Xi Production in pp Collisions at Different Multiplicity and Spherocity

Abstract: The protons and neutrons that make up the atoms in our Universe are part of a larger group of particles called hadrons. Hadrons have an inner structure consisting of the elementary particles called quarks. The interaction of the quarks is described by the quantum field theory quantum chromodynamics. Quantum chromodynamics predicts a confinement of the quarks within the hadrons, but at large temperatures and/or energy densities there will be a phase transition from hadrons into a deconfined state called quark gluon plasma. The study of these two regimes in quantum chromodynamics have, to a large extent, been separate. The deconfined QGP regime has mainly been considered in large systems (heavy-ion collisions). The formation of a possible medium or significant final state interactions have not been considered in small systems. The two different branches of research are now starting to overlap after similar quark gluon plasma-like signatures have been observed in both small and large systems. In this thesis, signatures of the quark gluon plasma has been studied in small systems. The goal was to show that the event estimator transverse spherocity could discriminate between the confined and deconfined regime of quantum chromodynamics. Transverse spherocity looks at the produced particle distribution to quantify the topology of the event. Events were most of the momentum is distributed along an axis (called jetty) indicates that there has been a hard parton-parton (a parton is either a quark or a gluon) interaction, and so it is presumably less likely that a quark gluon plasma was formed. If the momentum is instead isotropically distributed it would suggest an event with several soft parton interactions, where it seems more likely that a medium could be formed. This thesis uses the $\Xi^-$ hadron to control the hypothesis that transverse spherocity can select events where the quark gluon plasma-like effects are more or less pronounced. $\Xi^-$ was chosen since it is sensitive to a signature of the quark gluon plasma called strangeness enhancement. Strangeness enhancement is multiplicity dependent, and the data shows a larger production of $\Xi^{\mp}$ compared to charged particles in high multiplicity events with respect to minimum bias events. The data is compared to the monte carlo generator PYTHIA, which does not include any mechanism for strangeness enhancement, and the generated data does not show any strangeness enhancement. Comparison between the two transverse spherocity selections shows a larger $\Xi^{\mp}$ production compared to non-strange hadrons in isotropic events than in jetty events. However, there is also an observed bias of the spherocity selection which leads to a separation of isotropic and jetty events. Because of this it is not possible to conclude that transverse spherocity is able to discriminate between the quark gluon plasma regime and the confined regime.