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Abstract

The Quark-Gluon Plasma~(QGP) is a strongly interacting matter with high temperature and energy density, where partons are deconfined. It is hypothesised being the same state the universe was in just a few microseconds after the Big Bang. Experimentally, the QGP is studied at accelerator experiments using heavy-ions. The presence of a deconfined phase after the ultra-relativistic collisions is expected to influence the system evolution. The search for modifications induced on the particle production is carried out taking elementary particle collisions as reference measurement. \\ The research presented in this thesis focuses on the study of neutral meson and direct photon production in lead ion collisions at $\sqrt{s_{NN}}$~=~2.76~TeV with the ALICE detector at the Large Hadron Collider. The neutral pion and \e mesons are reconstructed via their photon decay channel, exploiting the photon conversions in the detector material. A modification of the meson spectra is observed and investigated further with the comparison to similar experimental results as well as theoretical models. The measurement of neutral mesons is essential for the study of direct photon production, since decay photons are the largest background for this signal. The photon excess signal on top of the decay photon background in the transverse momentum interval 1~$<$~\pT~$<$~1.6~\GeVc is measured with a significance of 1.5~$\sigma$. In this region, direct photons are expected to originate from a phase of the QGP where the system is thermalised. In an attempt to describe the phenomenon behind the observed signal, comparisons to several theoretical predictions have been performed.