by Denise Moreira de Godoy. Published: 13 October 2012

Denise Godoy, PhD student at the University of Sao Paolo in Brazil

Heavy-ion collisions at ultrarelativistic energies can achieve sufficiently high temperature and/or energy density to form the quark-gluon plasm (QGP). The QGP is a state of matter predicted by QCD theory in which quarks and gluons are deconfined. This hot and dense deconfined medium has been used to describe physical processes that occurred in the early universe.

One of the most important probes for the QGP formation is the elliptic azimuthal anisotropy, which can be related to the collective motion due to the pressure gradient created in early stages of non-central collisions of heavy ions. The elliptic azimuthal anisotropy is quantified by the second parameter v2 of the particle azimuthal angle distribution with respect to the reaction plane that is defined by the impact parameter direction and the beam direction. In addition, heavy quarks (charm and bottom) serve as a sensitive probe for the QGP properties since they are predominantly produced in initial hard scatterings and interact with the deconfined medium.

ALICE has measured the elliptic azimuthal anisotropy of electrons from heavy-flavour decays as a function of transverse momentum in non-central Pb-Pb collisions at center-of-mass energy √s = 2.76 TeV per colliding nucleon pair. The transverse momentum dependence of heavy-flavour decay electron v2 is sensitive to the properties of the QGP. A nonzero v2 of low transverse momentum electrons from semi-leptonic decays of heavy flavours indicates a collective motion of the heavy quarks in the QGP. Whereas, heavy-flavour decay electron v2 at high transverse momentum is sensitive to the path length dependence of heavy-quark energy loss within the created dense medium.

Inclusive electrons were identified using the Time Projection Chamber (TPC), Time-Of-Flight (TOF) and Electromagnetic calorimeter (EMCal) detectors. The identification using TPC is based on the specific energy loss per path length as a function of momentum. The TOF detector was used to reject hadrons in the ambiguous regions of electron energy loss at low momentum. Electron identification in the EMCal is based on the distribution of the energy to momentum ratio, which is around unity for electrons since they deposit all their energy in the EMCal detector.

Figure 1: Elliptic azimuthal anisotropy of inclusive electrons and estimated non-heavy flavor electron background as a function of transverse momentum.

The event plane method was used to obtain the elliptic azimuthal anisotropy of inclusive electrons. In this method, the reaction plane angle is estimated by the angle of symmetry (event plane angle) and correlated to the azimuthal angle of the inclusive electrons. Since the event plane determination is limited by the multiplicity, the inclusive electron v2 was corrected for the event plane resolution. The event plane angle was measured by the VZERO detector and the event plane resolution was obtained by correlating event plane angles measured by the VZERO and TPC detectors.

The contribution of non-heavy flavour electron v2 was estimated with a simulation using Monte Carlo event generator based on the measured spectrum and v2 of the main electron background sources. Figure 1 shows the measured inclusive electron v2 and the estimated non-heavy flavour electron v2. The subtraction of non-heavy flavour electron v2 from the inclusive electron v2 is weighted by the heavy-flavour decay electron to background ratio.

Figure 2: Elliptic azimuthal anisotropy of electrons from heavy-flavour decays as a function of transverse momentum.

Finally, figure 2 shows the elliptic azimuthal anisotropy of electrons from heavy-flavour decays as a function of transverse momentum. The result was compared to the results from the PHENIX experiment and theoretical predictions. Nonzero v2 of electrons from heavy-flavour decays was observed. At low transverse momentum, the result indicates strong interaction of heavy quarks in the created hot and dense partonic medium.