Abstract
The elliptic flow of electrons from beauty hadron decays at midrapidity ($|y|$ $<$ 0.8) is measured in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV with the ALICE detector at the LHC. The azimuthal distribution of the particles produced in the collisions can be parameterized with a Fourier expansion, in which the second harmonic coefficient represents the elliptic flow, $v_{\rm 2}$. The $v_{\rm 2}$ coefficient of electrons from beauty-hadron decays is measured for the first time in the transverse momentum ($p_{\rm{T}}$) 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 $c\tau \approx$ 500 $\mu$m compared to that of charm hadrons and most of the other background sources. The $v_{\rm 2}$ of electrons from beauty hadron decays at midrapidity is found to be positive with a significance of 3.75$\sigma$. The results provide insights on the degree of thermalization of beauty quarks in the medium. A model assuming full thermalization of beauty quarks is strongly disfavoured by the measurement at high $p_{\rm{T}}$, but is in agreement with the results at low $p_{\rm{T}}$. Transport models including substantial interactions of beauty quarks with an expanding strongly-interacting medium describe the measurement.
Highlights
Pb-Pb 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 main goal of the ALICE experiment [1] is the study of strongly interacting matter at the high energy density and temperature reached in ultrarelativistic heavy-ion collisions at the Large Hadron Collider (LHC)
The momentum anisotropies are characterized by the flow harmonic coefficients vn from the Fourier expansion of the particle azimuthal distribution with respect to the azimuthal
Summary
; ð1Þ where Nin and Nout are the number of beauty-decay electrons in two 90°-wide intervals of Δφ 1⁄4 φ − Ψ2: in plane (−π=4 < Δφ < π=4 and 3π=4 < Δφ < 5π=4) and out of plane (π=4 < Δφ < 3π=4 and 5π=4 < Δφ < 7π=4), respectively. [55], is used to obtain the impact parameter distributions of photon conversions and Dalitz decays. The uncertainty due to the slight remaining difference is assessed by exchanging the Dalitz template for one of the charged hadrons measured in data and scaling the resulting difference by an estimate of the contamination [22] relative to the fraction of electrons from Dalitz decays. This results in an uncertainty of 0.009 on the final v2 in the first pT interval, falling quickly with pT. The unaccounted contribution of these strange decay electrons is negligible within the applied d0 range of 1⁄2−0.1; 0.1 cm in the fitting procedure [52]
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