Search for cLFV in  \(\Upsilon (2S) \rightarrow \ell ^\mp \tau ^\pm \) ( \(\ell =e,\mu \) ) Decays at Belle

Despite the tremendous success of the Standard Model (SM) of Particle Physics, there are several unexplained phenomena such as neutrino masses and mixing, mass hierarchy problems, etc. The lepton flavor is one of the accidental symmetries of the SM. Charged lepton flavor-violating (cLFV) processes are forbidden in the SM, but some new physics models, such as the leptoquark model, predict these processes that could be observed in a high-energy physics experiment. Bottomonium system is a good place to study such processes. Belle experiment is a flavor physics experiment at KEKB asymmetric energy \(e^{-}e^{+}\) collider at KEK, Japan. It mainly collected the data at the energy of \(\Upsilon (4S)\) , but it also collected some data at \(\Upsilon (nS; n = 1, 2, 3)\) , and Belle has the world’s largest data sample for \(\Upsilon (2S)\) . We present a search for charged lepton flavor violation in \(\Upsilon (2S)\) to \(\ell ^\mp \tau ^\pm \) ( \(\ell =e,\mu \) ) decays where \(\tau \) is reconstructed from \(\tau ^{+} \rightarrow \ell ^{+} \nu _{\ell } \bar{\nu }_{\tau }\) and \(\tau ^{+} \rightarrow \pi ^{+}\pi ^{0}\bar{\nu }_{\tau }\) using \( 25~{fb^{-1}} \) data collected at \({\Upsilon (2S)}\) resonance with the Belle detector.