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Abstract

The goal of the ALICE experiment at CERN is to study strongly interacting matter under extreme conditions, which are generated in ultra-relativistic heavy-ion collisions at the LHC. It has been established that a medium with unconfined quarks and gluons, the quark-gluon plasma (QGP), exists shortly after these collisions. Detecting direct photons produced in heavy-ion collisions can help characterize the properties and dynamics of this medium. We present the first direct photon measurement using the highest available heavy-ion collision energy per nucleon-nucleon pair of 5.02 TeV. Photons are reconstructed from electron and positron track pairs, which emerge from the occurrence of pair production of photons in the detector material; this is the photon conversion method (PCM). Within the work of this thesis, new methods are developed and implemented in the analysis procedure. As a result, direct photon spectra and Rγ are presented in 0-20, 20-40, 40-60, and 60-80% classes of collision centrality. The neutral mesons π0 and η constitute the largest background source for direct photons, with their decays to two photons, which is one motivation for measuring their spectra as precisely as possible. The second goal is to study the energy loss of partons in the QGP by observing the suppression of high transverse momentum particle production in heavy-ion collisions with respect to pp collisions. Neutral pion spectra and nuclear modification factors RAA are presented in 0-10, 20-40, and 60-80% centrality classes, η meson spectra, RAA, as well as η/π0 ratios in 0-20, 20-40, and 60-80%.