Measurement of $CP$-Violating and Mixing-Induced Observables in $B_s^0\to \varphi \gamma$ Decays

A time-dependent analysis of the $B_s^0\to \varphi \gamma$ decay rate is performed to determine the $CP$ -violating observables $S_{\varphi \gamma }$ and $C_{\varphi \gamma }$ and the mixing-induced observable ${\mathcal{A}}_{\varphi \gamma }^{\mathrm{\Delta }}$. The measurement is based on a sample of $pp$ collision data recorded with the LHCb detector, corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$ at center-of-mass energies of 7 and 8 TeV. The measured values are $S_{\varphi \gamma }=0.43\pm 0.30\pm 0.11$, $C_{\varphi \gamma }=0.11\pm 0.29\pm 0.11$, and ${\mathcal{A}}_{\varphi \gamma }^{\mathrm{\Delta }}=-{0.67}_{-0.41}^{+0.37}\pm 0.17$, 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.

Figure 1

Fits to the mass distributions of the (top) $B_s^0\to \varphi \gamma$ and (bottom) $B^0\to K^{*0}\gamma$ candidates.

Figure 2

Decay-time fit projections. The top row corresponds to the tagged (left) $B_s^0\to \varphi \gamma$ and (right) ${\overline{B}}_s^0\to \varphi \gamma$ candidates, while the bottom plots show the (left) untagged $B_s^0\to \varphi \gamma$ and (right) $B^0\to K^{*0}\gamma$ candidates. The line is the result of the fit described in the text, including statistical uncertainties.