Neutral Mesons & Direct Photon Flow

Abstract

Ultra-relativistic collisions of protons and heavy-ions reveal the structure of matter at the smallest scales and help us to complete the picture of the universe. Transverse momentum dependent particle production spectra are expected to change for pp, p-A and A-A collisions because of the change in energy densities and temperature of the created system. The modification with respect to pp collisions is quantified by the nuclear modification factor that is able to give insight into low and high p T particle production mechanisms. In A-A collisions at LHC energies it is expected that a Quark Gluon Plasma is formed. This strongly interacting state of matter can be studied by measuring the elliptic flow of direct photons. Direct photons, i.e. all photons excluding the ones from hadronic decays, are produced during every stage of the heavy-ion collision. Direct photon flow is measured by subtracting the contribution of decay photon flow from the measured inclusive photon flow via the double ratio R, which defines the excess of direct photons over decay photons. This thesis presents the results of two analyses using the data recorded with the ALICE detector at the LHC. The neutral meson measurement is done using the Electro-Magnetic Calorimeter of the ALICE detector in p-Pb collisions at sqrt(s) = 5.023 TeV. The invariant cross sections of the neutral pion and η is extracted using the invariant mass method. In addition, the η to neutral pion ratio and the neutral pion R p−Pb is presented where the latter is compared to state of the art theoretical predictions. The direct photon flow measurement is performed using the photon conversion method in ALICE in Pb-Pb collisions at sqrt(s) = 2.76 TeV. The purity of the inclusive photon sample has been studied to identify the conversion photon background sources. In addition, a method has been developed to correct the inclusive photon flow for background flow. The direct photon flow is measured for collision centralities 0-20%, 20-40% and 40-80%. The results show a slight tension with being consistent with zero.