The measurement of direct photons is a unique tool for the study of early phases of ultra- relativistic nucleus-nucleus collisions. Since photons do not interact with the strong-coupling medium created in these collisions, they carry undistorted information about the system at their production time. During the hydrodynamic expansion of the fireball, pressure gradients turn inhomogeneities in the initial energy density distribution into azimuthal anisotropies in the produced particle spectra. Recent hydrodynamic calculations predict a substantial portion of direct photons from early phases of the collision, where the anisotropic flow has not fully devel- oped. Thus, the direct-photon azimuthal anisotropy is generally expected to be small compared to the anisotropy of hadrons. However, measurements by the PHENIX experiment at RHIC revealed a direct-photon anisotropic flow with a magnitude similar to that for pions. This thesis presents the first measurement of the direct-photon anisotropic flow in Pb-Pb collisions at a center-of-mass energy of 2.76TeV per nucleon at the LHC. In particular, its dependence on the collision centrality and the triangular component were measured for the first time. Photons were measured by their conversion in the ALICE detector material. Background contributions of photons from hadron decays were determined in a cocktail simulation and subtracted. The results provide evidence for a hadron-like direct-photon anisotropic flow and are thus qualitatively consistent with the observations at RHIC. These findings challenge our present theoretical understanding of the time evolution of heavy-ion collisions and might indicate a significantly enhanced direct-photon emission from late stages of the system evolution.
|Supervisor:||Reygers, PD Dr. Klaus|
|Date of thesis defense:||29 October 2013|
|Date Deposited:||04 Nov 2013 12:36|
|Faculties / Institutes:||The Faculty of Physics and Astronomy > Institute of Physics|