First observation of $CP$ violation and measurement of polarization in $B^{+}\to \rho (770{)}^0{K}^{\ast }(892{)}^{+}$ decays

Abstract

An amplitude analysis of the $B^{+}\to ({\pi }^{+}{\pi }^{-})(K_{\mathrm{S}}^0{\pi }^{+})$ decay is performed in the mass regions $0.30<m_{{\pi }^{+}{\pi }^{-}}<1.10\mathrm{GeV}/c^2$ and $0.75<m_{K_{\mathrm{S}}^0{\pi }^{+}}<1.20\mathrm{GeV}/c^2$, using $pp$ collision data recorded with the LHCb detector corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$. The polarization fractions and $CP$ asymmetries for $B^{+}\to \rho (770{)}^0{K}^{\ast }(892{)}^{+}$ decays are measured. Violation of the $CP$ symmetry in the decay $B^{+}\to \rho (770{)}^0{K}^{\ast }(892{)}^{+}$ is observed for the first time, with a significance exceeding nine standard deviations. The $CP$ asymmetry is measured to be ${\mathcal{A}}_{CP}=0.507\pm 0.062\text{(stat)}\pm 0.024\text{(syst)}$ and the $CP$-averaged longitudinal polarization fraction of $f_L=0.720\pm 0.028\text{(stat)}\pm 0.009\text{(syst)}$. The measurements help to shed light on the polarization puzzle of $B$ mesons decaying to two vector mesons.

All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://lbfence.cern.ch/alcm/public/analysis/full-details/4537/ (LHCb public pages)

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