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PDF, English - main document Download (16MB) | Lizenz: Measurement of Direct Photons and Neutral Mesons in Small Collisions Systems with the ALICE Experiment at the LHC by Bock, Friederike underlies the terms of Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 Germany

Abstract

The Quark-Gluon Plasma (QGP), a strongly interacting state of matter in which quarks and gluons are deconfined, is hypothesized to have existed merely a few micro-seconds after the Big Bang. This state can be studied experimentally in heavy ion collisions at the current high-energy particle accelerators, like the Large Hadron Collider (LHC). However, recently the size of the collision system, which is required to create a QGP has been been put into question and many measurements suggests that this might even be possible in high multiplicity pp or p–Pb collisions. The research study of direct carried out in this thesis focuses on the photons and neutral mesons in pp collisions at √s = 2.76 TeV and p–Pb collisions at √sNN = 5.02 TeV with the ALICE detector at the LHC. The neutral pion and η meson have been reconstructed in their two photon decay channels. For that up to four different reconstruction methods have been pursued within this thesis, out of which two are entirely new within ALICE, exploiting the full photon detection capabilities of ALICE. Like this the neutral pion spectra could be measured up to transverse momenta of 40 GeV/c in pp collision and a new level of precision could be reached for identified particle measurements in p–Pb collisions at √sNN = 5.02 TeV. For pp collisions new high precision inputs for the determination of the fragmentation functions for neutral pions and η mesons are provided and the current models are compared to the obtained results. For p–Pb collisions a small suppression can be observed beyond 4 GeV/c with respect to an interpolated pp reference at the same center-of-mass energy. It is investigated further through the comparison with various models and results from other identified particles, in order to test whether another ansatz, taking into account only cold nuclear matter effects, would yield a similar suppression. For the direct photons three partially independent reconstruction techniques are presented, two of which are entirely new in ALICE, as well as, their combined results for in minimum bias pp and p–Pb collisions. The combination of the results yields a significant direct photon excess in p–Pb collisions beyond 6 GeV/c, which is in agreement with the predicted excess from prompt photons in this collision system. Below these momenta and for pp collision at √s = 2.76 TeV upper limits on the possible direct photon production can be provided. In this region a possible thermal photon signal of 2 − 3% cannot be excluded within the current uncertainties.