First experimental study of photon polarization in radiative $B_s^0$ decays

The polarization of photons produced in radiative $B_s^0$ decays is studied for the first time. The data are recorded by the LHCb experiment in $pp$ collisions corresponding to an integrated luminosity of 3fb${}^{-1}$ at center-of-mass energies of $7$ and $8$TeV. A time-dependent analysis of the $B_s^0\to \varphi \gamma$ decay rate is conducted to determine the parameter ${\mathcal{A}}^{\mathrm{\Delta }}$, which is related to the ratio of right- over left-handed photon polarization amplitudes in $b\to s\gamma$ transitions. A value of ${\mathcal{A}}^{\mathrm{\Delta }}=-0.98_{-0.52-0.20}^{+0.46+0.23}$ is measured. This result is consistent with the Standard Model prediction within two standard deviations.