Study of strangeness production in pp collisions at √ s = 5.02 TeV at the LHC and the upgrade of the ALICE muon trigger for high luminosity

Delsanto, Silvia
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The ultimate aim of the ALICE (A Large Ion Collider Experiment) detec tor is to study relativistic heavy-ion collisions (A-A) provided by the CERN Large Hadron Collider (LHC). In these collisions, extraordinary conditions of temperature and energy density occur. Under these extreme conditions, the standard nuclear matter undergoes a transition from hadronic phase to an interacting quantum state called Quark-Gluon Plasma (QGP), where quarks and gluons are not confined anymore into hadrons but they are asymptot ically free. The LHC also provides proton-proton (pp) and proton-nucleus (p-A) collisions considered reference colliding systems for A-A collisions in ALICE because the formation of the QGP is not expected. Since the short-lived QGP cannot be observed directly, signatures/ ob servables affected by its formation are measured and analyzed in order to provide information on its characteristics. The so-called strangeness enhance ment was one of the first QGP signatures proposed. Strangeness enhance ment predicts an enhanced production of strange hadrons in A-A collisions with respect to pp collisions. However, recent experimental observations showed also an increase in strangeness production in high-multiplicity pp and p-Pb collisions at LHC energies. These results, together with other typical features of the heavy-ion collisions normally associated to the QGP, were recently observed also in the reference colliding systems. These unex pected developments have prompted unresolved questions about the nature of high-multiplicity pp and p-A collisions. The goal of this PhD thesis is to present the ALICE measurement per formed for the analysis of strange and multi-strange hadrons production as a function of charged-particle multiplicity in pp collisions at √ s = 5.02 TeV. The results obtained are compared to other ALICE measurements obtained in pp collisions at √ s = 7 and √ s = 13 TeV as well as to those obtained in p Pb collisions at √ sNN = 5.02 TeV. The √ s = 5.02 TeV pp results shown here are found to be in agreement within uncertainties either with √ s = 7 and √ s = 13 TeV pp analyses both with √ sNN = 5.02 TeV p-Pb analysis. This kind of studies is crucial in order to understand details of the strangeness production mechanism and the evolution of pp and p-A colliding systems. In addition, as part of the PhD service task and in the context of the ALICE upgrade for high luminosity, a new set of Resistive Plate Chambers (RPCs) of the ALICE muon trigger system was tested. The RPCs installed in ALICE in 2007 might present some ageing effects during the next operation period foreseen in 2021 on wards. Therefore, in order to guarantee the best performance of the muon trigger, some of the RPCs need to be replaced before the next ALICE activity Run. The new set available for tests showed ii major structural problems and/or low efficiency levels compared to the first RPCs group. These issues could be identified during the tests on the new RPCs. Hence, these defects were communicated to the company responsible for their production. A new set of RCPs was produced and it will be tested in the upcoming months
A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the Faculty of Science, School of Physics, University of the Witwatersrand, Johannesburg, 2021