Elliptic flow of electrons from heavy-flavour hadron decays at mid-rapidity in Pb-Pb collisions at $\mathbf{\sqrt{s_\mathrm{NN}}}$ = 2.76 TeV

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2.76 TeV
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The elliptic flow of electrons from heavy-flavour hadron decays at mid rapidity ($|y|$ $<$ 0.7) is measured in Pb--Pb collisions at \energy~ = 2.76 TeV with  ALICE at the LHC. The particle azimuthal distribution with respect to the reaction plane can be parametrized with a Fourier expansion, where the second coefficient ($v_{\rm 2}$) represents the elliptic flow. The $v_{\rm 2}$ coefficient of inclusive electrons is extracted in three centrality classes  (0-10\%, 10-20\% and 20-40\%) with the event plane and the scalar product method in the transverse momentum (\pt) interval 0.5 -- 13 GeV/$c$ and 0.5 -- 8 GeV/$c$, respectively. After subtracting the background a positive $v_{\rm 2}$ of heavy-flavour decay electrons is observed in all centrality classes, with a significance of $5.9$$\sigma$ in the interval $2$ \mbox{$<$ $p_{\rm T}$ $<$ $2.5$ GeV/$c$} in semi-central collisions (20-40\%). The mean value of $v_{\rm 2}$ decreases towards more central collisions at low and intermediate $p_{\rm T}$ (0.5 $<$ $p_{\rm T}$ $<$ 3 GeV/$c$). The $v_{\rm 2}$ of heavy-flavour decay electrons at mid rapidity is found to be similar to the one of heavy-flavour decay muons at forward rapidity \mbox{(2.5 $<$ $y$ $<$ 4)}. The results are compared to models describing the interactions of heavy quarks (charm and beauty) and open heavy-flavour hadrons with the high-density medium formed in heavy-ion collisions. The models including substantial interactions of heavy quarks with an expanding medium and/or hadronization via charm quark recombination with light quarks from the medium qualitatively describe the data within uncertainties.