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Search for the Lepton Flavor Violating Decays

Fresh from the print, the “Search for the Lepton Flavor Violating Decays B+ → K+ τ l (l = e, μ) at Belle” has been published online at An open-access version is available at

In this study, led by Shun Watanuki and Gaetano de Marino, we searched for decays involving charged B and K mesons and two leptons of different flavors: a tau lepton and an electron or muon. Unlike those with
same-type leptons like tau+ tau- or e+ e-, such processes are not allowed in the Standard Model of elementary particles and their interactions. Their observation would entail the presence of new mediators, like Leptoquarks, allowing for lepton flavor violation. No evidence of B+ → K+ τ l decays was found; nonetheless, Belle could set
the world’s most stringent upper limits on their branching fractions.

The event display shows how such a transition would appear in the Belle II detector.