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

Abstract

Abstract:The elliptic flow of electrons from heavy-flavour hadron decays at mid-rapidity (|y| < 0.7) is measured in Pb-Pb collisions at √ s NN = 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 2 ) represents the elliptic flow. The v 2 coefficient of inclusive electrons is measured in three centrality classes (0-10%, 10-20% and 20-40%) with the event plane and the scalar product methods in the transverse momentum (p T ) intervals 0.5-13 GeV/c and 0.5-8 GeV/c, respectively. After subtracting the background, mainly from photon conversions and Dalitz decays of neutral mesons, a positive v 2 of electrons from heavy-flavour hadron decays is observed in all centrality classes, with a maximum significance of 5.9σ in the interval 2 < p T < 2.5 GeV/c in semicentral collisions (20-40%). The value of v 2 decreases towards more central collisions at low and intermediate p T (0.5 < p T < 3 GeV/c). The v 2 of electrons from heavy-flavour hadron decays at mid-rapidity is found to be similar to the one of muons from heavy-flavour hadron decays at forward rapidity (2.5 < y < 4). The results are described within uncertainties by model calculations including substantial elastic interactions of heavy quarks with an expanding strongly-interacting medium. The ALICE collaboration 34 Keywords: Heavy Ion Experiments IntroductionThe main goal of the ALICE [1] experiment is the study of strongly-interacting matter at the high energy density and temperature reached in ultra-relativistic heavy-ion collisions at the Large Hadron Collider (LHC). In these collisions the formation of a deconfined state of quarks and gluons, the Quark-Gluon Plasma (QGP), is predicted by Quantum ChromoDynamic (QCD) calculations on the lattice [2][3][4][5][6]. Because of their large masses, heavy quarks, i.e. charm (c) and beauty (b) quarks, are produced at the initial stage of the collision, almost exclusively in hard partonic scattering processes. Therefore, they interact with the medium in all phases of the system evolution, propagating through the hot and dense medium and losing energy via radiative [7,8] and collisional scattering [9][10][11] processes. Heavy-flavour hadrons and their decay products are thus effective probes to study the properties of the medium created in heavy-ion collisions. Heavy-quark energy loss in strongly-interacting matter can be studied via the modification of the transverse momentum (p T ) spectra of heavy-flavour hadrons and their decay products in heavy-ion collisions with respect to the proton-proton yield scaled by the number of binary nucleon-nucleon collisions, quantified by the nuclear modification factor (R AA ). A strong suppression of open charm hadrons and heavy-flavour decay leptons is observed for p T > 3 GeV/c in central collisions, both at RHIC ( √ s NN = 200 GeV) [12][13][14][15][16] -1 - JHEP09(2016)028and LHC ( √ s NN = 2.76 TeV) [17][18][19][20] energies. The PHENIX and STAR Collabor...

Event-by-event correlations between soft hadrons and D0 mesons in 5.02 TeV PbPb collisions at the CERN Large Hadron Collider

“…The observation of large azimuthal anisotropy of open heavy flavor mesons [19][20][21] adds important new elements to the overall picture discussed above. Heavy quarks are produced by hard processes in the initial stages of the collision with a non-thermal transverse momentum spectrum, which is expected to relax towards a nearly thermal distribution within a relaxation time scale τ R ∼ M T η sT [22] (M is the heavy quark mass).…”

Measurements of low-pT electrons from semileptonic heavy-flavour hadron decays at mid-rapidity in pp and Pb-Pb collisions at $$ \sqrt{s_{\mathrm{NN}}}=2.76 $$ TeV

“…The interaction of charm and, to a lesser extent, beauty quarks of low transverse momentum with the medium may lead to the participation of heavy quarks in the collective expansion of the hot and dense system [28,29] and, eventually, to a partial or complete thermalization of heavy quarks in the system [30]. Moreover, while in pp collisions charm and beauty quarks hadronize via fragmentation, in heavy-ion collisions a competing hadronization mechanism through the coalescence with other quarks from the medium could become relevant and modify the phase-space distribution of heavy-flavour hadrons up to transverse momenta of a few GeV/c [31][32][33].…”

“…For the Pb-Pb analysis, the same data sample recorded at √ s NN = 2.76 TeV was used as for previous publications [28,42]. The events were collected with a MB interaction trigger using information from the coincidence of signals between the V0A and V0C detectors.…”

“…Central and semi-central Pb-Pb collisions were selected online by applying different thresholds on the V0 signal amplitudes resulting in central (0-10%) and semi-central (10-50%) trigger classes [50]. Events affected by pile-up from different bunch crossings have been rejected offline [28]. This selection removes up to 5% of the total number of events depending on the centrality of the collisions.…”

“…Candidate electrons tracks are required to fulfil the criteria summarized in table 2, similarly to what was done in refs. [28,47], in order to select good quality tracks. The rapidity range used in the analyses is restricted to |y| < 0.8 to exclude the edges of the detectors, where the systematic uncertainties related to particle identification increase.…”