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1. Search for lepton-flavor-violating τ decays into a lepton and a vector meson using the full Belle data sample

   https://doi.org/10.1007/JHEP06(2023)118  

   Tsuzuki, N. ; Inami, K. ; Adachi, I. ; Aihara, H. ; Asner, D. M. ; Atmacan, H. ; Aushev, T. ; Ayad, R. ; Babu, V. ; Banerjee, Sw. ; et al ( June 2023 , Journal of High Energy Physics)

   A bstract Charged-lepton-flavor-violation is predicted in several new physics scenarios. We update the analysis of τ lepton decays into a light charged lepton ( ℓ = e ± or μ ± ) and a vector meson ( V 0 = ρ 0 , ϕ , ω , K *0 , or $$ \overline{K} $$ K ¯ *0 ) using 980 fb − 1 of data collected with the Belle detector at the KEKB collider. No significant excess of such signal events is observed, and thus 90% credibility level upper limits are set on the τ → ℓV 0 branching fractions in the range of (1.7–4 . 3) × 10 − 8 . These limits are improved by 30% on average from the previous results. 

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2. Search for charged lepton flavor violating decays of ϒ (1S)

   https://doi.org/10.1007/JHEP05(2022)095  

   Patra, S. ; Bhardwaj, V. ; Trabelsi, K. ; Adachi, I. ; Aihara, H. ; Al Said, S. ; Asner, D. M. ; Atmacan, H. ; Aushev, T. ; Ayad, R. ; et al ( May 2022 , Journal of High Energy Physics)

   A bstract We present a search for the charged lepton-flavor-violating decays ϒ(1 S ) → ℓ ± ℓ ′ ∓ and radiative charged lepton-flavour-violating decays ϒ(1 S ) → γ ℓ ± ℓ ′ ∓ [ ℓ , ℓ ′ = e, μ, τ ] using the 158 million ϒ(2 S ) sample collected by the Belle detector at the KEKB collider. This search uses ϒ(1 S ) mesons produced in ϒ(2 S ) → π + π − ϒ(1 S ) transitions. We do not find any significant signal, so we provide upper limits on the branching fractions at the 90% confidence level. 

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3. Search for charged-lepton flavor violation in Υ(2S) → ℓ∓τ± (ℓ = e, μ) decays at Belle

   https://doi.org/10.1007/JHEP02(2024)187  

   Dhamija, R ; Nishida, S ; Giri, A ; Adachi, I ; Aihara, H ; Asner, D M ; Aushev, T ; Ayad, R ; Babu, V ; Bahinipati, S ; et al ( February 2024 , Journal of High Energy Physics)

   We report a search for the charged-lepton flavor violation in Υ(2S) →ℓ^∓τ^±(ℓ=e, μ) decays using a 25 fb⁻¹Υ(2S) sample collected by the Belle detector at the KEKBe⁺e⁻asymmetric-energy collider. We find no evidence for a signal and set upper limits on the branching fractions ($$ \mathcal{B} $$$B$) at 90% confidence level. We obtain the most stringent upper limits:$$ \mathcal{B} $$$B$(Υ(2S)→ μ^∓τ^±)<0.23×10⁻⁶and$$ \mathcal{B} $$$B$(Υ(2S)→ e^∓τ^±)<1.12×10⁻⁶.

    

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4. Search for the lepton-flavour violating decays B0 → K*0τ±μ∓

   https://doi.org/10.1007/JHEP06(2023)143  

   Aaij, R. ; Abdelmotteleb, A. S. ; Abellan Beteta, C. ; Abudinén, F. ; Ackernley, T. ; Adeva, B. ; Adinolfi, M. ; Afsharnia, H. ; Agapopoulou, C. ; Aidala, C. A. ; et al ( June 2023 , Journal of High Energy Physics)

   A bstract A first search for the lepton-flavour violating decays B 0 → K *0 τ ± μ ∓ is presented. The analysis is performed using a sample of proton-proton collision data, collected with the LHCb detector at centre-of-mass energies of 7, 8 and 13 TeV between 2011 and 2018, corresponding to an integrated luminosity of 9 fb − 1 . No significant signal is observed, and upper limits on the branching fractions are determined to be $$ \mathcal{B}\left({B}^0\to {K}^{\ast 0}{\tau}^{+}{\mu}^{-}\right)<1.0(1.2)\times {10}^{-5} $$ B B 0 → K ∗ 0 τ + μ − < 1.0 1.2 × 10 − 5 and $$ \mathcal{B}\left({B}^0\to {K}^{\ast 0}{\tau}^{-}{\mu}^{+}\right)<8.2(9.8)\times {10}^{-6} $$ B B 0 → K ∗ 0 τ − μ + < 8.2 9.8 × 10 − 6 at the 90% (95%) confidence level. 

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5. Test of lepton flavor universality and search for lepton flavor violation in B → Kℓℓ decays

   https://doi.org/10.1007/JHEP03(2021)105  

   Choudhury, S. ; Sandilya, S. ; Trabelsi, K. ; Giri, A. ; Aihara, H. ; Al Said, S. ; Asner, D. M. ; Atmacan, H. ; Aulchenko, V. ; Aushev, T. ; et al ( March 2021 , Journal of High Energy Physics)

   null (Ed.)

   A bstract We present measurements of the branching fractions for the decays B → Kμ + μ − and B → Ke + e − , and their ratio ( R K ), using a data sample of 711 fb − 1 that contains 772 × 10 6 $$ B\overline{B} $$ B B ¯ events. The data were collected at the ϒ(4 S ) resonance with the Belle detector at the KEKB asymmetric-energy e + e − collider. The ratio R K is measured in five bins of dilepton invariant-mass-squared ( q 2 ): q 2 ∈ (0 . 1 , 4 . 0) , (4 . 00 , 8 . 12) , (1 . 0 , 6 . 0), (10 . 2 , 12 . 8) and ( > 14 . 18) GeV 2 /c 4 , along with the whole q 2 region. The R K value for q 2 ∈ (1 . 0 , 6 . 0) GeV 2 /c 4 is $$ {1.03}_{-0.24}^{+0.28} $$ 1.03 − 0.24 + 0.28 ± 0 . 01. The first and second uncertainties listed are statistical and systematic, respectively. All results for R K are consistent with Standard Model predictions. We also measure CP -averaged isospin asymmetries in the same q 2 bins. The results are consistent with a null asymmetry, with the largest difference of 2.6 standard deviations occurring for the q 2 ∈ (1 . 0 , 6 . 0) GeV 2 /c 4 bin in the mode with muon final states. The measured differential branching fractions, $$ d\mathrm{\mathcal{B}} $$ d ℬ /dq 2 , are consistent with theoretical predictions for charged B decays, while the corresponding values are below the expectations for neutral B decays. We have also searched for lepton-flavor-violating B → Kμ ± e ∓ decays and set 90% confidence-level upper limits on the branching fraction in the range of 10 − 8 for B + → K + μ ± e ∓ , and B 0 → K 0 μ ± e ∓ modes. 

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