A timedependent analysis of the $B_s^0 \to \phi\gamma$ decay rate is performed to determine the $CP$violating observables $S_{\phi\gamma}$ and $C_{\phi\gamma}$, and the mixinginduced observable $\mathcal{A}^{\Delta}_{\phi\gamma}$. The measurement is based on a sample of $pp$ collision data recorded with the LHCb detector, corresponding to an integrated luminosity of 3 fb$^{1}$ at centerofmass energies of 7 and 8 TeV. The measured values are \begin{align*} S_{\phi\gamma} &= 0.43 \pm 0.30 \pm 0.11, \\ C_{\phi\gamma} &= 0.11 \pm 0.29 \pm 0.11, \\ \mathcal{A}^{\Delta}_{\phi\gamma} &= 0.67 ^{+0.37}_{0.41} \pm 0.17, \end{align*} where the first uncertainty is statistical and the second systematic. This is the first measurement of the observables $S$ and $C$ in radiative $B_s^0$ decays. The results are consistent with the Standard Model predictions.
Fits to the mass distributions of the (left) $ B_s^0 \rightarrow \phi\gamma$ and (right) $ B^0 \rightarrow K ^{*0} \gamma$ candidates. 
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Decaytime fit projections. The top row corresponds to the tagged (left) $ B_s^0 \rightarrow \phi\gamma $ and (right) $\overline{ B }{} {}^0_ s \rightarrow \phi\gamma$ candidates, while the bottom plots show the (left) untagged $ B_s^0 \rightarrow \phi\gamma $ and (right) $ B^0 \rightarrow K ^{*0} \gamma $ candidates. The line is the result of the fit described in the text, including statistical uncertainties. 
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Animated gif made out of all figures. 
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Created on 11 July 2020.