Measurement of the photon polarization in $\Lambda_{b}^{0}$ $\to$ $\Lambda$ $\gamma$ decays

The photon polarization in $b \to s \gamma$ transitions is measured for the first time in radiative b-baryon decays exploiting the unique spin structure of $\Lambda_{b}^{0}$ $\to$ $\Lambda$ $\gamma$ decays. A data sample corresponding to an integrated luminosity of $6\;fb^{-1}$ collected by the LHCb experiment in $pp$ collisions at a center-of-mass energy of $13\;TeV$ is used. The photon polarization is measured to be $\alpha_{\gamma}= 0.82^{\,+\,0.17\,+\,0.04}_{\,-\,0.26\,-\,0.13}$, where the first uncertainty is statistical and the second systematic. This result is in agreement with the Standard Model prediction and previous measurements in b-meson decays. Charge-parity breaking effects are studied for the first time in this observable and found to be consistent with $CP$ symmetry. NUMERICAL VALUES HAVE BEEN DIGITISED FROM THE PAPER.