CERN Accelerating science

In high energy heavy ion collisions, a state of matter composed of quarks and gluons has been hypothesized to exist. Charge dependent correlation is one of tools used to study the properties of the created matter. The correlation between charges and associated anticharges are studied with the use of balance function (BF) . In this thesis, we have described the measurement of BF in Pb+Pb collisions at the LHC energy by the ALICE experiment in detail. Balance function observable is an excelent observable which can give information regarding hadronisation time of produced system in high energy heavy ion collisions. A correlation in relative rapidity/pseudorapidity or in relative azimuthal angle between two charged particles is measured by balance function. Results on the BF for pions are presented at Pb+Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV in ALICE experiment. ALICE data is analyzed for 8 centrality bins. 0-5$\%$ centrality bin is called most central collisions and 60-70$\%$ centrality bin is called most peripheral collisions. The width of the balance function distribution (variance of distribution) is used to quantify the correlation strength between particles in pair. The widths of the BF of pions measured in $\Delta y$ and $\Delta\varphi$ decrease with increasing centrality i.e peripheral to central collisions. So for most central collisions, there is a small separation between balancing charges compared to peripheral collisions. It means balancing pairs are strongly correlated in phase space defined by $\Delta y$ and $\Delta\varphi$ for most central collisions. It suggests that charges are created at later times. It hints that there is a delayed hadronisation of the produced system. \mbox{} In BF distributions of pions, a dip at ($\Delta y, \Delta\varphi$) = (0,0) is observed. It is because of mainly coulomb force between balancing charges or quantum statistics correlations. The BF distributions of pions in Pb+Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV have narrower distribution than distributions from Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. The width of balance function has energy and system size dependence. It could be explained as a result of collective flow. Collective flow has larger value in large system size as well as for higher energy. It gives a hint that system produced in high energy heavy ion collisiions exist for a substantial amount of time. In this thesis, chiral magnetic effects are also studied using balance function as a basis. Chiral magnetic effect is the effect of charge separation in heavy ion collisions at the presence of external magnetic field (spectator nucleons produce magnetic field). The BF measures strength of charge separtion in balancing charges,therfore BF should be a good obervable for analysing CME. Elliptic flow also a good observable for describing chiral magntic effect because charge separtion takes place perpendicular to reaction plane. Charge separation means there is a certain distance between positive and negative charged particles. Therefore a dipole is formed. For analysing CME using BF of all charged particles produced by AMPT model, a dipole moment has been created by flipping momenta of some fraction of quarks perpendicular to reaction plane. This fraction has been taken as a variable. We have observed that BF shows a peak towards $\pi^c$ from $0^c$ when flipping fraction is 30$\%$ or greater. It gives that more balancing pairs are emitted in the direction perpendicular to the reaction plane for large flipping fraction. CME effect is the effect of parity violation in the strong interaction between quarks. CME is quantified by $\gamma$ correlator. We have observed that BF moments of $\gamma$ correlator have negative values when flipping fraction is 30$\%$ or greater. We also have observed that $\gamma$ correlator averaged with number of participants give negative values for same charges and positive values for opposite charges. Elliptic flow of pions shows a decreasing trend for higher flipping fractions. Therefore $v_2$ is sensitive to CME because of large out of plane charge separation.