Belle II Collaboration

17/03/2026

On the 13th of March, the experiment at KEK’s accelerator reached an important milestone for this Run: an integrated luminosity of 3 fb⁻¹ in a single day. This means that roughly three million B mesons pairs were produced in one day.

26/01/2026

Thanks to the hard work of the and teams, we are ready to start 2026 operations! The Belle II Solenoid has been turned on and we’ll begin Main Ring operation today after the 2025-26 winter shutdown. Here’s to a 2026 with stable operation and bountiful data!

Belle IIとSuperKEKBチームのお陰で我々が２０２６年度ランを始まる！Belle IIのソレノイドはONになり、メインリングオペレーションは今日から開始する。２０２５−６年の冬シャットダウンの後、データ豊かかつ安定なランニングの２０２６年へ！

22/12/2025

As we move towards the end of the year, Belle II and SuperKEKB have just finished the 2025 run schedule. The coming weeks will see inspections and possible tweaks to improve performance in 2026 and a well-deserved rest for the New Year's holiday. Thank you to the operations team and all shifters who made it possible to have a great run, and we'll be looking forward to doing even better in 2026!

26/11/2025

We have begun collecting collision events with the Belle II detector!
You can see what reactions are occurring inside Belle II by viewing the event display at the link below.
https://evdisp.belle2.org

What new results will emerge during this run? Stay tuned!

20/11/2025

Belle II 測定器での衝突事象の収集を開始しました！
Belle IIの中でどんな反応が起きているか、下記のリンクからイベントディスプレイを見ることができます。
https://evdisp.belle2.org

今期の運転では、どのような新しい成果が生まれるのか——ぜひご期待ください。

18/11/2025

Last month, we held the 2025 Belle II physics week. The theme of the week was Rare decays of beauty and charm hadrons with missing energy. During this open meeting Belle II there were school sessions and lectures, where Belle II members were able to discuss their work with invited theorists and members of other experimental collaborations: experiments including \; and the BES-III and KOTO experiments.
Over 130 physicists were able to attend the meeting, organized with the KEK theory centre, and sponsored by the Toshiko Yuasa Lab (TYL), the Kobayashi-Maskawa Institute, and the US–Japan Science and Technology Cooperation Program in High Energy Physics.

A report on the Physics Week, thanks to KEK IPNS (\.IPNS) can be found here: https://www2.kek.jp/ipns/en/news/7785/

11/11/2025

A number of Belle II collaborators recently attended the Tau 2025 Workshop in Marseille, France. Some of the highlights include showing, for the first time, the results on the search for the lepton-flavor violating (LFV) mode 𝝉→µγ from Belle II (talk by Marcela Garcia). This is one of the “golden modes” for tau LFV and represents the first ever search at Belle II, yielding competitive upper limits even with less data then Belle (427fb⁻¹ vs 988 fb⁻¹).

Collaborators also presented some recent and world-leading upper limits on searches for other LFV tau decays from Belle and Belle II (talks by Arthur Thaller and Johan Colorado Caicedo). This included searches on 𝝉→eℓℓ’, 𝝉→ℓKs, 𝝉→ℓɑ, and 𝝉→µµµ. The latest results in semileptonic B-decays and LFV meson decays with taus in the final state were also shown (talks by Paul Feichtinger and Petar Rados). The status and plans for upgrading SuperKEKB with polarized electron beams was shown (talk by Michael Roney). Finally a summary talk including prospects for tau leptons was given on the final day (talk by Laura Zani), where it is clear Belle II will be one of the major contributors to tau physics over the next ~decade.

Photo credits: Lisovskyi Vitalii and Petar Rados.

最近、Belle IIの研究者が仏・マルセイユのTau 2025ワークショップに参加した。発表した結果の中、ハイライトはレプトンフレーバー破れの崩壊モード𝝉→µγのBelle IIの初公開。これはτ粒子の一つの「黄金のチャンネル」であり、Belle IIの初めての探索。それに、Belleよりデータが少なくても、競争的な結果を得られた(427fb⁻¹ vs 988 fb⁻¹).

それ以上に、様々なLFVタウ粒子に関する最近と世界最先端探索期限を発表した（アーサー・サラー氏、ヨハン・コロラド・カイセド氏）。例えば、𝝉→eℓℓ’, 𝝉→ℓKs, 𝝉→ℓɑ, and 𝝉→µµµの探りを公開した。そして、セミレプトニックB中間子崩壊とLFV中間子崩壊も発表された（ポール・ファイフティンガー氏、ピター・ラドス氏）。SuperKEKBの状況もアップグレード予定（偏極電子ビームの予想も含めて）も発表した（マイケル・ロニー氏）。最終日、将来のτ粒子の可能性のまとめ発表がラウラ・ザニ氏に発表された。そのまとめでは、次に〜１０年間のうち、Belle IIはτ物理の中心実験となることが明らかにした。

23/05/2025

As Belle II prepares for its future high-luminosity phases, software, data production, and computing experts from across the collaboration gathered in Karlsruhe, Germany, for the 2025 Joint Computing, Data Production, and Software Workshop. Hosted at the Karlsruher Institut für Technologie (KIT) and jointly organized by the Institute for Experimental Particle Physics (ETP) and the Scientific Computing Center (SCC), the workshop brought together around 40 participants from various institutes, many of them from Germany and KIT, for a week of in-depth discussions and collaborative planning.

Spanning topics from calibration and analysis infrastructure to artificial intelligence and modern computing architectures, the workshop tackled the challenges of building a more robust and efficient software and computing ecosystem for Belle II. The goal: to prepare the experiment’s software and computing stack for the demands of the coming years, with improved tools, streamlined workflows, and stronger cross-group collaboration.

14/05/2025

We congratulate Belle II Collaboration Member Laura Zani for being awarded the 2025 EPS HEP Young Experimental Physicist Prize, for "...outstanding work on searches for physics beyond the standard model at B-Factories, in particular dark matter portals and lepton-flavor-violating decays of the tau lepton, and for remarkable contributions to the construction, commissioning and operation of the Belle II experiment."

Laura has been a member of Belle II since 2017, serving in a variety of critical roles: She authored the first Belle II physics analysis and later became coordinator of the Working Group for the Analysis of Tau Physics, and later also the system manager for the Silicon Vertex Detector. Thanks to her leadership and contributions in data analysis as well as hardware operation, Belle II has reached new levels of sensitivity and stability.

Laura earned her M.Sc. and Ph.D. at the University of Pisa, after which she worked in Marseille as a Postdoc. She is now a full-time researcher at the National Institute for Nuclear Physics (INFN) Rome.

For more information, please see INFN's press release: https://www.infn.it/il-young-experimental-physicist-prize-2025-alla-ricercatrice-infn-laura-zani/, and for information about all the 2025 HEP Prize winners, see here: https://eps-hepp.web.cern.ch/prizes.php

15/02/2025

Physics Today has a new article about and and our quest to reach higher luminosities:

Squeezing beams of electrons and positrons for the Belle II experiment at the SuperKEKB facility proceeds with halting progress.