Observation of $D^0\to \rho^0\gamma$ and search for $CP$ violation in radiative charm decays

TL;DR

This paper reports the first observation of the radiative charm decay $D^0 o ho^0 \gamma$ and searches for $CP$ violation in several charm radiative decays, finding results consistent with no $CP$ violation and providing improved branching fraction measurements.

Contribution

First observation of $D^0 o ho^0 \gamma$ decay and the most precise measurements of related branching fractions, along with a search for $CP$ violation in these decays.

Findings

Branching fraction for $D^0 o ho^0 \\gamma$ measured as $(1.77 \\pm 0.30 \\pm 0.07) imes 10^{-5}$.

$CP$ asymmetries in the studied decays are consistent with zero.

Improved measurements of branching fractions for $D^0 o \\phi \\gamma$ and $D^0 o \\overline{K}{}^{*0} \\gamma$.

Abstract

We report the first observation of the radiative charm decay $D^0 \to \rho^0 \gamma$ and the first search for $CP$ violation in decays $D^0 \to \rho^0 \gamma$, $\phi\gamma$, and $\overline{K}{}^{*0} \gamma$, using a data sample of 943 fb$^{-1}$ collected with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. The branching fraction is measured to be $\mathcal{B}(D^0 \to \rho^0 \gamma)=(1.77 \pm 0.30 \pm 0.07) \times 10^{-5}$, where the first uncertainty is statistical and the second is systematic. The obtained $CP$ asymmetries, $\mathcal{A}_{CP}(D^0 \to \rho^0 \gamma)=+0.056 \pm 0.152 \pm 0.006$, $\mathcal{A}_{CP}(D^0 \to \phi \gamma)=-0.094 \pm 0.066 \pm 0.001$, and $\mathcal{A}_{CP}(D^0 \to \overline{K}{}^{*0} \gamma)=-0.003 \pm 0.020 \pm 0.000$, are consistent with no $CP$ violation. We also present an improved measurement of the branching fractions $\mathcal{B}(D^0 \to \phi \gamma)=(2.76 \pm 0.19 \pm 0.10) \times 10^{-5}$ and $\mathcal{B}(D^0 \to \overline{K}{}^{*0} \gamma)=(4.66 \pm 0.21 \pm 0.21) \times 10^{-4}$.