Measurement of the photon polarization in radiative $B^0_s$ decays at LHCb

The photon polarization is studied for the first time in $B_s^0$ decays, using an integrated luminosity of 3 fb$^{-1}$ of proton-proton data recorded by the LHCb experiment. An untagged time-dependent analysis of $B_s^0 \rightarrow \phi\gamma$ decays allows to measure the CPV parameter $\mathcal{A}^{\Delta}$, which is sensitive to the left- and right-handed helicity amplitudes. The measured value $\mathcal{A}^{\Delta} = -0.98 \; ^{+0.46}_{-0.52}\text{(stat.)} ^{+0.23}_{-0.20}\text{(syst.)}$ is consistent with the Standard Model prediction within two standard deviations. This value can put constraints on the Wilson coefficients $\mathcal{C}_7$ and $\mathcal{C}_7^{'}$. With a tagged analysis of the same decay the parameter $S_{\phi\gamma}$, also sensitive to the photon polarization, could be measured.