This Letter presents a study of the flavor-changing neutral current radiative B±→K±π-π±γ decays performed using data collected in proton-proton collisions with the LHCb detector at 7 and 8 TeV 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±π- π± 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±π-π± 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 b→sγ transition is reported with a significance of 5.2σ. © 2014 CERN, for the LHCb Collaboration.
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