Measurement of branching fractions in radiative $B$ decays to $\eta K\gamma$ and search for $B$ decays to ${\eta }^{\prime }K\gamma$

We present measurements of the $B\to \eta K\gamma$ branching fractions and upper limits for the $B\to {\eta }^{\prime }K\gamma$ branching fractions. For $B^{+}\to \eta K^{+}\gamma$ we also measure the time-integrated charge asymmetry. The data sample, collected with the BABAR detector at the Stanford Linear Accelerator Center, represents $232\times {10}^6$ produced $B\overline{B}$ pairs. The results for branching fractions and upper limits at 90% confidence level in units of ${10}^{-6}$ are: $\mathcal{B}(B^0\to \eta K^0\gamma )={11.3}_{-2.6}^{+2.8}\pm 0.6$, $\mathcal{B}(B^{+}\to \eta K^{+}\gamma )=10.0\pm 1.3\pm 0.5$, $\mathcal{B}(B^0\to {\eta }^{\prime }{K}^0\gamma )<6.6$, $\mathcal{B}(B^{+}\to {\eta }^{\prime }{K}^{+}\gamma )<4.2$. The charge asymmetry in the decay $B^{+}\to \eta K^{+}\gamma$ is ${\mathcal{A}}_{\mathrm{ch}}=-0.09\pm 0.12\pm 0.01$. The first errors are statistical and the second systematic.

Figure 1

The $B$ candidate $m_{\mathrm{ES}}$ and $\Delta E$ projections for $\eta K^{+}\gamma$ (a), (b), $\eta K^0\gamma$ (c), (d), ${\eta }^{\prime }{K}^{+}\gamma$ (e), (f) and ${\eta }^{\prime }{K}^0\gamma$ (g), (h). Points with error bars (statistical only) represent the data, the solid line the full fit function, and the dashed line its background component.

Figure 2

Plot of $\eta K$ invariant mass for signal using the weighting technique described in the text for the combined subdecay modes: (a) charged modes, (b) neutral modes. Errors are statistical only.