Made in Germany and big in Japan: Pixel Vertex Detector installed in Belle II experiment

The Pixel Vertex Detector (PXD), at the size of just a soda can, forms the innermost detector layer of the international Belle II experiment. It has now been installed at its destination, the SuperKEKB electron-positron accelerator in Japan. The ultrasensitive device was developed at the German Belle II institutes, led by the Max Planck Institute for Physics (MPP), the Max Planck Semiconductor Laboratory and DESY. The small PXD is designed to detect a certain type of particle decays that is expected to shed light on the great mystery of the imbalance of matter and antimatter in the universe.

Installation of the complete version of the Pixel Vertex Detector in the Belle II experiment (Photo: B. Paschen/University of Bonn)

The Pixel Vertex detector: The silicon modules are built in two layers and are only 75 micrometers thick, like human hair. Here, the collisions of electrons and positrons are measured with extreme precision (Photo: B. Wankerl/MPP)

The pixel vertex detector surrounds the beamline and is located only 1.4 centimeters from the  interaction point, where particles collide (vertex). This allows the precise decay location of short-lived particles to be accurately determined. The PXD consists of 20 pixel detectors - 75-micrometer-thin silicon modules that are only as thick as a human hair. The new detector is based on DEPFET technology developed at the Max Planck Semiconductor Laboratory. It delivers up to 50,000 high-resolution images per second. Recorded are decay products of B-mesons, which are created when electrons and positrons collide in the SuperKEKB accelerator.

The B-meson system is an ideal experimental field to study the fundamental symmetries of nature: The violation of CP symmetry (parity charge symmetry) is one of three conditions that must be satisfied to explain why our present universe is almost entirely composed of matter. The high precision of the Belle II detector, combined with the statistical accuracy of the electron-positron collisions at SuperKEKB, provides the best conditions for studying CP violation and other interesting phenomena in unprecedented detail.

"We are very proud that the Munich groups contributed essential parts of the detector concept and development," says Hans-Günther Moser, head of the Belle II group at MPP. "With the help of DEPFET technology, we can exploit highly complex and ultrasensitive sensors in the Belle II experiment, which are also used in satellite experiments. This technology underscores the Semiconductor Laboratory's unique global expertise in radiation detectors."

A detector flies business class

Back in 2018, a first, still incomplete version of the PXD was installed in the Belle II detector and has already delivered valuable results. But only the new and complete version is capable of handling the high luminosity that will be achieved in SuperKEKB in the coming years. The transport to Japan was complex: First, the sensitive detector had to be transferred via normal traffic roads from its assembly site at the MPP in Munich to DESY for critical functional tests and optimization of the detector parameters.

After the successful test phase, the newly assembled 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 have easily broken one of the delicate silicone modules. To minimize these risks and keep vibrations low, the team specially packed the detector. The PXD traveled in business class, giving it enough room in its own seat while the team was able to 'babysit' it throughout the trip.

The freshly installed detector is scheduled to begin taking data at the beginning of 2024. The DESY, the Max Planck Institute of Physics and the Max Planck Semiconductor Laboratory, the Ludwig Maximilians University of Munich, the Technical University of Munich, the University of Bonn, the University of Giessen, The University of Göttingen, the University of Mainz and the Karlsruhe Institute of Technology were involved in the design and construction of the PXD.