Observation of time-dependent $CP$ violation and measurement of the branching fraction of $B^0 \to J/\psi \pi^0$ decays

TL;DR

This paper reports the first observation of mixing-induced $CP$ violation in $B^0 o J/ar{psi} ho^0$ decays, measuring the branching fraction and $CP$ asymmetries using Belle II data, confirming the Standard Model predictions.

Contribution

The study provides the first measurement of time-dependent $CP$ violation in $B^0 o J/ar{psi} ho^0$ decays, utilizing Belle II data from 2019-2022.

Findings

Branching fraction measured as (2.00 ± 0.12 ± 0.09)×10⁻⁵.

Mixing-induced $CP$ violation observed with 5.0 sigma significance.

First observation of $CP$ violation in this decay mode.

Abstract

We present a measurement of the branching fraction and time-dependent charge-parity ($CP$) decay-rate asymmetries in $B^0 \to J/\psi \pi^0$ decays. The data sample was collected with the Belle~II detector at the SuperKEKB asymmetric $e^+e^-$ collider in 2019-2022 and contains $(387\pm 6)\times 10^6$ $B\overline{B}$ meson pairs from $\Upsilon(4S)$ decays. We reconstruct $392\pm 24$ signal decays and fit the $CP$ parameters from the distribution of the proper-decay-time difference of the two $B$ mesons. We measure the branching fraction to be $B(B^0 \to J/\psi \pi^0)=(2.00 \pm 0.12 \pm 0.09)\times 10^{-5}$ and the direct and mixing-induced $CP$ asymmetries to be $C_{CP}=0.13 \pm 0.12 \pm 0.03$ and $S_{CP}=-0.88 \pm 0.17 \pm 0.03$, respectively, where the first uncertainties are statistical and the second are systematic. We observe mixing-induced $CP$ violation with a significance of $5.0$ standard deviations for the first time in this mode.