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Measurement of the $\Lambda^0_b\rightarrow J/\psi\Lambda$ angular distribution and the $\Lambda^0_b$ polarisation in $pp$ collisions

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Abstract

This paper presents an analysis of the $\Lambda^0_b\rightarrow J/\psi\Lambda$ angular distribution and the transverse production polarisation of $\Lambda^0_b$ baryons in proton-proton collisions at centre-of-mass energies of 7, 8 and 13 TeV. The measurements are performed using data corresponding to an integrated luminosity of 4.9 fb$^{-1}$, collected with the LHCb experiment. The polarisation is determined in a fiducial region of $\Lambda^0_b$ transverse momentum and pseudorapidity of $1 < p_{T} < 20$ GeV$/c$ and $2 < \eta < 5$, respectively. The data are consistent with $\Lambda^0_b$ baryons being produced unpolarised in this region. The parity-violating asymmetry parameter of the $\Lambda\rightarrow p\pi^-$ decay is also determined from the data and its value is found to be consistent with a recent measurement by the BES III collaboration.

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Definition of the five decay angles, $\theta$, $\theta_b$, $\phi_b$, $\theta_{l}$ and $\phi_l$ used to describe the kinematics of the $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda $ decay. The angles are described in the text.

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Mass distribution of selected $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda $ candidates in (top-left) the 2011, (top-right) the 2012, (bottom-left) the 2015 and (bottom-right) the 2016 data sets. The long and downstream categories have been combined. The results of fits to the distributions are overlaid.

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Angular distributions of $\cos\theta_b$, $\cos\theta_l$, $\cos\theta$, $\phi_b$ and $\phi_l$ for the background-subtracted candidates. The long and downstream categories for the different data-taking years have been combined. The result of the moment analysis, folded with the angular efficiency, is overlaid. The contribution from the long and downstream categories are indicated by the green and red lines, respectively.

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Posterior probability distributions of $|a_{\pm}|$, ${\rm arg}(a_{\pm})$, $|b_{-}|$, ${\rm arg}(b_{-})$ and the transverse production polarisation of the $\Lambda ^0_ b $ baryons, $P_{b}$, at centre-of-mass energies of 7, 8 and 13 $\text{ Te V}$ assuming uniform priors. The shaded regions indicate the 68% and 95% credibility intervals.

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Measured transverse production polarisation of the $\Lambda ^0_ b $ baryons, $P_{b}$, as a function of the centre-of-mass energy, $\sqrt{s}$, of the data set. The points indicate the most probable value and the shaded regions the 68% and 95% credibility level intervals.

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Posterior probability distribution of the parity-violating asymmetry parameter, $\alpha_b$. The shaded regions indicate the 68% and 95% credibility intervals.

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Posterior probability distribution for $\alpha_{\Lambda }$, assuming a uniform prior, with all external constraints removed. The shaded regions indicate the 68% and 95% credibility intervals.

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Statistical correlation between the moments determined at $\sqrt{s}$ of 7 $\text{ Te V}$ .

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Statistical correlation between the moments determined at $\sqrt{s}$ of 8 $\text{ Te V}$ .

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Statistical correlation between the moments determined at $\sqrt{s}$ of 13 $\text{ Te V}$ .

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Animated gif made out of all figures.

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Tables and captions

Angular functions parameterising the $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda $ angular distribution. The numbering scheme is the same as in Ref. [37].

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Signal yields in the long and downstream categories of the 2011, 2012, 2015 and 2016 data sets.

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Values of the 20 moments, $M_{i}$, measured in the data collected at 7, 8 and 13 $\text{ Te V}$ centre-of-mass energies. The long and downstream categories have been combined. The first and second uncertainties are statistical and systematic, respectively.

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Systematic uncertainties on the angular moments. The largest value amongst the moments is given for each source. The total systematic uncertainty varies from 0.002 to 0.010, depending on the moment considered. The sources are described in the text.

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Estimates for the magnitude and phase of the decay amplitudes and the transverse production polarisation of the $\Lambda ^0_ b $ baryons, extracted using the Bayesian analysis. The most probable value (MPV) and the shortest 68% interval containing the most probable value are given.

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Intervals at 95% credibility level on the amplitudes, the polarisation and $\alpha_{b}$ from the Bayesian analysis. The interval on $\alpha_{\Lambda }$, with the external constraint removed, is also provided.

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Supplementary Material [file]

Supplementary material full pdf

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This ZIP file contains supplementary material for the publication LHCb-PAPER-2020-005. The files are: supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps, *.C : The figures in various formats paperData.json : Numerical values for moments and covariance matrices

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Created on 12 April 2024.