CERN Accelerating science

At relativistic energies, the heavy-ion collisions provide a unique environment to study the properties of strongly interacting matter known as Quark Gluon Plasma (QGP) created just after the ``Big-Bang''. To explore the evolution of universe, A Large Ion Collider Experiment has been built at Large Hadron Collider (LHC) at CERN. ALICE is specially optimised for the study of heavy-ion collisions to characterize the properties of the QGP. The study of event-by-event fluctuations is the most effective way to address the properties of QGP matter. It helps to extract the dynamical fluctuations present in the system which might be weaken when studied as averaging over all events in an ensemble. The event-by-event study offers a chance to investigate each and every event, and extracts the rare physical events. The experiments at LHC and RHIC (Relativistic Heavy Ion Collider) provide a high multiplictiy environment to study the fluctuations in physical observables on event-by-event basis. A strong magnetic field created by the fast moving spectator protons induces the electric current that leads to the separation of positively and negatively charged particles along the magnetic field direction resulting in the phenomenon of Chiral Magnetic Effect (CME). The hunting for the existence of CME is possible via charge-dependent particle azimuthal correlation measurements. Several experiments at LHC and RHIC reported the charge separation measurement and the results are qualitatively in agreement with the CME expectations. For the charged-neutral correlation measurement, the STAR has studied the event-by-event fluctuations and found no significant deviation from the generic pion production. In this thesis, the results on study of charged-neutral fluctuations and charge separation in Pb-Pb collisions at $\sqrt{s}_{\mathrm{NN}}$ = 2.76 TeV using the ALICE detector at the LHC, are presented. An event-by-event fluctuations in charged-to-neutral particle multiplicities in nuclear-nuclear collisions might be the consequence of formation of DCC domains. The search for the DCC-type domains is performed using the Sliding Window Method (SWM) which locates the unusual charged-to-neutral particle fraction in an event over the azimuthal plane. The fluctuations in charged neutral particle multiplicties are also measured via variable, $\nu^{\gamma, ch}_{dyn}$ and factorial moments $r_{m,1}$. The simulation study is also performed using the Monte Carlo event generators viz., HIJING and AMPT. Results obtained are compared with those of data and provide a hint for the existence of non-statistical fluctuations in data. A new method of Sliding Dumbbell is developed to scan the full azimuthal plane to search event-by-event localised charge separation in $\eta-\phi$ phase space. The sum $Db_{\pm}$ of positive charge fraction on one side and negative charge fraction on other side of the dumbbell is computed and its maximum $Db^{max}_{\pm}$ value is obtained by sliding the dumbbell in steps of $1^\circ$ over the whole azimuthal plane. The $Db^{max}_{\pm}$ distributions in each centrality are further divided into ten bins, where 0-10$\%$ corresponds to higher $Db^{max}_{\pm}$ values, while the 90-100$\%$ corresponds to lower $Db^{max}_{\pm}$ values. Furthermore, the two- and three- particle azimuthal correlations are investigated in terms of $Db^{max}_{\pm}$ bins to extract the events exhibitng large charge separation to have a sample enriched with CME-type events. The study of charge separation effect using different Monte Carlo event generators is also performed. The particle azimuthal correlations are measured for HIJING and AMPT as a function of centrality as well as for different $Db^{max}_{\pm}$ bins. In order to examine the effect of CME signal, the different percentages of CME signal are introduced in the standard string melting AMPT and performed the detailed study via particle azimuthal correlations. Reasonably very good results are obtained using Sliding Dumbbell Method (SDM) regarding the extraction of CME type signal. SDM is applied to Pb-Pb collision data at $\sqrt{s}_{\mathrm{NN}}$ = 2.76 TeV to investigate the event-by-event charge separation. Thus, it is possible to get CME enriched sample events corresponding to top 10$\%$, 20$\%$, and 30$\%$ $Db^{max}_{\pm}$ values for 20-30$\%$, 30-50$\%$, and 50-70$\%$ collision centralities, respectively. The CME fraction ($f_{CME}$) is $\sim$35$\%$ in top 10$\%$, 20$\%$, and 30$\%$ $Db^{max}_{\pm}$ values in these centralities which corresponds to CME signal $\sim$ 5-10$\%$ in these collision centralities.