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Test of lepton universality with $\Lambda^{0}_{b} \to p K^- \ell^+ \ell^-$ decays

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Abstract

The ratio of branching fractions of the decays $\Lambda^{0}_{b}\to pK^{-}e^{+}e^{-}$ and $\Lambda^{0}_{b}\to pK^{-}\mu^{+}\mu^{-}$, $R^{-1}_{pK}$, is measured for the first time using proton-proton collision data corresponding to an integrated luminosity of 4.7 $fb^{-1}$ recorded with the LHCb experiment at center-of-mass energies of 7, 8 and 13 TeV. In the dilepton mass-squared range $0.1 < q^{2} < 6.0$ $GeV^{2}/c^{4}$ and the $pK^{-}$ mass range $m(pK^{-}) < 2600$ $MeV/c^{2}$, the ratio of branching fractions is measured to be $R^{-1}_{pK} = 1.17 ^{+0.18}_{-0.16} \pm 0.07$, where the first uncertainty is statistical and the second systematic. This is the first test of lepton universality with b baryons and the first observation of the decay $\Lambda^{0}_{b}\to pK^{-}e^{+}e^{-}$.

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Distributions of dilepton invariant mass squared, $ q^2$ , for $\Lambda ^0_ b $ candidates as a function of $pK^-\ell^+\ell^-$ invariant mass, in data, for (left) $\ell=\mu$ and (right) $\ell=e$. The complete selection is applied to both distributions, except for $ q^2$ and $ m_{\textrm{corr}}$ requirements, defined in Sec. 3.

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Invariant-mass distribution, with the $ { J \mskip -3mu/\mskip -2mu\psi }$ mass constraint applied, of $\Lambda ^0_ b \rightarrow p K ^- { J \mskip -3mu/\mskip -2mu\psi } ( \rightarrow \mu ^+\mu ^- )$ (left) and $\Lambda ^0_ b \rightarrow p K ^- { J \mskip -3mu/\mskip -2mu\psi } (\rightarrow e ^+ e ^- )$ (right) candidates, summed over trigger and data-taking categories. The black points represent the data, while the solid blue curve shows the sum of the fit to the different categories. The signal component is represented by the red curve and the shaded shapes are the background components, as detailed in the legend.

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Invariant-mass distribution of (left) $\Lambda ^0_ b \rightarrow p K ^- \mu ^+\mu ^- $ and (right) $\Lambda ^0_ b \rightarrow p K ^- e ^+ e ^- $ candidates summed over trigger and data-taking categories. The black points represent the data, while the solid blue curve shows the total PDF. The signal component is represented by the red curve and the combinatorial, $\overline{ B } {}^0 \rightarrow \overline{ K } {}^{*0} \ell^+ \ell^- $ and $ B ^0_ s \rightarrow K^{+} K^{-} \ell^+ \ell^- $ components by yellow, brown and green filled histograms. In the electron model, the grey and blue filled histograms represent the partially reconstructed and $\Lambda ^0_ b \rightarrow p K ^- { J \mskip -3mu/\mskip -2mu\psi } (\rightarrow e ^+ e ^- )$ backgrounds.

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Logarithm of the profile likelihood of the $ R_{ p K }^{-1}$ parameter in blue (red) including only statistical (total) uncertainty. The dashed line indicates the one standard deviation interval.

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Tables and captions

Resonant and nonresonant mode $ q^2$ and $ p K ^-\ell^+ \ell^- $ invariant-mass ranges. For the resonant modes, the four-body invariant mass is computed with a $ { J \mskip -3mu/\mskip -2mu\psi }$ mass constraint on the dilepton system.

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Systematic uncertainties in percent associated to the ratio of branching fractions, $r_{\mathcal{B} }$, for the different data taking periods. For uncertainties that are correlated between data taking periods, a single value is given.

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Systematic uncertainties in percent associated to the measurement of $ R_{ p K }^{-1}$ , for the different data taking periods and trigger categories. For uncertainties that are correlated between data taking periods and categories, a single value is given.

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Supplementary Material [file]

Supplementary material full pdf

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This ZIP file contains supplemetary material for the publication LHCb-PAPER-2019-040. The files are: Supplementary.pdf : "Test of lepton universality with Lb --> pK ll" decays Supplementary.tex

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Created on 06 March 2021.