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

Abstract

This paper presents an analysis of the ${\Lambda }_b^0\to J/\psi \Lambda$ angular distribution and the transverse production polarisation of ${\Lambda }_b^0$ 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 ${\Lambda }_b^0$ transverse momentum and pseudorapidity of $1<p_T<20$ GeV$/c$ and $2<\eta <5$, respectively. The data are consistent with ${\Lambda }_b^0$ baryons being produced unpolarised in this region. The parity-violating asymmetry parameter of the $\Lambda \to p{\pi }^{-}$ decay is also determined from the data and its value is found to be consistent with a recent measurement by the BES\,III collaboration.

All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2020-005.html (LHCb public pages)

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• Estimates of exchange topological contributions and $CP$-violating observables in ${\Lambda }_b\to \Lambda \varphi$ decay
• Time-reversal asymmetries in ${\Lambda }_b$ semileptonic decays
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