First look at time-dependent CP violation using early Belle II data

Abstract

Time dependent CP-violation phenomena are a powerful tool to precisely measure fundamental parameters of the Standard Model and search for New Physics. 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 $5.5\times10^{33}~cm^{-2}s^{-1}$ , and Belle II has recorded a data sample of about $0.5~fb^{-1}$ . Main operation of SuperKEKB has started in March 2019. This early data set is used to establish the performance of the detector in terms of reconstruction efficiency of final states of interest for the measurement of time dependent CP violation, such as $J/\psi K_0$ , $\eta' K_0$, and $\phi K_0$. A first assessment of the B flavor tagging capabilities of the experiment will be given, along with estimates of the Belle II sensitivity to the CKM angles $\phi_1/\beta$ and $\phi_2/\alpha$ and to potential New Physics contributions in penguin amplitudes dominated decays and in $b\to s\gamma$ transitions. In this paper we will present estimates of the sensitivity to $\phi_1$ in the golden channels $b\to c\bar{c} s$ and in the penguin-dominated modes $B_0\to \eta' K_0,\quad\phi K_0,\quad K_0\pi_0(\gamma)$. A study for the time-dependent analysis of $B_0\to\pi_0\pi_0$, relevant for the measurement of $\phi_2$, and feasible only in the clean environment of an $e^+e^-$ collider, will also be given.