Youngmin Yook

12 September 2021

Abstract: The Belle II experiment has accumulated data corresponding to $89.99~{\rm fb}^{-1}$ integrated luminosity in the past 2 years, and is performing very good. Waiting that the full planned data set will be recorded $(50~{\rm ab}^{-1})$, which will allow search for rare processes and will have a tremendous impact in the spectroscopy field, the Phase 3 data set allows to already perform analysis with high precision. We present here the analysis of $B\to K J/\psi \pi^+ \pi^-$ and $B\to K\psi(2S)$: in the former for the first time the evidence for the $X(3872)\to J/\psi \pi^+\pi^-$ has been found at Belle II, which is consistent with the observation at Belle of the same resonant state, performed in 2003. The re-discovery of the $X(3872)$ based on the early Phase3 data includes the efficiency and resolution study, calibration with $B\to K\psi(2S)$, and background check. When higher statistics will be available, Belle II is planning the even more interesting analysis of $B\to D\bar{D}\pi^0K$, and search for $X(3872)\to D\bar{D}\pi^0$.

Keyword(s): X(3872) ; Rediscovery ; HADRON2020