Search for the rare decay ${\overline{B}}^0\to D^{*0}\gamma$

We report on a search for the rare decay ${\overline{B}}^0\to D^{*0}\gamma$, which in the standard model is dominated by $W$-exchange. The analysis is based on a data sample comprising $87.8\times {10}^6$ $B\overline{B}$ pairs collected with the BABAR detector at the PEP-II asymmetric-energy $B$ Factory at SLAC. No significant signal is observed, and an upper limit on the branching fraction of $2.5\times {10}^{-5}$ at the 90% confidence level is obtained.

Figure 1

$W$-exchange is the leading contribution to the ${\overline{B}}^0\to D^{*0}\gamma$ decay in the standard model. The photon may be emitted from any quark line or the $W$.

Figure 2

Distribution of data events in the $\Delta E\mathrm{\text{−}}{m}_{\mathrm{ES}}$ plane. The lines indicate the regions of the signal box and of the grand sideband.

Figure 3

$\Delta E$ (left) and $m_{\mathrm{ES}}$ (right) distributions for data (points) and MC simulation (shaded histograms). All selection requirements are applied including the $m_{\mathrm{ES}}$ signal box requirement for the left plot and the $\Delta E$ signal box requirement for the right plot.

Figure 4

$1-\mathrm{\text{confidence}}$ level versus the assumed branching fraction. The shaded areas are the 68% and 95% probability regions. The 90% C.L. is marked with an arrow.