High-energy heavy-ion collisions allow the Quark-Gluon Plasma (QGP) production and study, a state of high-density QCD matter in which quarks and gluons are no longer confined inside hadrons for a very short time. Different observables reveal an enhanced production of strange hadrons, signatures of collective effects and opacity to jets due to the QGP formation. However, collective effects and strangeness enhancement are also observed in pp and p-Pb collisions (small systems) with high charged-particle multiplicity. These observations in small systems are very puzzling since the QGP is not expected to be formed since the small systems are considered too dilute and too short-lived.
In this thesis, I present a study on the production of π, K and p in pp collisions at √s = 13 TeV using data recorded by the ALICE experiment. To investigate the origins of collective effects and strangeness enhancement, I measure the particle production as a function of the Relative Transverse Activity (RT) and the Transverse Spherocity (S0). In the RT analysis, particle production is measured in different topological regions (toward, away, and transverse) with respect to the leading charged track per event. The toward and away regions mainly contain the fragmentation products of the jet, while the Underlying Event dominates the transverse. The transverse momentum spectra and K/π and p/π ratios are measured as a function of RT = NT/⟨NT⟩, where NT is the charged-particle multiplicity in the transverse region. It is observed that the K/π and p/π ratios increase with increasing RT in the toward region, while little RT dependence is observed in the transverse. Models tuned to e+e- results describe the low-RT particle ratios qualitatively.
The transverse spherocity is an event shape observable that can disentangle jet-like from isotropic topologies. I studied particle production in high-multiplicity events as a function of spherocity. I contrast the results obtained using a forward and a mid-pseudorapidity multiplicity estimator. The results show that the mid-pseudorapidity estimator combined with the spherocity selection allows selecting events with more significant variations in the average transverse momentum (⟨pT⟩) than the forward estimator. I present the pT spectra and particle ratios as a function of S0. Finally, all the data results are contrasted with microscopic models.
- Christiansen, Peter, Supervisor
- Ohlson, Alice, Assistant supervisor
- Silvermyr, David, Assistant supervisor
|Award date||2022 Jun 10|
|ISBN (electronic) ||978-91-8039-282-2|
|Publication status||Published - 2022|
Place: Rydbergsalen, Fysiska Institutionen, Professorsgatan 1, Lund.
Name: Calderon de la Barca Sanchez, Manuel
Affiliation: Department of Physics and Astronomy, UC Davis, USA
- Quark-Gluon Plasma
- Small Systems
- pp Collisions
- Underlying Event
- Transverse Spherocity
- Relative Transverse Activity