Constraints on the photon polarisation in b → sγ transitions using Bs0→ ϕe+e− decays

Abstract

An angular analysis of the Bs0$$ {B}s^0 $$→ ϕe+e− decay is performed using the proton-proton collision dataset collected between 2011 and 2018 by the LHCb experiment, corresponding to an integrated luminosity of 9 fb−1 at centre-of-mass energies of 7, 8 and 13 TeV. The analysis is performed in the very low dielectron invariant mass-squared region between 0.0009 and 0.2615 GeV2/c4. The longitudinal polarisation fraction of the ϕ meson is measured to be less than 11.5% at 90% confidence level. The ATReCP$$ {A}{\textrm{T}}^{\mathcal{R} eCP} $$ observable, which is related to the lepton forward-backward asymmetry, is measured to be 0.116 ± 0.155 ± 0.006, where the first uncertainty is statistical and the second systematic. The transverse asymmetries, AT2$$ {A}{\textrm{T}}^{(2)} $$ and ATImCP$$ {A}{\textrm{T}}^{\mathcal{I} mCP} $$, which are sensitive to the virtual photon polarisation, are found to be −0.045 ± 0.235 ± 0.014 and 0.002 ± 0.247 ± 0.016, respectively. The results are consistent with Standard Model predictions.