World’s Largest Data Sample of Entangled B Mesons Recorded

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In doing so, it has surpassed its predecessor, Belle, and opened a new chapter in precision measurements. Professor Thomas Kuhr, Professor of Experimental Physics at the Faculty of Physics, is among the researchers working on the experiment.

At the SuperKEKB accelerator at the KEK High Energy Accelerator Research Organization in Tsukuba, Japan, electrons and positrons are accelerated to energies of 7 and 4 gigaelectronvolts, respectively, and brought into collision. These collisions produce, among other particles, bound states consisting of a bottom quark and an antibottom quark, which decay into pairs of B mesons. B mesons consist of a bottom quark and a light up or down quark, and the two particles are produced in an entangled state. Because of this quantum-mechanical entanglement, the state of one B meson is determined when the other decays – without any direct exchange taking place between them.

This makes it possible to investigate differences in the behavior of B mesons and their antiparticles. Such differences between matter and antimatter are essential to explaining why our universe consists almost entirely of matter, with virtually no antimatter. However, the differences observed to date are not sufficient to account for the matter–antimatter asymmetry of the universe. This asymmetry is a fundamental prerequisite for the formation of galaxies and, ultimately, life. Investigating its origins is one of the active areas of research within the ORIGINS Cluster of Excellence.

In addition to producing entangled B mesons, the collision events recorded by Belle II offer precisely known initial conditions and are free from the background generated by strong-interaction processes at facilities such as the Large Hadron Collider (LHC). These exceptionally clean conditions enable unique measurements of decays involving „invisible“ particles, including neutrinos and potential dark-matter particles. Studies of so-called exotic states composed of more than three quarks also benefit from this environment.

Progress across Belle II’s broad research program depends on ever greater measurement precision. This requires recording larger numbers of events so that even subtle effects can be detected – effects that may provide clues to fundamental questions such as the origin of the universe’s matter – antimatter asymmetry or the identity of dark matter. SuperKEKB has already achieved a collision rate more than twice that of its predecessor. Recording the world’s largest data sample of entangled B-meson pairs marks another major milestone. Further information is available here.

„This outstanding achievement was possible only through many years of collaboration among scientists around the world“, says Professor Kuhr. „It is a milestone for the Belle II physics program. This data sample will enable measurements with unprecedented precision. Over the next few years, we will learn a great deal about where new physics may – or may not – be hiding.“

The SuperKEKB accelerator and the Belle II experiment are currently on pause for maintenance and minor improvements. Data collection is scheduled to resume early next year. In parallel, plans are being developed for a major upgrade of both the accelerator and the detector beginning in 2032. Further records – and exciting new physics results – are expected in the years ahead.

Research at the Belle II experiment is funded by the BMFTR and by the German Research Foundation (DFG) through the ORIGINS Cluster of Excellence.