Observation of $B^{+}\to {\rho }^{+}{K}^0$ and measurement of its branching fraction and charge asymmetry

We present the first observation of the decay $B^{+}\to {\rho }^{+}{K}^0$, using a data sample of $348{\mathrm{fb}}^{-1}$ collected at the ${\rm Y}(4S)$ resonance with the BABAR detector. The branching fraction and charge asymmetry are measured to be $({8.0}_{-1.3}^{+1.4}\pm 0.6)\times {10}^{-6}$ and $(-12.2\pm 16.6\pm 2.0)\%$, respectively, where the first uncertainty is statistical and the second is systematic. The charge asymmetry is defined by ${\mathcal{A}}_{\mathrm{ch}}=({\Gamma }_{B^{-}}-{\Gamma }_{B^{+}})/({\Gamma }_{B^{-}}+{\Gamma }_{B^{+}})$ with ${\Gamma }_{B^{\pm }}$ the $B^{\pm }$ decay rate. The significance of the observed branching fraction, including systematic uncertainties, is 7.9 standard deviations.

Figure 1

Distributions of (a) $\Delta E$, (b) $m_{\mathrm{ES}}$, (c) $m_{{\pi }^{+}{\pi }^0}$, (d) $\cos {\theta }_{\rho }$, (e) $\mathcal{F}$, and (f) $\Delta t/{\sigma }_{\Delta t}$. To improve visibility, a selection requirement on the likelihood ratio that retains 70.0% of the signal events has been applied. The points with uncertainties are the data. The curves are projections of the ML fit. The dashed curves show the sum of the continuum and $B\overline{B}$ background components. The dot-dashed curves show the signal component. The solid curves show the sum of the signal and background components.