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Angular moments of the decay $\Lambda_b^0 \rightarrow \Lambda \mu^{+} \mu^{-}$ at low hadronic recoil

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Abstract

An analysis of the angular distribution of the decay $\Lambda_b^0 \rightarrow \Lambda \mu^{+} \mu^{-}$ is presented, using data collected with the LHCb detector between 2011 and 2016 and corresponding to an integrated luminosity of approximately $5\,fb^{-1}$. Angular observables are determined using a moment analysis of the angular distribution at low hadronic recoil, corresponding to the dimuon invariant mass squared range $15 < q^{2} < 20\, GeV^2/c^4$. The full basis of observables is measured for the first time. The lepton-side, hadron-side and combined forward-backward asymmetries of the decay are determined to be \begin{align} A_{FB}^{l} & = -0.39 \pm 0.04\,\rm{stat} \pm 0.01\, \rm{syst}, \nonumber\\ A_{FB}^{h} & = -0.30 \pm 0.05\,\rm{stat} \pm 0.02\, \rm{syst}, \nonumber\\ A_{FB}^{lh} & = +0.25 \pm 0.04\,\rm{stat} \pm 0.01\, \rm{syst}. \nonumber \end{align} The measurements are consistent with Standard Model predictions.

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Distribution of $p\pi ^- \mu ^+\mu ^- $ invariant mass for (left) long- and (right) downstream-track $p\pi ^- $ categories in the (top) Run 1 data and (bottom) Run 2 data. The result of the fit to each sample of data is indicated by the solid blue line. The signal and background components are illustrated by the dotted green and dashed red lines, respectively.

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One-dimensional projections of the angular distributions of the candidates (black points), combining Run 1 and Run 2 data, as well as candidates reconstructed in the long- and downstream-track $p\pi ^- $ categories. The background is subtracted from the data but no efficiency correction is applied. The projection of each angular distribution obtained from the moment analysis multiplied by the efficiency distribution is superimposed. The large variation in $\phi_\ell$ is primarily due to the angular acceptance.

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Angular observables combining the results for the moments obtained from Run 1 and Run 2 data, as well as candidates reconstructed in the long- and downstream-track $ p \pi ^- $ categories. The blue line represents the SM predictions obtained using the \texttt{EOS} software. The thickness of the light-blue band represents the uncertainty on the SM predictions.

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Correlation coefficients between observables for the combined Run 1 and Run 2 data. The correlations are estimated using bootstrapped samples.

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Correlation coefficients between the observables (top) in the Run 1 data and (bottom) in the Run 2 data. The correlations are estimated using bootstrapped samples.

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Tables and captions

Angular observables combining the results of the moments obtained from Run 1 and Run 2 data, as well as candidates reconstructed in the long- and downstream-track $ p \pi ^- $ categories. The first and second uncertainties are statistical and systematic, respectively.

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Sources of systematic uncertainty on the $K_i$ angular observables, together with the mean and the range of uncertainty values assigned to the 34 $K_i$ parameters in each case. The variation of each source of systematic uncertainty between the different observables depends on the structure of the weighting functions used to extract the observable and its correlation with the angular efficiency.

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Measured values for the angular observables from the Run 1 data combining the results of the moments obtained from the candidates reconstructed in the long- and downstream-track $ p \pi ^- $ categories. The first and second uncertainties are statistical and systematic, respectively.

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Measured values for the angular observables from the Run 2 data combining the results of the moments obtained from the candidates reconstructed in the long- and downstream-track $ p \pi ^- $ categories. The first and second uncertainties are statistical and systematic, respectively.

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Supplementary Material [file]

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Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.