CERN Accelerating science

Particle colliders are an important tool to enhance our understanding of fundamental physics. The most powerful particle accelerator is the Large Hadron Collider (LHC), located at the European Organization for Nuclear Research (CERN) in Geneva. A major achievement by the scientists working at CERN was the discovery of the Higgs Boson in July 2012. While the LHC is able to collide two protons pp, two heavy-ions (A-A) or a combination of those (p-A), this thesis is focussing on pp collisions at a center-of-mass energy of $\sqrt{s}=13\,$TeV, measured with A Large Ion Collider Experiment (ALICE). ALICE is dedicated to the exploration of the Quark-Gluon Plasma (QGP), a state of matter in which quarks can be seen as quasi-free particles. This state was present until $10$µs after the Big Bang and is expected to be created at heavy-ion collisions. In this thesis, the measurement of the cross section of $\pi^{0}$, $\eta$ and $\omega$ meson production is presented. The measurement of the cross section of $\pi^{0}$ and $\eta$ mesons is making use of their decay into two photons ($\gamma \gamma$). The $\omega$ meson cross section measurement is using its decay into three pions ($\pi^{+}\pi^{-}\pi^{0}$). While $\pi^{+}$ and $\pi^{-}$ can be measured directly, using the tracking detectors of ALICE, $\pi^{0}$ and $\eta$ mesons have to be reconstructed from their decay products. On the one hand, the two photons of the $\pi^{0}$ and $\eta$ decay can directly be measured with one of the calorimeters available in ALICE: EMCal, DCal or PHOS. On the other hand, photons can convert into $e^{+}e^{-}$-pairs within the inner detector material. The resulting $e^{+}$ and $e^{-}$ can then be tracked analogous to the $\pi^{+}$ and $\pi^{-}$. The measurement method that utilizes this photon conversion is called Photon Conversion Method (PCM). Furthermore, it is possible to measure one decay photon with the help of a calorimeter and the second decay photon utilizing PCM. While the dedicated measurements of the cross section of $\pi^{0}$ and $\eta$ meson production in this thesis are making use of the PHOS detector and its combination with PCM, the measurement of the $\pi^{0}$ mesons used for the $\omega$ reconstruction is utilizing all methods mentioned. Measurements of the cross sections of neutral meson productions in pp collisions are valuable probes for various properties of the QGP, through their comparison to heavy-ion collisions, and particle production in general. The measurement of the cross section of neutral mesons is needed to test the understanding of Quantum Chromodynamics (QCD) and its implementation in QCD-based event generators like PYTHIA. The fragmentation function and parton distribution function is constrained by parametrizing measured data at different collision energies. Furthermore, the low-momentum regime is described by phenomenological models, which can be verified by meson measurements. In addition to that, photons are produced during different stages of the collision and do not interact strongly. Hence, they are ideal particles to probe the QGP. However, to acquire the fraction of direct photons from all generated photons, precise measurements of neutral meson spectra are needed. The three largest contributions of decay photons to the total amount of decay photons are given by the $\pi^{0}$, $\eta$ and $\omega$ mesons. As the QGP is expected to suppress hadron production in heavy-ion collisions compared to pp collisions, particle productions in pp collisions can furthermore be used to probe the QGP. In addition to that, experimental data of vector meson production, like the $\omega$ meson, at LHC energies is scarce. The measurement of the $\omega$ meson cross section in this thesis is performed over an unprecedented momentum range and provides important precision data for future theoretical calculations in vector meson fragmentation. This thesis will explain the basic theoretical concepts for particle productions and their measurement in chapter 3. These explanations are followed up by a description of the experimental setup of the ALICE detector in chapter 4. After a quick overview of the Service Task, that has been performed during this work, in chapter 5, the used data sets and the choice of selected events are explained in chapter 6. Finally, the measurements of charged pions and photons are described in chapter 7 and 8, while the measurements of $\pi^{0}$ and $\eta$ meson production cross sections are described in chapter 9. The $\omega$ meson production cross section measurement is described in chapter 10.