Evidence for $B^0 \to \chi_{c1} \pi ^0$ at Belle

Abstract

We present a measurement of the branching fraction for the Cabibbo- and color-suppressed $B^0 \to \chi_{c1}\pi^0$ decay based on a data sample of $657\times 10^6$ $B\bar B$ events collected at the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. We observe a signal of $40\pm9$ events with a significance of $4.7\sigma$ including systematic uncertainties. The measured branching fraction is $\mathcal {B}(B^0 \to \chi_{c1} \pi^0) = (1.12\pm 0.25(\rm {stat.})\pm 0.12({\rm syst.}))\times 10^{-5}$.