Particle physics: Focus on BELLE-II before the experiment restarts

Major improvements

After a year and a half of technical stop, the Belle II experiment, located in Tsukuba, Japan, is ready to resume data collection. This break has allowed several upgrades for both to be made to the SuperKEKb acceleratorl and the detector itself. In particular, shielding was added to the accelerator to reduce background noise from the beams. The major modification was to install the second layer of the pixel vertex detector (located at the centre of the detector), which will be key to measure for time-dependent CP violation in order to characterise the differences between matter and antimatter. Until December 2023, the detector will be gradually restarted, taking data from cosmic ray particles.  The accelerator, however, will restart at the beginning of 2024, producing collisions until July, with the goal of breaking the world record for instantaneous luminosity previously reached at 4.7.10³⁴ /cm²/s.

First analyses of BELLE-II data

During this technical stop, the experiment's physicists analysed the data recorded between 2019 and 2022, corresponding to a statistic of 424 fb^-1, around half the data recorded in the previous experiment, Belle. Numerous results were presented at the Summer 2023 conferences, including a first hint of the rare b-quark transition decaying into a strange quark and two neutrinos (b → s ν anti-ν) using the decay of the B+ meson into K+ ν anti-ν. These results, to which the IPHC Strasbourg team made a significant contribution, were presented in a seminar at CERN on 7 November (if we wait until the seminar to publish...). 

This decay is predicted with high precision by the Standard Model of particle physics, at a rate of the order of 5.10^-6. A very low rate which represents however a challenge from an experimental point of view, as the only visible particle in the final state is a kaon, which is difficult to identify from the multitude of kaons commonly produced by the experiment in the decays of beauty mesons. Belle-II offers unique experimental conditions for the search for such a final state, with a large invisible part, thanks in particular to the excellent knowledge of the total energy and momentum of the lepton collision, making it possible to kinematically constrain the invisible part. 

Changes in collaboration

Finally, on the management side, Karim Trabelsi, former researcher at IJClab and current director of the TYL international research laboratory), was elected spokesperson for the Belle-II collaboration last June for a 2-year term. At the IN2P3 level, he has been replaced by CPPM researcher Justine Serrano as scientific manager of the Belle II Master Project.