Observation of photon polarization in the $b\to s{\hat{I}}^3$ transition

Abstract

This Letter presents a study of the flavor-changing neutral current radiative $B^{\pm }\to K^{\pm }\ddot{I}{€}^{\mp }\ddot{I}{€}^{\pm }{\hat{I}}^3$ 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 $3{\text{fb}}^{-1}$, nearly $14000$ signal events are reconstructed and selected, containing all possible intermediate resonances with a $K^{\pm }\ddot{I}{€}^{\mp }\ddot{I}{€}^{\pm }$ final state in the $[1.1,1.9]$GeV/$c^2$ 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 $K^{\pm }\ddot{I}{€}^{\mp }\ddot{I}{€}^{\pm }$ 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 $b\to s{\hat{I}}^3$ transition is reported with a significance of $5.2\ddot{I}ƒ$.