Evidence for $B\to K\eta \prime \gamma$ decays at Belle

We present the results of a search for the radiative decay $B\to K{\eta }^{\prime }\gamma$ and find evidence for $B^{+}\to K^{+}{\eta }^{\prime }\gamma$ decays at the 3.3 standard deviation level with a partial branching fraction of $(3.6\pm 1.2\pm 0.4)\times {10}^{-6}$, where the first error is statistical and the second systematic. This measurement is restricted to the region of combined $K{\eta }^{\prime }$ invariant mass less than $3.4\mathrm{GeV}/c^2$. A 90% confidence level upper limit of $6.4\times {10}^{-6}$ is obtained for the partial branching fraction of the decay $B^0\to K^0{\eta }^{\prime }\gamma$ in the same $K{\eta }^{\prime }$ invariant mass region. These results are obtained from a $605{\mathrm{fb}}^{-1}$ data sample containing $657\times {10}^{6}B\overline{B}$ pairs collected at the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider.

Figure 1

Projections to the signal region from the fits to $605{\mathrm{fb}}^{-1}$ of data. Each plot is the distribution of one 2D fit variable within the signal region of the second 2D fit variable. In each plot the solid line is the combined background plus signal PDF, the dotted line is the $e^{+}{e}^{-}\to q\overline{q}$ PDF, the dot-dashed the $b\to c$ PDF, the long dashed the signal PDF, and the short dashed the $b\to u$, $d$, $s$ PDF.