experiment’s Υ(4S) dataset and accumulated about 864 fb⁻¹ of total data.

India’s Contribution to Project Belle II

India has participated in the Belle scientific programme since the period of the original Belle experiment.

Indian researchers are involved in:

• Detector design and construction
• Development of the silicon vertex detector
• Experimental data analysis
• Theoretical studies
• Software development
• Grid computing and data processing
• Training students and researchers in high-energy physics

The Tata Institute of Fundamental Research, or TIFR, played an important role in the development of Layer 4 of Belle II’s silicon-strip Vertex Detector.

The TIFR team contributed to the design, prototyping and construction of this detector layer. It produced 12 high-quality Silicon Vertex Detector modules, of which ten were installed and two were retained as spares.

TIFR also hosts a grid-computing facility for Belle II. Such facilities help process, store and analyse the enormous quantities of data generated by the experiment.

India’s contribution demonstrates the country’s growing capability in advanced detector technology, electronics, precision engineering, computing and international mega-science collaboration.

Why Is Belle II Important for Science?

Belle II is important because it explores some of the deepest unanswered questions in modern physics.

It may help scientists understand:

• Why matter dominates over antimatter
• Whether unknown particles influence rare decays
• Whether the Standard Model remains valid at very high precision
• Whether dark matter has weak interactions with known particles
• Whether all three charged leptons interact universally
• How quarks combine to form conventional and exotic particles
• Whether additional sources of CP violation exist

Even when no new particle is discovered, precise measurements can restrict possible theories and guide future experiments.

Significance of Belle II for India

India’s participation has scientific, technological and strategic importance.

Development of advanced technology

Indian institutions gain experience in semiconductor detectors, precision instrumentation, electronics, software and distributed computing.

Training of skilled human resources

Students and researchers receive exposure to frontier-level experimental physics and international scientific cooperation.

Participation in mega-science projects

Belle II strengthens India’s presence in large international research programmes and complements Indian involvement in CERN, ITER, FAIR, LIGO and other global projects.

Domestic scientific capacity

The expertise gained from detector construction and data analysis can support future scientific facilities and experiments in India.

Technological spillovers

Technologies developed for particle physics may later find applications in medicine, imaging, radiation detection, computing and materials research.

Challenges Before the Belle II Experiment

Despite its achievements, Belle II faces several challenges.

Beam instability

Operating SuperKEKB at extremely high luminosity can result in sudden beam losses. These may interrupt operations and expose sensitive detector components to radiation.

Detector backgrounds

High collision rates produce unwanted signals that can interfere with the identification of rare particle decays.

Data-management requirements

Belle II generates enormous quantities of data. Processing and analysing it requires a global network of computing facilities.

Rare-event identification

Potential new-physics processes may occur only a few times among billions of ordinary events.

Statistical uncertainty

An unusual result may arise from a random fluctuation. Scientists therefore need much larger datasets before claiming a discovery.

Long-term operation

Reaching the target of 50 ab⁻¹ requires continued improvements in accelerator stability, detector performance and international funding.

Project Belle II UPSC Prelims Facts

• Location: KEK, Tsukuba, Japan
• Accelerator: SuperKEKB
• Type: Electron–positron collider experiment
• Detector: Belle II
• Main particles studied: B mesons, D mesons and tau leptons
• Primary field: Particle physics and flavour physics
• Main objective: Search for physics beyond the Standard Model
• Important concept: CP violation
• Collider category: Luminosity-frontier collider
• 2026 luminosity record: Around 5.2 × 10³⁴ cm⁻²s⁻¹
• Latest accumulated data: Approximately 864 fb⁻¹
• Next data milestone: 1 ab⁻¹
• Long-term target: 50 ab⁻¹
• Major Indian contribution: Layer 4 of the Silicon Vertex Detector led by TIFR
• Predecessor: Belle experiment at KEKB

Frequently Asked Questions

What is Project Belle II?

Project Belle II is an international particle physics experiment at the SuperKEKB electron–positron collider in Japan. It studies B mesons, charm particles and tau leptons to search for physics beyond the Standard Model.

Where is the Belle II experiment located?

It is located at the KEK research facility in Tsukuba, Japan.

What is the current status of Project Belle II?

Belle II is operational. It began Run 2 in 2024, achieved a world-record peak luminosity in March 2026 and had collected approximately 864 fb⁻¹ of data by the end of its latest run.

What is the purpose of the Belle II experiment?

Its purpose is to study CP violation, rare particle decays, lepton-flavour universality, exotic hadrons and possible dark-sector particles.

Is India participating in Belle II?

Yes. Indian scientists have contributed to detector construction, data analysis, software and computing. TIFR led the construction of Layer 4 of the silicon-strip Vertex Detector.

What is the difference between SuperKEKB and Belle II?

SuperKEKB is the accelerator that collides electrons and positrons. Belle II is the detector that records and analyses the particles produced in those collisions.

Why are B mesons important?

B mesons help scientists study CP violation, matter–antimatter differences and rare processes that may contain indirect signs of new physics.

Has Belle II discovered new physics?

Belle II has obtained several important and unusual results, but no observation has yet been confirmed as definitive evidence of physics beyond the Standard Model. Larger datasets and independent confirmation are required.

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