Abstract
The ratio of branching fractions of the decays {Lambda}_b^0 → pK−e+e− and {Lambda}_b^0 → pK−μ+μ−, {R}_{pK}^{-1} , 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 < q2< 6.0 GeV2/c4 and the pK− mass range m(pK−) < 2600 MeV/c2, the ratio of branching fractions is measured to be {R}_{pK}^{-1}={1.17}_{-0.16}^{+0.18}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}_b^0 → pK−e+e−.
Highlights
Ratios allow for very precise tests of LU, as hadronic uncertainties cancel in their theoretical predictions
The most precise measurements of RK in the q2 range between 1.1 and 6.0 GeV2/c4 and RK∗0 in the regions 0.045 < q2 < 1.1 GeV2/c4 and 1.1 < q2 < 6.0 GeV2/c4 have been performed by the LHCb collaboration and, depending on the theoretical prediction used, are respectively 2.5 [10], 2.1–2.3 and 2.4–2.5 [11] standard deviations below their SM expectations [5, 12–21]
This paper presents the first test of LU in the baryon sector, through the measurement of the ratio of branching fractions for pK − μ+ μ−
Summary
The LHCb detector [30, 31] is a single-arm forward spectrometer covering the pseudorapidity range 2 < η < 5, designed for the study of particles containing b or c quarks. The hardware muon trigger selects events containing at least one muon with significant pT (with thresholds ranging from ∼ 1.5 to ∼ 1.8 GeV/c, depending on the data-taking period). The hardware electron trigger requires the presence of a cluster in the ECAL with significant transverse energy, ET, (from ∼ 2.5 to ∼ 3.0 GeV, depending on the data-taking period). The software trigger requires a two-, three- or four-track secondary vertex, with a significant displacement from any primary pp interaction vertex. Samples of simulated Λ0b → pK−μ+μ−, Λ0b → pK−e+e−, Λ0b → pK−J/ψ(→ μ+μ−) and Λ0b → pK−J/ψ(→ e+e−) decays, generated according to the available phase space in the decays, are used to optimise the selection, determine the efficiency of triggers, reconstruction and signal event selection, as well as to model the shapes used in the fits to extract the signal yields. The interactions of the generated particles with the detector, and its response, are implemented using the Geant toolkit [37] as described in ref. [38]
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