Event shape engineering and study of high-multiplicity proton+proton collisions AT √s=13 TeV using Pythia 8.2

Abstract

Earlier pp-collision was considered as the baseline in measurements for heavy-ion collisions. But in the small systems collectivity was observed at LHC energies added to the strangeness enhancement, ridge structure, hardening of pT -spectra in high-multiplicity events. This suggests a possibility of medium formation in small systems like pp-collision. Color reconnection and Multi- Partonic Interactions can produce medium-like effects and are responsible for collectivity in small systems. It is interesting to look into the QGP-like features, usually found in heavy ion collisions in high-multiplicity proton+proton collisions. Event shape observables are the tools that can be used to study the geometric shapes of energy distribution in any collision experiment. The motivation of this project is to further investigate whether the collective behaviour observed in high energy pp-collisions can be imputed to the formation of Quark-Gluon Plasma (QGP). This is done by using transverse spherocity for the double differential study of several observables with reference to charged-particle multiplicity in high-multiplicity pp-collisions at ps= 13 TeV. The different multiplicity ppevents were divided into different transverse spherocity bins in order to probe whether the particle production is dominated by events that are geometrically jet-like or isotropic. This analysis is carried out using PYTHIA 8.2. The final state charged-particles have been chosen in the acceptance of V0 detector in ALICE at the LHC with pseudorapidity coverage of V0A (2.8 < < 5.1) and V0C (−3.7 < < −1.7) and these events are divided into ten multiplicity (V0M) classes. To disentangle the jetty (hard) and isotropic (soft) events from the average-shaped events we have applied the spherocity cuts on generated events. The spherocity distribution is selected in the pseudorapidity range of | |< 0.8. The transverse momentum (pT) spectra for identified particles like pion, kaon, proton and lambda are found in mid-rapidity, | |< 0.5 as a function of spherocity and multiplicity. The ratio of jetty and isotropic to spherocity integrated pT-spectra was found to be multiplicity dependent and the crossing point shifts towards higher values of pT as we move from low- to highmultiplicity. We observe a strong dependence on spherocity and multiplicity for all particles. Further we are interested in the thermodynamics of the system and to obtain the kinetic freeze-out temperature we use thermodynamically consistent Tsallis distribution function and Boltzmann-Gibbs Blastwave function for hadrons in pp-collisions. We analyse the transverse momentum (pT) spectra using these models and study the multiplicity dependence of temperature (Tkin), q (non-extensive parameter) from Tsallis and Tkin, (radial flow) from BGBW model. We observe dependence of extracted parameters on spherocity and charged-particle multiplicity for all particles. With increasing multiplicity, we found that kinetic freeze-out temperature also increases. For heavier particles, this increase was more steeper than lighter particles. In addition to this, we also study the dependence of these parameters on particle mass to observe any mass ordering. It was interesting to see that Tkin and were found to be dependent on mass for highest multiplicity, which is an indication of differential freeze-out scenario. The thesis is organized as follows: after a brief introduction and motivation to study pp-collisions at the LHC energies in viewof QGP-like behaviour in highmultiplicity pp-collisions, small discussion on the QCD, QGP, basic kinematics are discussed in chapter 1. In chapter 2, we discuss about the event shape observable like spherocity and its correlation with multiplicity. Multi-partonic interaction and their role in pp-collisions is also discussed in this chapter with its correlation to multiplicity. In chapter 3 we present the analysis methodology, PYTHIA event generator and its underlying physics, application of Tsallis nonextensive statistics andBGBWin pp-collisions. At the end, chapter 5 summarises our results with important findings.