CERN Accelerating science

The main goal of studying nucleus-nucleus collisions at ultra-relativistic is to characterize the dynamical processes by which QGP (Quark Gluon Plasma) like system is produced and to study the properties this hot and dense matter exhibits. However, experimentally, it is very challenging to study the QCD matter at high temperature and density because of very short spatio-temporal dimensions of the system produced. The study of bulk matter properties requires a very good understanding of the dynamics and chemistry of the collision, which can only be acquired by coordinated analysis of experimental data and theory. The ALICE (A Large Ion Collider Experiment) experiment at the LHC (Large Hadron collider) at CERN is a dedicated experiment to study the hot and dense matter created in ultra-relativistic Heavy Ion Collisions. It provides an opportunity to study the properties of the equilibrated system of de-confined state of quarks and gluons known as Quark Gluon Plasma via various experimental probes. The tool to characterize the spatio-temporal properties of the collision region at femtometer scale is known as $Femtoscopy$ and this study is essential to address the dynamical equilibration process through which the QCD matter proceeds. The correlations of two final-state particles at small relative momentum are the best source to provide the direct link to the size and lifetime of the smaller system. The work in this thesis presents the first ever measurement of the emission asymmetry at the LHC energies using the ALICE detector. The study has been performed using the data collected by ALICE experiment at the LHC in Pb$-$Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV. The analysis was carried out in different centrality classes namely 0-5\%, 5-10\%, 10-20\%, 20-30\%, 30-40\% and 40-50\%. The thesis describes the detailed steps of the femtoscopic technique required to determine the size of the homogeneity region of pion-kaon emission and the emission asymmetry. The analysis reported in this thesis was performed with the cartesian coordinate formalism. The method involved the construction of correlation function and the Double Ratio of the pion-kaon pairs. The pions and kaons were identified by the combined information provided by the TPC and the TOF detector in different momentum ranges. The selected pairs were corrected for the two-track merging effects due to detector resolution and $\gamma$ conversions. The correlation functions and double ratios were obtained for all charge combinations of pion-kaon pairs in different centrality classes. The obtained correlation function was also corrected for the background pairs, originating from non-femtoscopic sources like elliptic flow, resonance decay etc. The relevant parameters of the source, namely the size and the emission asymmetry was extracted by fitting the correlation function using the CorrFit software. The double ratio deviated from unity in the $Out$ direction for all combinations of pion-kaon pairs. This observation suggested that the space-time position of pion and kaon emissions are not same and the pions are emitted closer to the centre of the source compared to kaons. The kaons are emitted earlier than pions. The average size of the pion-kaon homogeneity region and the emission asymmetry was found to decrease for all charge combinations from most central to peripheral collisions. This observation was consistent with the previous measurements of source size using identical particle femtoscopy. The value of emission asymmetry in $Out$ direction decreased from most central collisions to peripheral collisions and the trend of emission asymmetry with respect to centrality is consistent with the previous observation at RHIC energies. The obtained results are also compared to the expectations of Therminator2 event generator coupled with (3+1)-dimensional viscous hydrodynamic calculations. The pion-kaon emission asymmetry obtained with the model agreed with the experimental observation when an additional time delay of 2.1 $fm/c$ was introduced for kaons. The results obtained in this analysis are consistent with the hydrodynamic-induced evolution of the system created in heavy ion collisions which favors a strong radial flow in central collisions. The origin of emission asymmetry can be understood by the strong radial flow hypothesis and the interplay between the collective and thermal movement of the dense matter created in collisions of heavy ions.