Observation of photon polarization in the b → sγ transition at LHCb

<!--HTML-->The Standard Model predicts that the photons emitted in flavor-changing neutral current b to s gamma transitions are predominantly left-handed. However, the photon polarization has never been observed in a direct measurement and remains largely unexplored. Several extensions of the Standard Model predicting a significantly different photon polarization have not been yet ruled out by other measurements, such as the inclusive B to Xs gamma decay rate.<br /> <br /> This talk will focus on the recent study of the radiative B to K pi pi gamma decays performed using data collected in pp collisions with the LHCb detector at 7 and 8 TeV center-of-mass energies. The distribution of the angle of the photon direction with respect to the plane defined by the final-state hadrons in their rest frame is studied in intervals of K pi pi mass and the asymmetry between the number of signal events found on each side of the plane is presented.<br /> <br /> This approach is conceptually very similar to the historic Wu measurement of parity violation from 1957. The first direct observation of the photon polarization in the b to s gamma transition is reported with a significance of 5.2 standard deviations. Future prospects for the extension of this measurement and for the extraction of photon polarization in LHCb will also be discussed.