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

Kavli Affiliate: T. Higuchi

| First 5 Authors: Belle II Collaboration, I. Adachi, L. Aggarwal, H. Ahmed, H. Aihara

| Summary:

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 $(387pm 6)times 10^6$
$Boverline{B}$ meson pairs from $Upsilon(4S)$ decays. We reconstruct $392pm
24$ 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 $B(B^0 to J/psi pi^0)=(2.02 pm 0.12 pm 0.10)times 10^{-5}$
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.

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