Studying Hyperon Polarization at the Large Hadron Collider Beauty Experiment

Abstract

$Lambda$ hyperons are baryons consisting of up, down, and strange (uds) valence quarks. Spontaneous transverse polarization of $Lambda$ hyperons produced in unpolarized hadronic collisions was first observed over four decades ago and contradicted expectations from early leading-order perturbative calculations in quantum chromodynamics (QCD). Recent studies have linked the polarization to spin-momentum correlations in the process of hadronization. In theoretical calculations, hyperon polarization from unpolarized proton-proton and proton-nucleus collisions comes from including spin-momentum correlations either as higher-order twist-3 collinear multi-parton correlation matrix elements or as a convolution of a twist-2 transverse-momentum-dependent parton distribution function with a transverse-momentum-dependent fragmentation function. Complementary measurements of hyperon polarization from unpolarized hadronic collisions, electron-positron annihilation, and semi-inclusive deep-inelastic lepton-nucleon scattering measurements can advance our understanding of this effect in QCD. The Large Hadron Collider beauty (LHCb) experiment at the Large Hadron Collider studies proton-proton, proton-nucleus, and nucleus-nucleus collisions at TeV energy scales. It is unique among hadronic collider experiments in its forward detector geometry, providing complementary kinematic coverage. The $Lambda$ and $bar{Lambda}$ polarization transverse to the production plane is measured in proton-lead collisions at a center-of-mass energy of $sqrt{s_{NN}} = 5.02$ TeV, using minimum bias data collected by the LHCb detector in the year 2013. The measured transverse polarization averaged over transverse momentum, $p_T$, from 0 to 6 GeV/c and Feynman-$x$, $x_F$, from 0.005 to 0.04 is $0.005pm0.005 text{(stat)}pm0.007text{(sys)}$ for $Lambda$ and $0.007pm0.005text{(stat)}pm0.006$text{(sys)} for $bar{Lambda}$. The measured transverse polarization averaged over $p_T$ from 0 to 6 GeV/c and $x_F$ from -0.10 to -0.01 is $0.004pm0.004 text{(stat)}pm0.010text{(sys)}$ for $Lambda$ and $-0.005pm0.005text{(stat)}pm0.006$text{(sys)} for $bar{Lambda}$. The polarization is additionally measured as a function of $x_F$ and $p_T$ and is consistent with zero. The results are compared to prior measurements, including a concurrent measurement using LHCb fixed-target proton-neon data. The LHCb results are compatible with the trends of previous measurements, adding additional data at different energies and collision configurations in a kinematic region that was previously poorly explored.