Novel Method to Reliably Determine the Photon Helicity in B → K1γ

A sizable right-handed photon polarisation in b sy is a clear signal for new physics. In this Letter, we point out that the photon helicity in b -> s gamma can be unambiguously extracted by combining the measurements in B -> K-1 gamma and the Cabibbo-favored D -> K(1)e(+)nu decay. We propose a ratio of up-down asymmetries in D -> K(1)e(+)nu 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 BES111, Belle-ll 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.