Abstract
AbstractThe differential branching fraction of the rare decayΛb0 → Λμ+μ−is measured as a function ofq2, the square of the dimuon invariant mass. The analysis is performed using proton-proton collision data, corresponding to an integrated luminosity of 3.0 fb−1, collected by the LHCb experiment. Evidence of signal is observed in theq2region below the square of theJ/ψmass. Integrating over 15< q2<20 GeV2/c4the differential branching fraction is measured as$$ \mathrm{d}\mathrm{\mathcal{B}}\left({\varLambda}_b^0\to \varLambda {\mu}^{+}{\mu}^{-}\right)/d{q}^2=\left({1.18}_{-0.08}^{+0.09}\pm 0.03\pm 0.27\right)\times {10}^{-7}{\left({\mathrm{GeV}}^2/{c}^4\right)}^{-1}, $$dℬΛb0→Λμ+μ−/dq2=1.18−0.08+0.09±0.03±0.27×10−7GeV2/c4−1,where the uncertainties are statistical, systematic and due to the normalisation mode,Λb0 → J/ψΛ, respectively. In theq2intervals where the signal is observed, angular distributions are studied and the forward-backward asymmetries in the dimuon (AFBℓ) and hadron (AFBh) systems are measured for the first time. In the range 15< q2<20 GeV2/c4they are found to be$$ \begin{array}{l}{A}_{\mathrm{FB}}^{\ell }=-0.05\pm 0.09\left(\mathrm{stat}\right)\pm 0.03\left(\mathrm{syst}\right)\;\mathrm{and}\hfill \\ {}{A}_{\mathrm{FB}}^h=-0.29\pm 0.07\left(\mathrm{stat}\right)\pm 0.03\left(\mathrm{syst}\right).\hfill \end{array} $$AFBℓ=−0.05±0.09stat±0.03systandAFBh=−0.29±0.07stat±0.03syst.
Highlights
Background parametrisationAs there is ambiguity in the choice of parametrisation for the background model, in particular for regions with low statistical significance in data, simulated experiments are generated from a probability density functions (PDFs) corresponding to the best fit to data, for each q2 interval
The angular analysis is performed using the same q2 intervals as those used in the branching fraction measurement
Results are reported for each q2 interval in which the statistical significance of the signal is at least three standard deviations
Summary
The LHCb detector [22, 23] 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 trigger [28] consists of a hardware stage, based on information from the calorimeter and muon systems, followed by a software stage in which a full event reconstruction is carried out. In the subsequent software trigger, at least one of the final-state charged particles is required to have both pT > 0.8 GeV/c and impact parameter greater than 100 μm with respect to all of the primary pp interaction vertices (PVs) in the event. The tracks of two or more of the final-state particles are required to form a vertex that is significantly displaced from the PVs. Simulated samples of pp collisions are generated using Pythia [29] with a specific LHCb configuration [30]. The model used in the simulation of Λ0b → Λμ+μ− decays includes q2 and angular dependence as described in ref. For the Λ0b → J/ψ Λ decay the simulation model is based on the angular distributions observed in ref. [38]
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