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Installation of the Pixel Vertex Detector in the Belle II experiment, July 2024 (Photo: Botho Paschen/Universität Bonn)

Next step in the matter-antimatter mystery: World's largest B-meson dataset

The Belle II experiment at the SuperKEKB accelerator in Japan has accumulated a vast number of B-meson events in its search for physics beyond the Standard Model. With this unique dataset, the international collaboration, including researchers from the Max Planck Institute for Physics (MPP), has achieved a major milestone toward a new era of precision measurements. These studies may help answer one of the most fundamental questions in physics: Why does the universe contain so much matter but almost no antimatter?

Physicists hope to gain new insights from B mesons, particles that are produced together with their antiparticles when electrons and positrons collide. The SuperKEKB accelerator acts as a kind of "factory" for B mesons: it collides matter (electrons) and antimatter (positrons) at the resonance energy of the Y(4S) (Upsilon-4S). At this energy, the particle decays produced in electron–positron collisions are at their cleanest. This means that B mesons can be generated in an exceptionally low-background environment and measured with high precision.

Since the experiment began operation in 2019, scientists have continuously optimized the accelerator and achieved stable operation at significantly higher luminosities than its predecessor, Belle at KEKB. High luminosity - the number of particle collisions per unit area and time - enables researchers to collect larger datasets and perform measurements with greater precision.

Three new records

At the end of May, the dataset collected at the Υ(4S) energy for studies of B-meson decays became the world's largest. In addition, Belle II achieved two further milestones:

  • A new peak luminosity approximately 2.5 times higher than Belle's previous record. 
  • A data-taking rate roughly three times faster than the best year of the Belle experiment. 
Arrangement of the silicon modules in the pixel detector. The modules are installed in two layers and are only 75 micrometers thick, like human hair. There, the collisions of electrons and positrons are measured with extreme precision. (Photo: MPP)

The main contribution of the MPP Belle II group led by Dr. Hans-Günther Moser was the design and construction of the pixel vertex detector, together with DESY and several major German universities. With an active sensitive-layer thickness of just 75 µm, like human hair, this detector remains the thinnest operating silicon detector and is essential for Belle II to achieve the highest-precision measurements. Furthermore, MPP physicists contributed to the development of a neural-network trigger with superior background-rejection capabilities.

The Belle II collaboration comprises more than 1,200 members from 28 countries and regions worldwide. Its physics program focuses on precision studies of heavy quarks and leptons in a field known as flavor physics. In particular, researchers are searching for evidence of physics beyond the Standard Model through extremely rare and experimentally challenging processes. By comparing highly precise measurements with theoretical predictions, Belle II aims to uncover possible signs of new particles or fundamental forces that lie beyond current theories of particle physics.

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