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Tests of lepton universality using $B^0\to K^0_S \ell^+ \ell^-$ and $B^+\to K^{*+} \ell^+ \ell^-$ decays

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Abstract

Tests of lepton universality in $B^0\to K^0_S \ell^+ \ell^-$ and $B^+\to K^{*+} \ell^+ \ell^-$ decays where $\ell$ is either an electron or a muon are presented. The differential branching fractions of $B^0\to K^0_S e^+ e^-$ and $B^+\to K^{*+} e^+ e^-$ decays are measured in intervals of the dilepton invariant mass squared. The measurements are performed using proton-proton collision data recorded by the LHCb experiment, corresponding to an integrated luminosity of $9 \mathrm{fb}^{-1}$. The results are consistent with the Standard Model and previous tests of lepton universality in related decay modes. The first observation of $B^0 \to K^0_S e^+ e^-$ and $B^+ \to K^{*+} e^+ e^-$ decays is reported.

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Distributions of (top left) $ { J \mskip -3mu/\mskip -2mu\psi } (\mu ^+ \mu ^- ) K ^0_{\mathrm{S}} $ mass, (top right) $ { J \mskip -3mu/\mskip -2mu\psi } ( e ^+ e ^- ) K ^0_{\mathrm{S}} $ mass, (bottom left) $ { J \mskip -3mu/\mskip -2mu\psi } (\mu ^+ \mu ^- ) K ^0_{\mathrm{S}} \pi ^+ $ mass and (bottom right) $ { J \mskip -3mu/\mskip -2mu\psi } ( e ^+ e ^- ) K ^0_{\mathrm{S}} \pi ^+ $ mass with the fit models used to determine the control mode yields.

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Distributions of (top left) $ K ^0_{\mathrm{S}} \mu ^+ \mu ^- $ and (top right) $ K ^0_{\mathrm{S}} e ^+ e ^- $ mass with the fit models used to determine the $ { B ^0 \rightarrow K ^0_{\mathrm{S}} \mu ^+ \mu ^- } $ yield and $ R^{-1}_{ K ^0_{\mathrm{S}} }$ , and (bottom left) $ K ^0_{\mathrm{S}} \pi ^+ \mu ^+ \mu ^- $ and (bottom right) $ K ^0_{\mathrm{S}} \pi ^+ e ^+ e ^- $ mass with the fit models used to determine the $ { B ^+ \rightarrow K ^{*+} \mu ^+ \mu ^- } $ yield and $ R^{-1}_{ K ^{*+} }$ .

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Total selection efficiencies as a function of $ q^2$ , for (top left) $ { B ^0 \rightarrow K ^0_{\mathrm{S}} \mu ^+ \mu ^- } $ , (top right) $ { B ^0 \rightarrow K ^0_{\mathrm{S}} e ^+ e ^- } $ , (bottom left) $ { B ^+ \rightarrow K ^{*+} \mu ^+ \mu ^- } $ , and (bottom right) $ { B ^+ \rightarrow K ^{*+} e ^+ e ^- } $ decays. The efficiencies are evaluated using simulated candidates that have been corrected to improve agreement with data. These efficiencies account for detector resolution effects including the effect of bremsstrahlung, which may cause a candidate to be reconstructed in a different bin of $ q^2$ than would be dictated by its `true' value.

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Single ratios (left) $ r^{-1}_{ { J \mskip -3mu/\mskip -2mu\psi } K ^0_{\mathrm{S}} }$ and (right) $ r^{-1}_{ { J \mskip -3mu/\mskip -2mu\psi } K ^{*+} }$ as a function of the opening angle of the two leptons, normalised to their average value.

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Single ratios (left) $ r^{-1}_{ { J \mskip -3mu/\mskip -2mu\psi } K ^0_{\mathrm{S}} }$ and (right) $ r^{-1}_{ { J \mskip -3mu/\mskip -2mu\psi } K ^{*+} }$ as a function of the output of an MVA trained to separate the signal and control modes, normalised to their average value.

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Scans of the profile likelihoods as a function of (left) $ R^{-1}_{ K ^0_{\mathrm{S}} }$ , and (right) $ R^{-1}_{ K ^{*+} }$ .

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Scans of the profile likelihoods as a function of the differential branching fractions for (left) $ { B ^0 \rightarrow K ^0 e ^+ e ^- } $ decays, and (right) $ { B ^+ \rightarrow K ^{*+} e ^+ e ^- } $ decays.

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Measurements of $ R_{ K ^0_{\mathrm{S}} }$ and $ R_{ K ^{*+} }$ performed by the LHCb and Belle collaborations \cite{Belle:2019oag, BELLE:2019xld}.

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Distributions of (top left) $ { J \mskip -3mu/\mskip -2mu\psi } (\mu ^+ \mu ^- ) K ^0_{\mathrm{S}} $ mass, (top right) $ { J \mskip -3mu/\mskip -2mu\psi } ( e ^+ e ^- ) K ^0_{\mathrm{S}} $ mass, (bottom left) $ { J \mskip -3mu/\mskip -2mu\psi } (\mu ^+ \mu ^- ) K ^0_{\mathrm{S}} \pi ^+ $ mass and (bottom right) $ { J \mskip -3mu/\mskip -2mu\psi } ( e ^+ e ^- ) K ^0_{\mathrm{S}} \pi ^+ $ mass with the fit models used to determine the control mode yields.

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Distributions of (top left) $\psi {(2S)} (\mu ^+ \mu ^- ) K ^0_{\mathrm{S}} $ mass, (top right) $\psi {(2S)} ( e ^+ e ^- ) K ^0_{\mathrm{S}} $ mass, (bottom left) $\psi {(2S)} (\mu ^+ \mu ^- ) K ^0_{\mathrm{S}} \pi ^+ $ mass and (bottom right) $\psi {(2S)} ( e ^+ e ^- ) K ^0_{\mathrm{S}} \pi ^+ $ mass with the fit models used to determine the $\psi {(2S)}$ control mode yields.

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Distributions of (top left) $\psi {(2S)} (\mu ^+ \mu ^- ) K ^0_{\mathrm{S}} $ mass, (top right) $\psi {(2S)} ( e ^+ e ^- ) K ^0_{\mathrm{S}} $ mass, (bottom left) $\psi {(2S)} (\mu ^+ \mu ^- ) K ^0_{\mathrm{S}} \pi ^+ $ mass and (bottom right) $\psi {(2S)} ( e ^+ e ^- ) K ^0_{\mathrm{S}} \pi ^+ $ mass with the fit models used to determine the $\psi {(2S)}$ control mode yields.

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Supplementary Material [file]

Supplementary material full pdf

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This ZIP file contains supplementary material for the publication LHCb-PAPER-2021-038. The files are: supplementary.pdf : A document containing the extra figures and equations. supplementary.tex : the tex source for supplementary.pdf *.pdf, *.png, *.eps *.py *.json *.C : The figures in various formats

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Created on 13 April 2024.