World record at the Belle II experiment
Bianca Scavino, Roman Sultanov and Karin Schönning in front of the Belle II detector.
The Belle II experiment in Japan has collected a world-leading sample of ϒ(4S) particles, i.e. short-lived particles consisting of a bottom quark and its antiquark, formed in electron-positron collisions between electrons and positrons. This marks the beginning of a new era of precision measurements that can challenge the Standard Model of particle physics. One hope is to find evidence of hitherto unknown subatomic processes that can explain why our universe consists of matter but almost no antimatter.
Our entire visible universe, including us, consists of atoms made up of protons, neutrons and electrons. The protons and neutrons in turn consist of quarks, which can also combine in other ways, thus forming other types of particles. One example is the ϒ(4S), which consists of a heavy and unstable bottom quark and an antibottom quark. The ϒ(4S) particles are formed in collisions between electrons and positrons from the SuperKEKB accelerator, where the Belle II detector is located. They decay immediately into B mesons, particles that in addition to a bottom quark, also consist of a light up or down quark of the same type as those found in the proton. B mesons and their antiparticles are uniquely suited to investigate small differences between matter and antimatter, so-called CP violation or charge conjugation and parity violation. This is a necessary ingredient to explain why our universe consists of matter, but almost no antimatter. Belle II's predecessor Belle, together with the BaBar experiment in the USA, was the first to demonstrate CP violation in B meson decay, and was mentioned in the press release for the 2008 Nobel Prize.
“Electron-positron collisions provide a uniquely clean environment, since the ϒ(4S) particles are formed without any other recoil particles. This means that the background level is relatively low. At the same time, the resolution of the measured particle properties is excellent. With the new, record-breaking amount of data, we can study rare types and combinations of particles that were previously produced in too small quantities and therefore beyond reach. Thanks to the versatile detector, it is also possible to detect particles that are invisible to many other experiments. With Belle II, it is also possible to search for dark matter and exotic quark systems,” says Karin Schönning, professor at the Department of Physics and Astronomy at Uppsala University.
The main focus of research at Uppsala University is to investigate proton-like particles with the bottom quark's slightly lighter sibling, the charm quark. These are also formed in abundant quantities and in comparatively clean processes. Of special interest are their spin properties, i.e. their inner magnets, because they provide better sensitivity to CP violation. Since these measurements require high statistical precision, the Uppsala researchers will benefit greatly from the new, large amount of data. In addition, the Uppsala group contributes to improving data quality by developing methods that filter and track the particles as they move through the detector. Here, researcher Bianca Scavino plays a key role as the coordinator for the working group in particle tracking.
“Tracking is a fascinating field: together with colleagues from around the world, we follow the tracks of charged particles to reveal the physics behind them,” says Bianca Scavino.
Facts
Belle II is an experiment at the SuperKEKB accelerator facility in Tsukuba, Japan. It collides electrons and positrons at an energy corresponding to the mass of the ϒ(4S) particle and uses a new technique to achieve higher rates of particle collisions than its predecessor Belle. In this way, Belle II recently managed to collect more ϒ(4S) data than Belle did in its entire lifetime. Uppsala University has been involved in Belle II since 2022.
Belle II@UU is the largest research group within the nuclear physics program at the Department of Physics and Astronomy at Uppsala University. Researcher Bianca Scavino coordinates the Belle II particle tracking working group, and postdoctoral fellow Markus Reif coordinates the amplitude analysis working group, developing methods that enable both detailed studies of previously unknown hadrons and more precise CP tests. The local group also consists of postdoctoral fellow Adeel Akram, doctoral students William Lejon and Roman Sultanov, research engineer Pawel Marciniewski, and Professor Karin Schönning, who leads the local group and also sits on the collaboration's executive committee.
Particle tracking: The particles formed in a collision move at speeds close to that of light through a detector. They then interact with the material in the detector, which gives rise to measurable signals. By applying a strong magnetic field, the charged particles' trajectories are bent so that their charge and momentum can be determined – but for this to work, the signals from the different detector systems first need to be pieced together into coherent tracks. This is done using pattern recognition, with techniques that can be either classical or based on machine learning.