Strong constraints on the $b\to s\gamma$ photon polarisation from $B^0\to K^{\ast 0}{e}^{+}{e}^{-}$ decays

Abstract

An angular analysis of the $B^0\to K^{\ast 0}{e}^{+}{e}^{-}$ decay is performed using a data sample corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$ of $pp$ collisions collected with the LHCb experiment. The analysis is conducted in the very low dielectron mass squared ($q^2$) interval between $0.0008$ and $0.257{\mathrm{GeV}}^2$, where the rate is dominated by the $B^0\to K^{\ast 0}\gamma$ transition with a virtual photon. The fraction of longitudinal polarisation of the $K^{\ast 0}$ meson, $F_{\mathrm{L}}$, is measured to be $F_{\mathrm{L}}=(4.4\pm 2.6\pm 1.4)\%$, where the first uncertainty is statistical and the second systematic. The $A_{\mathrm{T}}^{\mathrm{Re}}$ observable, which is related to the lepton forward-backward asymmetry, is measured to be $A_{\mathrm{T}}^{\mathrm{Re}}=-0.06\pm 0.08\pm 0.02$. The $A_{\mathrm{T}}^{(2)}$ and $A_{\mathrm{T}}^{\mathrm{Im}}$ transverse asymmetries, which are sensitive to the virtual photon polarisation, are found to be $A_{\mathrm{T}}^{(2)}=0.11\pm 0.10\pm 0.02$ and $A_{\mathrm{T}}^{\mathrm{Im}}=0.02\pm 0.10\pm 0.01$. The results are consistent with Standard Model predictions and provide the world's best constraint on the $b\to s\gamma$ photon polarisation.

arXiv commentAll figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2020-020.html (LHCb public pages)

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