Observation of photon polarization in the bsgamma transition

This Letter presents a study of the flavor-changing neutral current radiative B±K±pi∓pi±gamma decays performed using data collected in proton-proton collisions with the LHCb detector at 7 and 8TeV center-of-mass energies. In this sample, corresponding to an integrated luminosity of 3fb-1, nearly 14000 signal events are reconstructed and selected, containing all possible intermediate resonances with a K±pi∓pi± final state in the [1.1,1.9]GeV/c2 mass range. 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 obtained. The first direct observation of the photon polarization in the bsgamma transition is reported with a significance of 5.2sigma.