CERN Accelerating science

The main goal of ultra-relativistic heavy-ion collisions is to create and characterize quark-gluon plasma (QGP). It is a deconfined state of quarks and gluons that can be realized at high density and temperature. The existence of QGP was predicted by quantum chromodynamics. Such a state of matter is expected to exist microseconds after the Big Bang. Thus, studying its properties and evolution could give a better understanding of the existence of matter in the present universe. In the laboratory, QGP is expected to be formed by the collisions of heavy ions using particle colliders. Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) are the dedicated state-of-the-art experimental facilities to this end and are focussed on understanding the properties of QGP. In central heavy-ion collisions, a large number of particles are produced through the multiple interactions of participants in the overlap region. Although most of the observed particles produced in such collisions are the results of the fragmentations of pieces of the colliding nuclei, a considerable amount of particle creation occurs by high incident energies ($\approx$ TeV). While the particles created are mostly pions, the production of relatively heavy particles than pions and heavy flavor (strange and charm) quark matter also takes place. However, in pp collisions, it was expected that the final state particles are only the result of the fragmentations of pieces of the two protons. And hence, historically, the proton on proton (pp) collisions were considered a baseline for forming QGP in heavy-ion collisions due to their significantly smaller size compared to the studying later. Recent observation of heavy-ion-like features in small systems like pp collisions from the experiment at the LHC has generated considerable interest in the scientific community. For example, the discovery of collective-like phenomena and strangeness enhancement are a few among them. These developments have significant consequences on the results obtained from heavy-ion collisions, as pp collisions have been used as a benchmark for heavy-ion collisions to understand a possible medium formation. These open up entirely new directions for theoretical and experimental studies of characterizing QGP-like properties and understanding the origin of such observations in small collision systems. Although hydrodynamics calculations describe data qualitatively, other approaches suggest that these can be initial state effects. To understand the recent measurements in high-multiplicity pp collisions, it is vital to perform multi-differential studies with event shape observables and charged-particle multiplicity. One of the event-shape techniques, called transverse spherocity (S$_{0}$) can disentangle events dominated by soft/hard-QCD processes based on their geometrical structure. Soft-QCD and hard-QCD processes could govern particle production in small system collisions like pp. While the study of bulk properties of the system would give an understanding of the underlying mechanism of the soft-QCD process, the study of jets could reveal the physics of hard-QCD processes. The detailed study in the light of transverse spherocity and multiplicity could provide deeper insight into understanding the underlying production dynamics of a particle in high multiplicity pp collisions; moreover, it could also help in tuning various models. Recent results reported by ALICE have observed enhanced production of strange and multi-strange particles in high-multiplicity proton+proton (pp collisions and observation of evidence of collectivity in pp collisions by CMS, which was traditionally considered as one of the signatures of QGP formation. These observations compel one to ask whether high-multiplicity pp collisions create QGP-droplets$?$ Extensive investigation using resonance particles containing strange quarks could provide hints towards the possible formation of QGP-like medium in pp collisions (specifically high-multiplicity events). Resonances are commonly known as hadrons, which are more massive than their ground state particle and have different excited quantum states but identical quark content. These particles usually have a short lifetime ($\tau$) as they decay strongly, and it is in the order of a few fm/$\rm{c}$, a typical proton diameter. Because of its short lifetime, reconstructed hadronic resonances through their decay products in a detector can be used to study the hadronic medium between the chemical and the kinetic freeze-out. Experimentally measured typical lifetime of hadronic resonances ranges for 1.3 fm/$\it{c}$ to 46.3 fm/$\it{c}$. As hadronic resonances of varying masses (770 - 1019 MeV/$\rm{c}^{2}$ ), hadron class (meson and baryon), strangeness (0 - 2), and lifetimes are available, they can be used to study the properties of the hadronic phase and its different stages of evolution. A comprehensive study of hadronic resonances plays a vital role in understanding ultra-relativistic heavy-ion collisions. In such collisions, expansion of the produced fireball can be probed by the hadronic resonances, as their lifetime is comparable to the lifetime of the fireball ($\tau \approx$ 10 fm/$\it{c}$ at LHC energies) created in the heavy-ion collisions. This helps to understand in-medium phenomena like rescattering (interaction of decay daughters with other in-medium particles, results in suppression of resonances when reconstructed, as the invariant mass of the daughter particles mismatches with the parent particle) and regeneration (enhancement of resonances because of pseudo-elastic collisions in the hadronic phase). Resonance particle-like $\phi$(1020) having $\tau \approx$ 46.3 fm/$\it{c}$ might not go through the above mentioned processes. However, resonance particle like $\rm{K}^{*}(892)^{\pm}$ meson has a lifetime ($\tau \approx$ 3.6 fm/$\it{c}$) which is comparable to the hadronic phase lifetime. This allows one to explore the hadronic phase. The sensitivity of hadronic resonances to rescattering and regeneration processes in the hadronic phases is depicted in Figure~\ref{fig_hadronic_phase}. The major objectives of this thesis are to understand the interplay of various processes in the hadronic phase with event shape and high-multiplicity dependence study of $\rm{K}^{*\pm}$ meson production using ALICE detectors at the LHC (CERN). To have a complete birds-eye view of the dynamics of particle production in pp collisions, we have also studied the event topology dependence of heavy-flavored hadrons using a pQCD-inspired model. This work revealed the importance of events topology in the production of charmed-flavored and strange-flavored hadrons. Also, we phenomenologically attempt to explore the possibility of a thermalized medium formation in pp collisions through geometric, statistical, and Monte-Carlo approach. In the case of the geometric approach, we have tried to explain the experimental results by taking the proton structure as consisting of three valence quarks connected by gluons. We describe the densities of quarks and gluons as a Gaussian type assuming a spherically symmetric distribution of quark densities from their respective centers and cylindrically symmetric gluon densities about the line joining two adjacent quarks. With this consideration, we could explain charged-particle multiplicity distribution and elliptic flow obtained in $pp$ collisions at $\sqrt{s}$ = 7 and 13 TeV, respectively, using a Glauber approach. In another work, considering final state multiplicity as a proxy of the number of constituents particles involved in the collisions, we have studied the thermodynamical quantities like heat capacity, trace anomaly, speed of sound, etc., using experimental inputs from ALICE and contrasted these results with those obtained from PYTHIA8 (devoid of thermalization). This work hints at the possible onset of thermalization in a small system like pp after a certain threshold in the final state charged multiplicity. We further extended these studies to include the event topology, linking to the thesis's analysis part and motivating future potential measurements.