Speaker
Description
The Belle II experiment at the SuperKEKB energy-asymmetric $e^+ e^-$ collider is a substantial upgrade of the B factory facility at the Japanese KEK laboratory. The design luminosity of the machine is $8\times 10^{35}$ cm$^{-2}$s$^{-1}$ and the Belle II experiment aims to record 50 ab$^{-1}$ of data, a factor of 50 more than its predecessor. From February to July 2018, the machine has completed a commissioning run, achieved a peak luminosity of $5.5\times 10^{33}$ cm$^{-2}$s$^{-1}$, and Belle II has recorded a data sample of about 0.5 fb$^{-1}$. Main operation of SuperKEKB has started in March 2019. In this presentation we report a measurement of the time-dependent CP violation parameter for $B^0(\bar B^0)\to J/\psi K^0_S$ using this early data set. One neutral $B$ meson is reconstructed in the $J/\psi K^0_S$ $CP$-eigenstate decay channel and the flavor of the accompanying $B$ meson is identified to be either $B^0$ or $\bar B^0$ from its decay products. We present a new concept for the time-dependent $CP$ violation fit together with initial results for the parameters of $B^0$ mixing-induced phenomena and the lifetime of $B^0$.
