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The B-meson super factory, located in the Japanese city of Tsukuba, has surpassed the data volume accumulated by its predecessor, Belle. Coinciding with this milestone, Cristina Castro, a researcher from the University of Valencia at the Institute of Corpuscular Physics (IFIC), a joint CSIC–UV centre, has been appointed chief engineer for the experiment’s upgrade scheduled for 2032.
Belle II is an experiment conducted at the SuperKEKB electron–positron collider, located in Tsukuba, Ibaraki prefecture, in Japan. It is designed to search for new physics beyond the Standard Model. The Belle II collaboration comprises more than 1,200 members from 28 countries and regions. IFIC (CSIC–UV) and the University of the Basque Country (EHU) are the only participating institutions from Spain.
The experiment’s physics programme focuses on precision studies of heavy quarks and leptons — a field known as flavour physics — through extremely rare and experimentally challenging processes. By comparing highly precise measurements with theoretical predictions, Belle II aims to uncover possible signs of new particles or forces beyond current theories.
The experiment has recently reached a major milestone by surpassing the total data volume accumulated by its predecessor, Belle. At the same time, IFIC researcher Cristina Castro Sequeiro has been appointed head of integration and mechanical design for the Belle II and SuperKEKB upgrade.
Record-breaking luminosity
Belle II studies the collisions produced at SuperKEKB, which accelerates electrons and positrons to high energies before bringing them into collision. These interactions produce, among other particles, pairs of B mesons and anti-B mesons, whose behaviour is of great interest for testing the Standard Model of particle physics with unprecedented precision. Analysing this behaviour requires the accumulation of vast amounts of experimental data, and luminosity is the key indicator of this data volume.
The luminosity of a collider is a quantity proportional to the number of collisions it can produce. It is a fundamental parameter because the higher the luminosity, the greater the likelihood of observing unusual events that may reveal new phenomena in nature. SuperKEKB was designed to achieve record-breaking luminosities through its nano-beam collision scheme, which significantly increases the collision rate by using extremely small beam sizes at the collision point.
Belle, the predecessor of Belle II, operated from 1999 to 2010 and recorded 772 million B-meson pair events, corresponding to an integrated luminosity of 711 fb⁻¹ (inverse femtobarns). Belle played a crucial role in establishing the Standard Model description of CP violation in quarks, leading discoveries that contributed decisively to the 2008 Nobel Prize in Physics awarded to Makoto Kobayashi and Toshihide Maskawa.
On 17 May, Belle II reached a major milestone in data production by surpassing Belle’s total number of recorded collisions. As of 4 June, Belle II had accumulated an integrated luminosity of 862 fb⁻¹, and the figure continues to rise. This achievement comes during a period marked by record instantaneous luminosities, sustained high-efficiency data collection and continuous performance improvements, enabling Belle II to collect data three times faster than its predecessor.
Carlos Mariñas, IFIC researcher and deputy spokesperson for the experiment, explains the significance of this achievement and IFIC’s role in it: “We have reached a milestone by surpassing the total amount of data collected by the original Belle project with the new experiment. IFIC has played a fundamental role in this historic achievement, which takes us deep into unexplored territory in the search for new physics. Our team has not only actively participated in the development of the current detector and the optimisation of data taking, but we are also leading the developments for the experiment’s upgrade planned for the early 2030s”.
Belle II’s strong performance is significant on a broader scale because electron–positron collisions are among the most promising avenues for discovering new physics. For this reason, the recently updated European Strategy for Particle Physics has identified a larger collider of this type — the Future Circular Collider (FCC-ee) — as the preferred option for CERN’s next flagship project. The expertise in advanced instrumentation developed for Belle II will be invaluable in making projects of this scale a reality.
“The experience gained from designing and operating the Belle II experiment at a particle accelerator using nano-beams and high currents such as SuperKEKB also places IFIC in a leading position for the experimental developments associated with the Higgs factory, the next major international project in particle physics”, adds Mariñas.
The current data-taking run of SuperKEKB and Belle II is scheduled to continue until the end of June and will resume after the summer shutdown. It is expected to surpass the milestone of 1,000 fb⁻¹ of integrated luminosity in the near future.
Cristina Castro, UV researcher at IFIC, appointed chief engineer
The high level of performance achieved requires exceptionally stable operation of the accelerator at high luminosity, presenting a major engineering challenge for the collaboration. This involves producing high-intensity electron and positron beams using the linear accelerator, efficiently injecting them into the main storage rings through the injector system, and precisely tuning and controlling them within the main rings. Operational stability has been limited by sudden beam-loss phenomena, in which a portion of the beam is lost with little or no warning. These previously unknown challenges have been gradually resolved, allowing sustained operation close to the new record for instantaneous luminosity.
A major upgrade of SuperKEKB and Belle II is planned for 2032 during an extended technical shutdown. The upgrade is expected to further increase performance and will also represent a significant technical challenge. Cristina Castro Sequeiro, a University of Valencia researcher at IFIC, has been appointed to lead these operations.
Her responsibilities will include project planning and the mechanical integration of the detection systems. “The Belle II upgrade will involve a substantial restructuring of the detector, with a direct impact on the experiment’s future capabilities. Leading this upgrade represents not only a highly interesting technical challenge but also a major responsibility at a key stage in the detector’s development. It is a unique opportunity made possible by IFIC’s experience and commitment to the project. I am grateful for the trust placed in me by the collaboration in selecting me for this position and I approach this new challenge with enthusiasm and determination to ensure the upgrade is a success”, says Castro.
The UV researcher will work closely with the teams responsible for upgrading both the accelerator and the detector to ensure full compatibility between the designs, while coordinating contributions from the various institutes within the collaboration.
Commenting on her role, she adds: “My position covers the mechanical design of various detector components, including structural and thermal aspects, the integration of new subsystems both within the detector and with adjacent accelerator systems, as well as overall project management. It is a complex task that requires constant collaboration between the different groups involved”.
Cristina Castro’s appointment highlights the importance of the contributions that IFIC has been making to the Belle II experiment.
