Search for lepton-flavor-violating decay modes $B^0 \to K^0_S \tau^{\pm}\ell^{\mp}$ with hadronic B tagging at Belle and Belle II

Adachi, Ichiro (author), Adamczyk, Karol (author), Bračko, Marko (author), Korpar, Samo (author), Križan, Peter (author), Pestotnik, Rok (author), Podobnik, Tomaž (author), Prudiiev, Ihor (author), Šantelj, Luka (author), Starič, Marko (author), Zhukova, Valentina (author)

We present the first search for the lepton-flavor-violating decay modes $B^0 \to K^0_S\tau^{\pm}\ell^{\mp}(\ell = \mu, e)$ using the 711 and 365 fb$^{-1}$ data samples recorded by the Belle and Belle II detectors, respectively. We use a hadronic $B$-tagging technique to fully reconstruct a $B$ meson and search for signal decays in the system recoiling against the tagged meson, considering $\tau$ decays to either light leptons, one charged hadron, or one charged hadron and a neutral pion. We find no evidence for $B^0 \to K^0_S\tau^{\pm}\ell^{\mp}$ decays and set 90 % confidence level upper limits on the branching fractions in the range of [0.8,3.6] × 10$^{-5}$.

• Language: English
• Keywords: experimental physics, physics of elementary particles, particle decays
• Work type: Article
• Typology: 1.01 - Original Scientific Article
• Organization: FMF - Faculty of Mathematics and Physics
• Publication status: Published
• Publication version: Version of Record
• Year: 2025
• Number of pages: Str. 041801-1-041801-8
• Numbering: Vol. 135, iss. 4
• PID: 20.500.12556/RUL-178825
• UDC: 539.1
• ISSN on article: 0031-9007
• DOI: 10.1103/yzvh-v3hm
• COBISS.SI-ID: 266690819
• Publication date in RUL: 30.01.2026

Record is a part of a journal

• Title: Physical review letters
• Shortened title: Phys. rev. lett.
• Publisher: American Physical Society
• ISSN: 0031-9007
• COBISS.SI-ID: 1282575

Licences

• License: CC BY 4.0, Creative Commons Attribution 4.0 International
• Link: http://creativecommons.org/licenses/by/4.0/
• Description: This is the standard Creative Commons license that gives others maximum freedom to do what they want with the work as long as they credit the author.

Secondary language

• Language: Slovenian
• Keywords: eksperimentalna fizika, fizika osnovnih delcev, razpadi delcev

Projects

• Funder: ARIS - Slovenian Research and Innovation Agency
• Project number: J1-9124
• Name: Napredne metode za identifikacijo hadronov pri eksperimentu Belle II

• Funder: ARIS - Slovenian Research and Innovation Agency
• Project number: P1-0135
• Name: Eksperimentalna fizika osnovnih delcev