Experimental study of electrons from heavy flavor hadrons decays in Au+Au collisions at 200, 62.4 and 39 GeV in the STAR experiment at RHIC.

다국어 초록 (Multilingual Abstract)

Zero baryon density Lattice QCD calculations confirm the existence of a deconfined state of partonic matter at very high temperatures and energy densities, such a state, called the Quark Gluon Plasma (QGP), is argued to feature a major phase of the expanding matter created in Heavy Ion Collisions (HIC) at RHIC and the LHC experiments. Many developments on the quantitative description of QGP have been worked out by the different experimental groups, however the picture is far from complete.
Unlike light quarks, heavy quarks are created at the very early stages of the collision, their numbers are "almost" conserved throughout the expansion and their interactions with the medium is amenable to perturbative QCD calculations. Therefore, comparing their kinematics in heavy-ion collisions to theoretical calculations offers a unique opportunity to extract medium properties and constrain the dynamics of the high transverse momentum (high- pT) partons interaction with the medium. So far, experiments studied two observables of heavy quarks. Firstly, the modification of heavy quarks production in presence of the QGP compared to the baseline production in p+p and d+Au collisions. Secondly, to understand the involvement of heavy quarks in the collective motion of the bulk medium expansion, azimuthal anisotropy measurements are invoked.
Our research focuses on simultaneous measurements of azimuthal anisotropy and production of electrons from heavy flavor semi-leptonic decay channels. The strong modification of the production of these electrons in Au +Au collisions compared to p+ p at sNN = 200 GeV surprised the community; early calculation of medium-induced gluon radiation, which describes the energy loss of light quarks, predicted much less suppression for heavy flavor quarks. This led to a variety theoretical proposals for energy loss mechanisms to account for this strong suppression. Understanding which theoretical model(s) provide the most accurate description of experimental measurements has many physical implications on the flavor dependence of energy loss, heavy quarks-medium interaction strength and their degree of thermalization, constraining the decoupling temperature of heavy quarks from the bulk matter, in addition to delineating the importance of the different hadronization mechanisms to the different heavy quarks kinematical domains. At this stage it is evident that experiments at RHIC and the LHC need to provide higher precision differential measurements to better distinguish between the plethora of energy loss models currently available.
To this end, in this work, we present a simultaneous measurement of mid-rapidity heavy flavor electron differential invariant yields and azimuthal anisotropy in Au+Au collisions at sNN = 200 GeV. The novelty of the new invariant yield measurement lies in the highly improved statistical precision it provides. This new measurement allows us to recalculate the heavy flavor electron nuclear modification factor using published STAR p+p results as a baseline. We also extend heavy quarks production studies to lower center-of-mass collision energies. In this quest, we seek to see if the energy loss of heavy quarks is lessened or turned off at lower energies or constrain the onset collision energy of heavy quarks suppression and flow. Therefore, we also present measurements of mid-rapidity differential invariant yield at sNN = 62.4 and azimuthal anisotropy at sNN = 39 and 62.4 GeV.