# Measurement of $CP$-violating and mixing-induced observables in $B_s^0 \to \phi\gamma$ decays

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## Abstract

A time-dependent 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 mixing-induced 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 center-of-mass 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.

## Figures and captions

 Fits to the mass distributions of the (left) $B_s^0 \rightarrow \phi\gamma$ and (right) $B^0 \rightarrow K ^{*0} \gamma$ candidates. Fig1a.pdf [23 KiB] HiDef png [613 KiB] Thumbnail [355 KiB] *.C file Fig1b.pdf [32 KiB] HiDef png [550 KiB] Thumbnail [329 KiB] *.C file Decay-time 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. Fig2a.pdf [20 KiB] HiDef png [168 KiB] Thumbnail [139 KiB] *.C file Fig2b.pdf [21 KiB] HiDef png [174 KiB] Thumbnail [146 KiB] *.C file Fig2c.pdf [20 KiB] HiDef png [170 KiB] Thumbnail [131 KiB] *.C file Fig2d.pdf [17 KiB] HiDef png [160 KiB] Thumbnail [135 KiB] *.C file Animated gif made out of all figures. PAPER-2019-015.gif Thumbnail

Created on 27 February 2021.