Measurement of the ratio of branching fractions ${\cal B}(B^0 \to K^{\ast 0} \gamma)/{\cal B}(B^0_s \to \phi \gamma)$

The ratio of branching fractions of the radiative $B$ decays $B^0\to K^{*0}\gamma$ and $B^0_s\to \phi\gamma$ has been measured using $0.37\,$fb$^{-1}$ of $pp$ collisions at a centre of mass energy of $\sqrt{s}=7\,$TeV, collected by the LHCb experiment. The value obtained is \begin{equation} \frac{{\cal B}(B^0\to K^{*0}\gamma)}{{\cal B}(B^0_s\to \phi\gamma)} = 1.12 \pm 0.08^{+0.06}_{-0.04}\phantom{.}^{+0.09}_{-0.08},\nonumber \end{equation} where the first uncertainty is statistical, the second systematic and the third is associated to the ratio of fragmentation fractions $f_s/f_d$. Using the world average for ${\cal B}(B^0\to K^{*0}\gamma) = (4.33\pm 0.15)~\times10^{-5}$, the branching fraction ${\cal B}(B^0_s\to \phi\gamma)$ is measured to be $(3.9\pm0.5)\times10^{-5}$, which is the most precise measurement to date.