Novel Method to Reliably Determine the Photon Helicity in B→K_{1}γ

A sizable right-handed photon polarization in b→sγ is a clear signal for new physics. In this Letter, we point out that the photon helicity in b→sγ can be unambiguously extracted by combining the measurements in B→K_{1}γ and the Cabibbo-favored D→K_{1}e^{+}ν decay. We propose a ratio of up-down asymmetries in D→K_{1}e^{+}ν to quantify the hadronic effects. A method for measuring, in experiment, the involved partial decay widths in the ratio is discussed, and experimental facilities like BESIII, Belle-II and LHCb are likely to measure this ratio. We also give the angular distribution that is useful for extracting the photon polarization in the presence of different kaon resonances.