The elliptic flow of electrons from beauty hadron decays at midrapidity ($|y|<0.8$) is measured in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ 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 $1.3–6\mathrm{GeV}/c$ in the centrality class 30%–50%. The measurement of electrons from beauty-hadron decays exploits their larger mean proper decay length $c\tau \approx 500\mu \mathrm{m}$ 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 $3.75\sigma$. 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.

• Received 23 June 2020
• Revised 27 January 2021
• Accepted 9 March 2021

DOI:https://doi.org/10.1103/PhysRevLett.126.162001

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