Search for the New Physics in Bottomonium Decays in Belle and Belle II

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 problem, etc. The conservation of lepton flavor is one of the accidental symmetries of the SM. Charged lepton flavor violating processes are forbidden in the SM but some new physics models, such as 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 $$e^{-}$$ - $$e^{+}$$ collider, at KEK, Japan. It mainly collected the data at the energy of $$\mathrm {\Upsilon (4S)}$$ , but it also collected some data at $$\mathrm {\Upsilon (nS;n=1,2,3}$$ ), so it is possible to study the decay of $$\mathrm {\Upsilon (nS)}$$ . Belle has the world’s largest data sample available of $$\mathrm {\Upsilon (2S)}$$ . The Belle II experiment is an upgrade version of the Belle experiment which aims to collect $$\mathrm {50 ~ab^{-1}}$$ of data. We will present the study of $$\mathrm {\Upsilon (2S)} \rightarrow \textrm{l}(\textrm{l}=e,\mu )\tau $$ at Belle. We also mention on the future prospects at Belle II.