Made in Germany, big in Japan: Pixel Vertex detector installed in Belle II experiment

Concettina Sfienti's research group at the JGU Institute of Nuclear Physics involved in design and construction

5 September 2023

After years of development work, the new Pixel Vertex Detector (PXD2) has now been installed successfully in the international Belle II experiment at the SuperKEKB electron-positron collider in Japan. The research group of Professor Concettina Sfienti at the Institute of Nuclear Physics at Johannes Gutenberg University Mainz (JGU) made important contributions to the design and construction of PXD2. The Mainz-based scientists programmed key sections of the control software, implemented real-time monitoring of data quality, and tested sensor modules at the Mainz accelerator MAMI.

The Pixel Vertex Detector (PXD2) is just about the size of a soda can and forms the innermost detector layer of the international Belle II experiment at the Japanese SuperKEKB collider. It surrounds the beamline and, due to its very compact design, is located only 1.4 centimeters away from the collision point. Here it can deliver 50,000 high-resolution images per second. This allows the exact decay location of short-lived particles, especially B mesons, to be determined very precisely, and the decay products can be recorded with high precision. B mesons are created when electrons and positrons collide in the SuperKEKB. In combination with the high collision rate, fundamental phenomena such as CP violation can thus be studied in high detail – with the goal of understanding the imbalance between matter and antimatter in the universe.

The decay products of B mesons have relatively low energy and are easily disturbed as they pass through matter. Therefore, the first detector elements of the Belle II detector have to be as thin as possible: Thus, PXD2 consists of a total of 20 pixel detectors – 75 micrometer thin silicon modules, which are only as thick as a human hair. They are based on DEPFET technology, which is also used in satellite experiments.

The detector travels business class

Already in 2018, a first, still incomplete version of the PXD was installed at the Belle II experiment. It has already delivered valuable results, but only the new and complete version is able to handle the high luminosity to be achieved in SuperKEKB in the next years.

Due to the extremely thin detectors, PXD2 is particularly fragile and extremely sensitive to handle, which is why the transport to Japan was very complex: First, the sensitive detector had to be transferred via normal traffic roads from its assembly site at the Max Planck Institute for Physics in Munich to DESY in Hamburg in order to carry out critical performance tests and optimization of the detector parameters.

Following successful testing, the reassembled detector was sent on its next journey – this time many thousands of kilometers to the east, to Japan. The flight presented new challenges. Unexpected turbulence and improper storage during transit could easily have broken one of the sensitive silicon sensors. To guard against such hazards and keep vibrations to an absolute minimum, the detector was specially packed and traveled in business class, getting enough space in its own seat.

PXD2 starts working at the beginning of 2024

Arriving well in Japan, installation and commissioning began. Especially because of the very limited space inside the Belle II detector, the installation was an extremely challenging task that required close collaboration with several other detector groups – and has now been successfully mastered. "The new detector is expected to start taking data in early 2024," said Professor Concettina Sfienti, who made important contributions to the development of the PXD with her research group at Mainz University. "Not only was the real-time monitoring of data quality implemented under JGU leadership, we also programmed key sections of the software controlling the PXD2 and tested sensor modules on our electron accelerator MAMI for radiation hardness," added Dr. Matthias Hoek, a scientist in Sfienti's group.

PXD2 is teamwork

Involved in the PXD's design and construction have been the University of Bonn, DESY, the University of Gießen, the University of Göttingen, the Karlsruhe Institute of Technology, Johannes Gutenberg University Mainz, the Max Planck Institute for Physics in Munich, the University of Munich, the Technical University of Munich, and the Semiconductor Laboratory of the Max Planck Society. Other institutes in Spain and the Czech Republic also contributed.

The German research groups in the Belle II experiment are funded by the following institutions and programmes:

  • the Alexander von Humboldt Foundation
  • the German Federal Ministry of Education and Research (BMBF)
  • the German Research Foundation (DFG), in particular within the framework of the Excellence Strategy of the German Federal Government and the Federal States:
    • "ORIGINS": EXC-2094 - 390783311
    • "Quantum Universe": EXC-2121 - 390833306
  • the European Research Council via the European Union's Horizon 2020, grant agreement No 822070
  • the Helmholtz Association
  • the Max Planck Society