New world record: Thinnest-ever pixel detector installed

The Belle II cooperation project at the Japanese research center KEK is facilitating global research in particle physics. An important milestone has been achieved as a new pixel detector has been successfully installed in Japan, enhancing the international experiment.

The newly installed detector, with a size comparable to a soda can, is designed to detect signals from specific particle decays. These signals can provide insights into the origin of the observed matter-antimatter asymmetry in the universe. The installation process went smoothly, marking a significant achievement in the evolution of the experiment and the German-Japanese research collaboration.

Belle II, situated at the SuperKEKB accelerator in Japan’s KEK research center, is an international collaborative project involving researchers from around the world. The experiment aims to unravel unanswered questions about the universe by searching for new phenomena in physics and undiscovered particles beyond the established Standard Model of particle physics. The project has over 1,200 members from the international Belle II collaboration.

The installation signifies the completion of a long journey for the detector. Starting in Munich, its components traveled through various German institutes, including the University of Bonn, before finally reaching Hamburg at the Deutsches Elektronen-Synchrotron (DESY), where they were assembled. The final leg of the journey involved a several thousand-kilometer trip to Japan and the SuperKEKB electron-positron collider, which serves as Belle II’s ultimate destination.

The transportation by air introduced additional challenges due to the detector’s high sensitivity to vibrations. To minimize any potential damage, a specially designed case protected the detector during the journey, ensuring its safe arrival in Japan.

The University of Bonn played a significant role in the installation process. Botho Paschen, the Technical Coordinator of the pixel detector project and a researcher at the University of Bonn, expressed satisfaction with the installation, highlighting the dedication and hard work of the team that developed and prepared the detector for installation.

The tight space presented challenges during the installation process, which was successfully overcome. Paschen emphasized the importance of starting the detector’s operation to capture new collision data in early 2024, marking the next significant step for the experiment.

Prof. Dr. Florian Bernlochner, the Belle II Group Leader at the University of Bonn, highlighted the significance of the new detector for the experiment’s physics objectives. Bernlochner stated that the pixel detector is crucial in accurately measuring the lifetimes of heavy quarks. These measurements will contribute to the investigation of charge-parity symmetry violation, which is one of the fundamental symmetries in nature and crucial in explaining the dominance of matter in the universe.

The decay products of heavy quarks possess low energy and can be easily disrupted when passing through the detector material. Hence, the closest detector elements to the collision point of the particle beams need to be lightweight. The pixel detector, composed of 20 silicon strips with a thickness equivalent to a human hair, is exceptionally fragile and posed installation challenges. Now installed, Belle II boasts the thinnest pixel detector in the world.

The innovative pixel detector technology, utilizing DEPFET (DEPleted Field Effect Transistor) sensors, was developed at the Max Planck Society’s semiconductor laboratory. The detector’s purpose is to generate up to 50,000 high-resolution images per second of decaying heavy quarks, abundant at the SuperKEKB. Additionally, the DEPFET sensor technology of the pixel detector has potential applications in multiple fields, including X-ray satellite missions, the search for sterile neutrinos or dark matter, and medical imaging.

The University of Bonn’s Research and Technology Center for Detector Physics (FTD) provides an ideal environment for researchers studying matter structure at ultra-small length scales. Equipped with cutting-edge infrastructure, the FTD is home to the University of Bonn’s Silicon Lab (SILAB), where the pixel detector modules were extensively tested and studied over several years.

Prof. Dr. Jochen Dingfelder, FTD Co-Speaker and SILAB Director, expressed his delight at the successful installation of the detector, attributing it to fruitful collaboration among institutions and research centers involved. The University of Bonn’s team extends its gratitude to all those involved, both in Japan and across Europe and Germany, for their dedicated efforts in achieving this groundbreaking milestone.