Search for the lepton-flavor-violating $\tau^- \to e^{\mp}\ell^{\pm}\ell^-$ decays at Belle II

Adachi, Ichiro (author), Borah, Jyotirmoi (author), Bračko, Marko (author), Korpar, Samo (author), Križan, Peter (author), Pestotnik, Rok (author), Podobnik, Tomaž (author), Šantelj, Luka (author), Starič, Marko (author)

We present the result of a search for the charged-lepton-flavor violating decays $\tau^- \to e^{\mp}\ell^{\pm}\ell^-$, where $\ell$ is a muon or an electron, using a data sample with an integrated luminosity of 428 fb$^{-1}$ recorded by the Belle II experiment at the SuperKEKB $e^+e^-$ collider. The selection of $e^+e^- \to \tau^+\tau^-$ events containing a signal candidate is based on an inclusive-tagging reconstruction and on a boosted decision tree to suppress background. Upper limits on the branching fractions between 1.3 and 2.5 × 10$^{-8}$ are set at the 90 % confidence level. These results are the most stringent bounds to date for four of the modes.

• Language: English
• Keywords: experimental physics, flavour physics, physics of elementary particles
• 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: 23 str.
• Numbering: Vol. 2025, art. no. 169
• PID: 20.500.12556/RUL-178842
• UDC: 539.1
• ISSN on article: 1029-8479
• DOI: 10.1007/JHEP12(2025)169
• COBISS.SI-ID: 266617603
• Publication date in RUL: 30.01.2026

Record is a part of a journal

• Title: The journal of high energy physics
• Shortened title: J. high energy phys.
• Publisher: SISSA
• ISSN: 1029-8479
• COBISS.SI-ID: 1314148

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 okusov, fizika osnovnih delcev

Projects

• Funder: ARIS - Slovenian Research and Innovation Agency
• Project number: J1-50010
• Name: Eksperimentalno preverjanje leptonske univerzalnosti in izboljšava metod identifikacije delcev pri eksperimentu Belle II

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