Strong constraints on the b → sγ photon polarisation from B ⁰ → K ^*0 e ⁺ e ⁻ decays

An angular analysis of the B⁰ → K*⁰e⁺e⁻ decay is performed using a data sample corresponding to an integrated luminosity of 9 fb⁻¹ of pp collisions collected with the LHCb experiment. The analysis is conducted in the very low dielectron mass squared (q²) interval between 0.0008 and 0.257 GeV², where the rate is dominated by the B⁰ → K^*0γ transition with a virtual photon. The fraction of longitudinal polarisation of the K^*0 meson, F_L, is measured to be F_L = (4.4 ± 2.6 ± 1.4)%, where the first uncertainty is statistical and the second systematic. The ATRe observable, which is related to the lepton forward-backward asymmetry, is measured to be ATRe = −0.06 ± 0.08 ± 0.02. The AT(2) and ATIm transverse asymmetries, which are sensitive to the virtual photon polarisation, are found to be AT(2) = 0.11 ± 0.10 ± 0.02 and ATIm = 0.02 ± 0.10 ± 0.01. The results are consistent with Standard Model predictions and provide the world’s best constraint on the b → sγ photon polarisation. [Figure not available: see fulltext.]