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 protonproton collision data corresponding to an integrated luminosity of 4.7 $fb^{1}$ recorded with the LHCb experiment at centerofmass energies of 7, 8 and 13 TeV. In the dilepton masssquared 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 $\Lambda^{0}_{b}\to pK^{}e^{+}e^{}$.
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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Invariantmass 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 datataking 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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Invariantmass 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 datataking 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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Animated gif made out of all figures. 
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Resonant and nonresonant mode $ q^2$ and $ p K ^\ell^+ \ell^ $ invariantmass ranges. For the resonant modes, the fourbody 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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Created on 06 March 2021.