CERN Accelerating science

Production of charmonia (bound states of $c$ and $\bar{c}$ quarks) and bottomia (boundstates of $b$ and $\bar{b}$) in heavy-ion collisions is considered as an ideal probe to explore the possibility of the Quark-Gluon Plasma (QGP) formation in the laboratory. The J/𝜓 suppression is one of the major key to probe the presence of primordial condition of earlier universe. The quarkonium measurements in the hadron-hadron collisions provide an essential tool to test the Quantum Chromo-Dynamics (QCD), as it contains the full energy regime studied by perturbative QCD to the lattice QCD. The studies involving multiplicity of charged-particles produced in high energy collisions is taken as a simple yet important variable for understanding the collision dynamics. The multiplicity measurement is useful in studying the general properties of particles collisions. The quarkonium production as a function of charged-particle multiplicity in proton-proton and proton-nucleus collisions is considered as an interesting observable to comprehend multi-parton interactions and to explore the presence of collective behaviour (fluid like) in small systems. Such studies can play an important role in understanding the production mechanism of soft scale processes and its relation with heavy quarks produced in the hard processes. The multiplicity dependence study of hard probes also provides important ground to look for the contribution of jets (hard 𝑝$_{𝑇}$ particles) in the underlying events, which are mainly dominated by the soft probes like pion, kaon. The self-normalized quarkonium yield as a function of self-normalized charged-particle multiplicity is a good observable where most of the efficiency corrections cancel out in the normalization process. In this way, the results become independent of the experiments from where it has been carried out and a comparative study is possible among the various experiments. Similarly, the evolution of the first moment of the mean transverse momentum of J/𝜓(⟨$𝑝$$_{T}^{J}$$^{/\psi}$⟩) as a function of multiplicity provides information about the collective nature of the collision system. The multiplicity dependence of J/𝜓 production has been previously studied in pp collisions at $\sqrt{s}$ = 7 and 13 TeV and p-Pb collisions at $\sqrt{\textit{s}_{\rm NN}}$ = 5.02 TeV at forward and mid-rapidity using the ALICE detector. A similar study was previously carried out for open charm (e.g. D mesons) particles. An increase of the relative J/𝜓 yields with the relative charged-particle multiplicity has been reported. The mean transverse momentum of the J/𝜓 as a function of relative charged-particle pseudo-rapidity density were reported in p-Pb and Pb-p collisions at $\sqrt{\textit{s}_{\rm NN}}$ = 5.02 TeV. The increasing trend of mean transverse momentum of J/𝜓 at low multiplicity with saturation towards high multiplicity was observed. Therefore, in this thesis, the study of J/𝜓 production as a function of charged-particles has been carried out in pp collisions at $\sqrt{s}$ = 2.76 and 5.02 TeV with the data from the ALICE detector at LHC. The findings have been compared with the results obtained in pp collisions at $\sqrt{s}$ = 7 and 13 TeV at forward and mid-rapidity to explore the energy and rapidity dependence of the correlation between soft and hard physics processes. The results are also compared with the theoretical model calculations. An extensive study with PYTHIA8 events generator has been done to explain the experimental data. The measurement of the inclusive production cross-section of J/𝜓 has been performed for consistency. The relative J/𝜓 yield has been measured as function of relative 𝑑𝑁$_{𝑐ℎ}$/𝑑𝜂 in pp collisions at $\sqrt{s}$ = 2.76 and 5.02 TeV at forward rapidity. The estimation of charged-particles multiplicity using the various approaches in pp collisions at $\sqrt{s}$ = 2.76 and 5.02 TeV have been reported. The analysis is carried out for inelastic > 0 event class. The new analysis techniques were used to calculate the efficiency correction factors related to the events. The charged-particle multiplicity (𝑑𝑁$_{𝑐ℎ}$/𝑑𝜂) increases as a function of the center of mass (CM) energies. The results of integrated charged-particle pseudo-rapidity (⟨𝑑𝑁$_{𝑐ℎ}$/𝑑𝜂⟩) is consistent with the in-dependent analysis reported by ALICE collaboration. The J/𝜓 yield scales with multiplicity linearly at both the CM energies. The comparison study reveals that the process is independent of the CM energy and largely depends on the rapidity. All the theoretical models e.g. higher fock states, percolation, PYTHIA8 etc., describe the data qualitatively, however, models provide best description at low multiplicity while more investigations are needed for high multiplicity. The evolution of ⟨$𝑝$$_{T}^{J}$$^{/\psi}$⟩ as a function of self-normalised 𝑑𝑁$_{𝑐ℎ}$/𝑑𝜂 in pp collisions at $\sqrt{s}$ = 5.02 TeV has been reported. The experimental results have been compared with other available results from ALICE to investigate the energy, rapidity and collision system dependencies. An increase of ⟨$𝑝$$_{T}^{J}$$^{/\psi}$⟩ has been observed at low multiplicity, while at high multiplicity a saturation is observed. The observed trend is found to be independent of the energy and collision system. However, the rate of the growth of ⟨$𝑝$$_{T}^{J}$$^{/\psi}$⟩ varies between the collision systems. The comparison of ⟨$𝑝$$_{T}^{J}$$^{/\psi}$⟩ results with the PYTHIA8 model calculation shows well description of the data. An extensive calculation of multiplicity dependence of J/𝜓 production has been carried out using 4C tune PYTHIA8.2 model. The double differential study in terms of event shapes and multiplicity has been performed in pp collisions at $\sqrt{s}$ = 5.02 and 13 TeV at mid and forward rapidities. The thermodynamic properties of J/𝜓 production has also been studied using non-extensive Tsallis distribution. The finding reveals that at mid-rapidity there is more contribution of jet in the J/𝜓 production while at forward rapidity it is more isotropic in nature. The non-extensive Tsallis temperature provides indication of collectiveness in the small system like pp collisions. The conventional way to investigate the QGP formation is well established with the heavy-ion collisions where pp collisions data are mainly used as a baseline. The formation of hot and dense fireball consisting of hadronic and (or) partonic matter is most likely to happen in heavy-ion collisions. To check for such thermalised medium formation, a systematic analysis of the experimental data on heavy-ion collisions at AGS and SPS energies, available in the laboratory has been carried out to study the correlations and event-by-event fluctuations. The main motivation behind such studies is that the fluctuations in physical observables in heavy-ion collisions serve as an indicator for the possible phase transition and QGP formation. There are some events, produced in $^{32}$S-AgBr and $^{32}$S-Gold collisions at 200 A GeV/c which indicate the presence of narrow phase space regions. Analysis of densely such events has been carried out separately which indicates the presence of dominant cluster or jet-like phenomena in one dimensional 𝜂 and 𝜑 and two dimensional 𝜂 − 𝜑 space. The study of intermittent pattern in term of scaled factorial moments suggests that it may seem serve as an tool to select a special class of events having high density phase regions for further analysis to search for the signals of some exotic states like disoriented chiral condensate (DCC) or QGP. In context to data at RHIC and LHC energies, it may be remarked that by applying the d$_{𝑖𝑘}$ cuts, used in the present study, rare events with large particle density in narrow phase space bins may be identified for further studies; analysis of such an individual event would be statistically reliable because of its high multiplicity. The findings reveals that the identification of large dynamical fluctuations may possible if particle density fluctuations are studied using selection of events with densely populated narrow regions or spikes. Formation of jet-like structure in multi-hadronic final states or clusters in the selected spiky events has also been investigated and compared with the predictions of a multi phase transport (AMPT) and independent emission hypothesis models by carrying out Monte Carlo simulation. The findings suggest that clustering or jet-like algorithm adopted in the present study may also serve as an important tool for triggering different classes of events. Furthermore, the possibility of occurrence of long range correlations has been investigated by analyzing the experimental data in $^{16}$O-AgBr and $^{32}$S-AgBr collisions at 200A GeV/c and the results are compared with the AMPT model predictions. The results show that the observed forward-backward (F-B) multiplicity correlations are mainly of short-range in nature. The range of F-B correlations increases with increasing projectile mass. The observed extended range of F-B correlations might be due to overall multiplicity fluctuations arising because of nuclear geometry. The findings are not sufficient for making any definite conclusions regarding the presence of long-range correlations. In addition, event-by-event fluctuations of hadronic patterns have been searched in terms of voids by analyzing the experimental data on 4.5, 14.5 and 60A GeV/c$^{16}$O-AgBr collisions. Two lowest moments of event-by-event fluctuations of voids, <𝐺𝑞> and 𝑆𝑞 as function of phase space bin width show power-law behaviour. The findings reveal that scaling exponent estimated from the observed power-law behavior of the voids may be used to characterize the various aspects of hadronic phase transition. The results also reject occurrence of second order quark-hadron phase transition at the projectile energies considered.