Measurement of the ratio of branching fractions $B(B^0 \to K^{\ast 0} \gamma) / B(B^0_s \to \phi \gamma)$ and the direct $C\!P$ asymmetry in $B^0 \to K^{\ast 0} \gamma$

The ratio of branching fractions of the radiative $B$ decays $B^0\rightarrow K^{*0}\gamma$ and $B^0_s\rightarrow\phi\gamma$ has been measured using an integrated luminosity of $1.0\mbox{fb}^{-1}$ of $pp$ collision data collected by the LHCb experiment at a centre-of-mass energy of $\sqrt{s}=7\mathrm{\,Te\kern -0.1em V}$. The value obtained is \begin{equation*} \frac{{\cal B}(B^0\rightarrow K^{*0}\gamma)}{{\cal B} (B^0_s\rightarrow\phi\gamma)} = 1.23 \pm 0.06\mathrm{\,(stat.)} \pm 0.04\mathrm{\,(syst.)} \pm 0.10\,(f_s/f_d)\,, \end{equation*} where the first uncertainty is statistical, the second is the experimental systematic uncertainty and the third is associated with the ratio of fragmentation fractions $f_s/f_d$. Using the world average value for ${\cal{B}}(B^0\rightarrow K^{*0}\gamma)$, the branching fraction ${\cal{B}}(B^0_s\rightarrow\phi\gamma)$ is measured to be $(3.5\pm 0.4)\times10^{-5}$. The direct CP asymmetry in $B^0\rightarrow K^{*0}\gamma$ decays has also been measured with the same data and found to be \begin{equation*} \mathcal{A}_{C\!P}(B^0\rightarrow K^{*0}\gamma) = (0.8\pm1.7\mathrm{\,(stat.)}\pm0.9\mathrm{\,(syst.)})\%\,. \end{equation*} Both measurements are the most precise to date and are in agreement with the previous experimental results and theoretical expectations.