CERN Accelerating science

Femtoscopy is an analysis technique that allows to obtain information about space-time characteristics of the particle-emitting source, such as source size $R$. Correlation functions reveal, among others, effects of Bose-Einstein correlations and Coulomb interactions. The aim of this thesis was to calculate correlation functions for pairs of pions $\pi^+\pi^+$, $\pi^-\pi^-$ and $\pi^+\pi^-$. Analysed data comes from proton-proton collisions collected by the ALICE experiment at the Large Hadron Collider at CERN laboratory, mainly from 2022, but also from 2018. The main tool of analysis was O2, recently developed ALICE framework designed to work with new data format. Part of this work was to understand and develop its code. I also carried out purity, efficiency and QA (Quality Assurance) analyses to improve the selection of pions from the particles produced in the collisions. I also analysed Monte Carlo (PYTHIA 8) simulations. They don't include effects of Bose-Einstein correlations but reproduce other effects and can be treated as background to obtain femtoscopic correlation functions. They were the basis to extract information about the source parameters. For identically charged pairs correlation functions can be fitted with Bowler-Sinyukov formula, both exponential and Gaussian, and source size $R$, as well as correlation strength $\lambda$, can be found. I have carried out the calculations in different intervals of multiplicity and pair transverse momentum $k_T$ in order to examine dependance of source size on these variables. Obtained results are in line with expectations.