Measurement of the $B^0\rightarrow \rho(770)^{0}\gamma$ branching fraction

Abstract

The ratio between the branching fractions of the $B^0\rightarrow \rho(770)^{0}\gamma$ and $B^{0}\rightarrow K^{*}(892)^{0}\gamma$ decays is measured with proton-proton collision data collected by the LHCb experiment at centre-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9 fb${}^{-1}$. The measured value is \begin{equation*} \frac{{{\cal B}}(B^0\rightarrow \rho(770)^{0}\gamma)}{{{\cal B}}(B^0\rightarrow K^{*}(892)^{0}\gamma)}=0.0189\pm 0.0007\pm 0.0005, \end{equation*} where the first uncertainty is statistical and the second systematic. The branching fraction for $B^0\rightarrow \rho(770)^{0}\gamma$ decays is hence obtained as \begin{equation*} {\cal{B}}(B^0\rightarrow \rho(770)^{0}\gamma) =(7.9\pm 0.3\pm 0.2\pm 0.2) \times 10^{-7}, \end{equation*} where the last uncertainty is due to the branching fraction of the normalisation mode. This result assumes that both the $\rho(770)^0$ and $K^{*}(892)^0$ decays saturate the dihadron mass spectra considered in the analysis. It is consistent with the current world-average value and by far the most precise measurement to date.