The ratio of branching fractions of the radiative B decays B0 > K*0 gamma and Bs0 > phi gamma has been measured using 0.37 fb1 of pp collisions at a centre of mass energy of sqrt(s) = 7 TeV, collected by the LHCb experiment. The value obtained is BR(B0 > K*0 gamma)/BR(Bs0 > phi gamma) = 1.12 +/ 0.08 ^{+0.06}_{0.04} ^{+0.09}_{0.08}, where the first uncertainty is statistical, the second systematic and the third is associated to the ratio of fragmentation fractions fs/fd. Using the world average for BR(B0 > K*0 gamma) = (4.33 +/ 0.15) x 10^{5}, the branching fraction BR(Bs0 > phi gamma) is measured to be (3.9 +/ 0.5) x 10^{5}, which is the most precise measurement to date.
Result of the fit for the $ B ^0 \rightarrow K ^{*0} \gamma $ (left) and $ B ^0_ s \rightarrow \phi \gamma $ (right). The black points represent the data and the fit result is represented as a solid line. The signal is fitted with a Crystal Ball function (light dashed line) and the background is described as an exponential (dark dashed line). Below each invariant mass plot, the Poisson $\chi^2$ residuals [21] are shown. 
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Animated gif made out of all figures. 
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Correction factors and corresponding uncertainties affecting the signal yields, in percent, induced by peaking backgrounds, partially reconstructed backgrounds, signal crossfeed and multiple candidates. The total uncertainty is obtained by summing the individual contributions in quadrature. 
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Summary of contributions to the relative systematic uncertainty on the ratio of branching fractions. Note that $f_s/f_d$ is quoted as a separate systematic uncertainty. 
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Created on 20 February 2021.