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Search for Lepton-Flavor-Violating Decay Modes 𝐵 0 →𝐾$^0_𝑆$⁢𝜏 ± ⁢ℓ ∓ with Hadronic 𝐵 Tagging at Belle and Belle II

Adachi, I. (ORCID:0000000322870173)·Adamczyk, K. (ORCID:0000000162080876)·Aggarwal, L. (ORCID:0000000209097537)·Ahmed, H. (ORCID:0000000339767498)·Aihara, H. (ORCID:0000000219075964)·Akopov, N. (ORCID:0000000244252096)·Alhakami, M. (ORCID:0000000222348628)·Aloisio, A. (ORCID:0000000238836693)·Althubiti, N. (ORCID:0000000315130409)·Ky, N. Anh·Asner, D. M. (ORCID:0000000215865790)·Atmacan, H. (ORCID:000000032435501X)·Aushev, V. (ORCID:0000000285885308)·Aversano, M. (ORCID:0000000199800953)·Ayad, R. (ORCID:0000000334669290)·Babu, V. (ORCID:0000000304196912)·Bae, H. (ORCID:0000000313938631)·Baghel, N. K. (ORCID:0009000878064422)·Bahinipati, S. (ORCID:0000000237445332)·Bambade, P. (ORCID:0000000173784852)·Banerjee, Sw. (ORCID:0000000188522409)·Bansal, S. (ORCID:0000000319920336)·Barrett, M. (ORCID:000000022095603X)·Bartl, M. (ORCID:0009000278350855)·Baudot, J. (ORCID:0000000155850991)·Baur, A. (ORCID:0000000313603292)·Beaubien, A. (ORCID:000000019438089X)·Becherer, F. (ORCID:0000000305624616)·Becker, J. (ORCID:0000000250825487)·Bennett, J. V. (ORCID:0000000254402668)·Bernlochner, F. U. (ORCID:0000000181532719)·Bertacchi, V. (ORCID:0000000199711176)·Bertemes, M. (ORCID:000000015038360X)·Bertholet, E. (ORCID:0000000237922450)·Bessner, M. (ORCID:0000000317760439)·Bettarini, S. (ORCID:0000000177422998)·Bhardwaj, V. (ORCID:0000000188578621)·Bhuyan, B. (ORCID:0000000162543594)·Bianchi, F. (ORCID:0000000215246236)·Biswas, D. (ORCID:0000000275433471)·Bobrov, A. (ORCID:0000000157358386)·Bodrov, D. (ORCID:0000000152794787)·Bolz, A. (ORCID:0000000240339223)·Boschetti, A. (ORCID:0000000160303087)·Bozek, A. (ORCID:0000000259151319)·Bračko, M. (ORCID:0000000224950524)·Branchini, P. (ORCID:0000000222709673)·Briere, R. A. (ORCID:0000000152291039)·Browder, T. E. (ORCID:0000000173579007)·Budano, A. (ORCID:0000000208561131)·Bussino, S. (ORCID:0000000238299592)·Campagna, Q. (ORCID:0000000231092046)·Campajola, M. (ORCID:0000000325187134)·Cao, L. (ORCID:0000000183325668)·Casarosa, G. (ORCID:000000034137938X)·Cecchi, C. (ORCID:0000000221928233)·Cerasoli, J. (ORCID:000000019777881X)·Chang, M.-C. (ORCID:0000000286506058)·Chang, P. (ORCID:000000034064388X)·Cheaib, R. (ORCID:0000000157298926)·Cheema, P. (ORCID:0000000184725727)·Cheon, B. G. (ORCID:0000000288034429)·Chilikin, K. (ORCID:0000000176202053)·Chirapatpimol, K. (ORCID:0000000320997760)·Cho, H.-E. (ORCID:0000000270083759)·Cho, K. (ORCID:0000000317057399)·Cho, S.-J. (ORCID:0000000216735664)·Choi, S.-K. (ORCID:0000000327478277)·Choudhury, S. (ORCID:0000000198410216)·Cochran, J. (ORCID:000000021492914X)·Corona, L. (ORCID:0000000225779909)·Cui, J. X. (ORCID:0000000223983754)·De La Cruz-Burelo, E. (ORCID:0000000274696974)·De La Motte, S. A. (ORCID:0000000339056805)·de Marino, G. (ORCID:0000000265097793)·De Nardo, G. (ORCID:0000000220479675)·De Pietro, G. (ORCID:000000018442107X)·de Sangro, R. (ORCID:0000000238085455)·Destefanis, M. (ORCID:0000000319976751)·Dey, S. (ORCID:0000000329973829)·Dhamija, R. (ORCID:0000000170523163)·Di Canto, A. (ORCID:0000000312333876)·Di Capua, F. (ORCID:0000000190765936)·Dingfelder, J. (ORCID:0000000157672121)·Doležal, Z. (ORCID:0000000256623675)·Jiménez, I. Domínguez·Dong, T. V. (ORCID:0000000330431939)·Dong, X. (ORCID:0000000185749624)·Dorigo, M. (ORCID:0000000206816946)·Dossett, D. (ORCID:0000000256705582)·Dubey, S. (ORCID:0000000213450970)·Dugic, K. (ORCID:000900066056546X)·Dujany, G. (ORCID:0000000213458163)·Ecker, P. (ORCID:0000000268176868)·Feichtinger, P. (ORCID:0000000339667497)·Ferber, T. (ORCID:0000000268490427)·Fillinger, T. (ORCID:0000000197957412)·Finck, C. (ORCID:0000000250685453)·Finocchiaro, G. (ORCID:0000000239362151)·Fodor, A. (ORCID:000000022821759X)
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Abstract

We present the first search for the lepton-flavor-violating decay modes 𝐵 0 →𝐾$^0_𝑆$⁢𝜏 ± ⁢ℓ ∓ (ℓ=𝜇,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 𝜏 decays to either light leptons, one charged hadron, or one charged hadron and a neutral pion. We find no evidence for 𝐵 0 →𝐾$^0_𝑆$⁢𝜏 ± ⁢ℓ ∓ decays and set 90% confidence level upper limits on the branching fractions in the range of [0.8,3.6] ×10 −5 .

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72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDSBottom mesonsElectroweak interactionFlavor changing neutral currentsHypothetical gauge bosonsLepton collidersLeptonic, semileptonic & radiative decaysLeptoquarksMonte Carlo methodsSignatures with missing energySignatures with tau leptons

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Adachi, I. (ORCID:0000000322870173), Adamczyk, K. (ORCID:0000000162080876), Aggarwal, L. (ORCID:0000000209097537), Ahmed, H. (ORCID:0000000339767498), Aihara, H. (ORCID:0000000219075964), Akopov, N. (ORCID:0000000244252096), Alhakami, M. (ORCID:0000000222348628), Aloisio, A. (ORCID:0000000238836693), Althubiti, N. (ORCID:0000000315130409), Ky, N. Anh, Asner, D. M. (ORCID:0000000215865790), Atmacan, H. (ORCID:000000032435501X), Aushev, V. (ORCID:0000000285885308), Aversano, M. (ORCID:0000000199800953), Ayad, R. (ORCID:0000000334669290), Babu, V. (ORCID:0000000304196912), Bae, H. (ORCID:0000000313938631), Baghel, N. K. (ORCID:0009000878064422), Bahinipati, S. (ORCID:0000000237445332), Bambade, P. (ORCID:0000000173784852), Banerjee, Sw. (ORCID:0000000188522409), Bansal, S. (ORCID:0000000319920336), Barrett, M. (ORCID:000000022095603X), Bartl, M. (ORCID:0009000278350855), Baudot, J. (ORCID:0000000155850991), Baur, A. (ORCID:0000000313603292), Beaubien, A. (ORCID:000000019438089X), Becherer, F. (ORCID:0000000305624616), Becker, J. (ORCID:0000000250825487), Bennett, J. V. (ORCID:0000000254402668), Bernlochner, F. U. (ORCID:0000000181532719), Bertacchi, V. (ORCID:0000000199711176), Bertemes, M. (ORCID:000000015038360X), Bertholet, E. (ORCID:0000000237922450), Bessner, M. (ORCID:0000000317760439), Bettarini, S. (ORCID:0000000177422998), Bhardwaj, V. (ORCID:0000000188578621), Bhuyan, B. (ORCID:0000000162543594), Bianchi, F. (ORCID:0000000215246236), Biswas, D. (ORCID:0000000275433471), Bobrov, A. (ORCID:0000000157358386), Bodrov, D. (ORCID:0000000152794787), Bolz, A. (ORCID:0000000240339223), Boschetti, A. (ORCID:0000000160303087), Bozek, A. (ORCID:0000000259151319), Bračko, M. (ORCID:0000000224950524), Branchini, P. (ORCID:0000000222709673), Briere, R. A. (ORCID:0000000152291039), Browder, T. E. (ORCID:0000000173579007), Budano, A. (ORCID:0000000208561131), Bussino, S. (ORCID:0000000238299592), Campagna, Q. (ORCID:0000000231092046), Campajola, M. (ORCID:0000000325187134), Cao, L. (ORCID:0000000183325668), Casarosa, G. (ORCID:000000034137938X), Cecchi, C. (ORCID:0000000221928233), Cerasoli, J. (ORCID:000000019777881X), Chang, M.-C. (ORCID:0000000286506058), Chang, P. (ORCID:000000034064388X), Cheaib, R. (ORCID:0000000157298926), Cheema, P. (ORCID:0000000184725727), Cheon, B. G. (ORCID:0000000288034429), Chilikin, K. (ORCID:0000000176202053), Chirapatpimol, K. (ORCID:0000000320997760), Cho, H.-E. (ORCID:0000000270083759), Cho, K. (ORCID:0000000317057399), Cho, S.-J. (ORCID:0000000216735664), Choi, S.-K. (ORCID:0000000327478277), Choudhury, S. (ORCID:0000000198410216), Cochran, J. (ORCID:000000021492914X), Corona, L. (ORCID:0000000225779909), Cui, J. X. (ORCID:0000000223983754), De La Cruz-Burelo, E. (ORCID:0000000274696974), De La Motte, S. A. (ORCID:0000000339056805), de Marino, G. (ORCID:0000000265097793), De Nardo, G. (ORCID:0000000220479675), De Pietro, G. (ORCID:000000018442107X), de Sangro, R. (ORCID:0000000238085455), Destefanis, M. (ORCID:0000000319976751), Dey, S. (ORCID:0000000329973829), Dhamija, R. (ORCID:0000000170523163), Di Canto, A. (ORCID:0000000312333876), Di Capua, F. (ORCID:0000000190765936), Dingfelder, J. (ORCID:0000000157672121), Doležal, Z. (ORCID:0000000256623675), Jiménez, I. Domínguez, Dong, T. V. (ORCID:0000000330431939), Dong, X. (ORCID:0000000185749624), Dorigo, M. (ORCID:0000000206816946), Dossett, D. (ORCID:0000000256705582), Dubey, S. (ORCID:0000000213450970), Dugic, K. (ORCID:000900066056546X), Dujany, G. (ORCID:0000000213458163), Ecker, P. (ORCID:0000000268176868), Feichtinger, P. (ORCID:0000000339667497), Ferber, T. (ORCID:0000000268490427), Fillinger, T. (ORCID:0000000197957412), Finck, C. (ORCID:0000000250685453), Finocchiaro, G. (ORCID:0000000239362151), Fodor, A. (ORCID:000000022821759X). 2025-07-25. Search for Lepton-Flavor-Violating Decay Modes 𝐵 0 →𝐾$^0_𝑆$⁢𝜏 ± ⁢ℓ ∓ with Hadronic 𝐵 Tagging at Belle and Belle II. https://doi.org/10.1103/yzvh-v3hm

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