Elliptic Flow of Electrons from Beauty-Hadron Decays in Pb-Pb Collisions at √𝑠_𝑁⁢𝑁 = 5.02  TeV

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, 𝑣₂. The 𝑣₂ 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 𝑣₂ 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.