Elliptic Flow of Electrons from Beauty-Hadron Decays in Pb-Pb Collisions at sNN =5.02 TeV

Abstract

The elliptic flow of electrons from beauty hadron decays at midrapidity ( | y | < 0.8 ) is measured in Pb-Pb collisions at √ s N N = 5.02 TeV with the ALICE detector at the LHC. The azimuthal distribution of the particles produced in the collisions can be parametrized with a Fourier expansion, in which the second harmonic coefficient represents the elliptic flow, v 2 . The v 2 coefficient of electrons from beauty hadron decays is measured for the first time in the transverse momentum ( p T ) range 1.3 – 6 GeV / c in the centrality class 30%–50%. The measurement of electrons from beauty-hadron decays exploits their larger mean proper decay length c τ ≈ 500 μ m compared to that of charm hadrons and most of the other background sources. The v 2 of electrons from beauty hadron decays at midrapidity is found to be positive with a significance of 3.75 σ . The results provide insights into the degree of thermalization of beauty quarks in the medium. A model assuming full thermalization of beauty quarks is strongly disfavored by the measurement at high p T , but is in agreement with the results at low p T . Transport models including substantial interactions of beauty quarks with an expanding strongly interacting medium describe the measurement within uncertainties.