Multiplicity-dependent Charmonia Production with ALICE and Collective Phenomena in O-O Collisions at LHC Energies

The study of ultra-relativistic heavy-ion collisions has emerged as a powerful approach to explore the fundamental properties of nuclear matter under extreme conditions of temperature and energy density. One of the primary goals of such studies is to create and understand the characteristics of a deconfined state of matter known as the quark-gluon plasma (QGP), where quarks and gluons are no longer confined within hadrons. This state is believed to have existed in the early Universe, microseconds after the Big Bang. The ALICE experiment at the CERN Large Hadron Collider (LHC) provides a unique opportunity to investigate this QCD matter through collisions of protons and heavy ions at unprecedented energies.

This thesis is focused on probing the QGP through two major studies: (i) experimental analysis on  a multiplicity dependent charmonia production with ALICE and (ii) a phenomenological investigation of collective dynamics in oxygen-oxygen (O-O) collisions at LHC energies using a multi-phase transport model (AMPT). The first part addresses the emergence of heavy-ion-like signatures in small collision systems, while the second part systematically characterizes light-flavor particle production, flow coefficients, and nuclear modification effects in O-O collisions.

Traditionally, pp collisions have been regarded as a reference or baseline for heavy-ion studies due to their expected lack of QGP formation. However, recent results at the LHC have challenged this view, showing that high-multiplicity pp events exhibit phenomena similar to those seen in heavy-ion collisions, such as strangeness enhancement and long-range correlations. This raises the intriguing possibility of medium effects even in small systems. Among the most sensitive probes of QGP are heavy quarkonia bound states of heavy quark-antiquark pairs, particularly charmonium states such as \(J/\psi\) and its excited state \(\Psi(2S)\).

In this thesis, we investigate the multiplicity dependence study of $J/\psi$  and $\psi(2S)$-over-$J/\psi$ ratio in pp collisions at \(\sqrt{s} = 13.6\) TeV using the ALICE detector. The analysis involves careful event and track selection, muon identification, background suppression, and signal extraction from invariant mass distributions. Monte Carlo simulations are used to correct for detector efficiencies and acceptance effects. Multiplicity is estimated using tracks from the central barrel detectors. 

To complement the experimental analysis, we conduct a detailed phenomenological study of collective behavior in O-O collisions at \(\sqrt{s_{\mathrm{NN}}} = 7\) TeV, focusing on light-flavor observables. Small collision systems like O-O serve as an intermediate bridge between pp and Pb-Pb collisions and are ideal for disentangling initial-state geometry from final-state interactions. For this purpose, we employ the AMPT model, which incorporates fluctuating initial conditions, partonic scattering, hadronization via quark coalescence, and subsequent hadronic rescattering.

Three distinct nuclear density profiles are considered for the initial geometry: Woods-Saxon, harmonic oscillator, and \(\alpha\)-clustered configurations. These profiles influence key observables such as eccentricity, triangularity, and impact parameter distributions. The analysis covers global observables like charged-particle multiplicity, transverse energy, Bjorken energy density, pseudorapidity distributions, and speed of sound. The kinetic freeze-out temperature and radial flow velocity are extracted using Boltzmann-Gibbs blast-wave fits to the \(p_T\) spectra of \(\pi\), \(K\), and \(p\).

The anisotropic flow is investigated using the two-particle correlation method, focusing on flow harmonics \(v_2\) (elliptic) and \(v_3\) (triangular). The effects of initial geometry on final-state momentum anisotropies are quantified via normalized symmetric cumulants and the number-of-constituent-quark (NCQ) scaling of \(v_2\) for identified particles. Our findings show that \(\alpha\)-clustered geometries produce stronger elliptic flow, consistent with the idea that initial-state fluctuations significantly impact collective dynamics.

In addition, the nuclear modification factor \(R_{\mathrm{AA}}\) is computed by comparing O-O spectra with a pp reference, adjusted for the number of binary nucleon-nucleon collisions. We find notable suppression of high-\(p_T\) hadrons in central O-O collisions, suggesting that medium-induced energy loss effects, typically associated with QGP, are present even in these lighter systems. In Run 3 of the LHC, ALICE has collected O-O collisions data. Our studies involving various density profiles of Oxygen nuclei, when confronted with the experimental data, will pave the way forward to understand the nuclear density profiles and their impact on the final state observables in TeV collisions.