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After completing its first long shutdown (LS1), lasting for about 1.5 years from the summer of 2022, SuperKEKB, an electron-positron collider, resumed operations in January 2024, and ran throughout the year, except during the summer.  Operations for 2024 were completed on the 27^th of December. On the final day, SuperKEKB set a new world record for luminosity (a measure how well the particles can collide) with a value of 5.1×10³⁴ cm⁻² s⁻¹, breaking its own record.

 

The Belle II experiment explores new physics, which is key to solving the mysteries of the universe by precisely studying particles such as B mesons produced by SuperKEKB. To unravel the existence of new physics, it is essential to accurately measure rare and unique phenomena that occur with extremely low probability. An enormous amount of experimental data is necessary to measure such rare events, and increasing luminosity is crucial to achieve this.

Increasing luminosity requires accurate control of the accelerator and stable conditions. By adjusting the pulsed magnets installed in the linear accelerator during LS1, which inject electrons and positrons into SuperKEKB, it became possible to control two beam bunches per shot separately. This enabled stable and consistent injection into the accelerator. Research using machine learning to identify optimal settings in real time has also progressed, resulting in improved injection efficiency.

 

In the main ring, where the electron and positron beams circulate, a phenomenon where the beam behavior suddenly becomes unstable has been observed. This phenomenon, known as Sudden Beam Loss (SBL), has been one of the obstacles to stable operation and increased luminosity of SuperKEKB, with the potential to damage both the accelerator and the Belle II detector. Investigating and resolving the causes of SBL has been a critical challenge for the success of the project, and the Belle II Collaboration has been working with the accelerator group to address this issue.

During operations in 2024, it was discovered that dust, likely the degraded vacuum sealing materials used in the beam pipe coupling sections, was deeply involved in SBL. After removing the degraded sealing materials in certain sections, SBL was significantly reduced, allowing for stable beam operation and various adjustments. As a result, the luminosity was improved, and on the final day of operations, a luminosity of 5.1×10³⁴ cm⁻² s⁻¹ was achieved, setting a new world record. (*)

Professor Kodai Matsuoka of the IPNS-Belle group stated, “Moving forward, we aim to further stabilize the accelerator in the next run by addressing the remaining causes of SBL and continuing the analysis of accelerator data, with the goal of achieving a luminosity of 10×10³⁴ cm⁻² s⁻¹. The Belle II Collaboration will continue the physics analysis, adding the collision data obtained during this period, aiming for the discovery of new physics.”

*At the time of the record, the accelerator was undergoing adjustment operations for luminosity improvement, and some of the Belle II subdetectors were temporarily excluded from data taking as a precaution to protect the detectors.