Pair angular correlations for pions, kaons and protons in proton-proton collisions in ALICE

This thesis presents the correlation functions in $\Delta\eta\, \Delta\phi$ space for pairs of pions, kaons and protons. The studies were carried out on the set of proton-proton collisions at the centre-of-mass energy $\sqrt{s}$ = 7 TeV, obtained in ALICE, A Large Ion Collider Experiment at CERN, the European Organization for Nuclear Research. The analysis was performed for two charge combinations (like-sign pairs and unlike-sign pairs) as well as for three multiplicity ranges. Angular correlations are a rich source of information about the elementary particles behaviour. They result in from the interplay of numerous effects, including resonances’ decays, Coulomb interactions and energy and momentum conservation. In case of identical particles quantum statistics needs to be taken into account. Moreover, particles differ in terms of quark content. Kaons, carrying the strange quark obey the strangeness conservation law. In the production of protons baryon number must be conserved. These features are reflected in the shapes of correlation functions. The obtained correlation functions for pions, kaons and protons show significant differences. In case of unlike-sign (particle-antiparticle) pairs, the correlation is stronger for kaons, slightly weaker for protons and much weaker for pions. Considering the like-sign (identical) pairs of particles, the kaons and pions correlations are comparable, while for protons there is a large anticorrelation. For all the particles the strength of the correlation is decreasing with increasing multiplicity. In order to quantitatively analyse the angular correlations, a fitting procedure was performed. It enables a separate description of the components of the correlation function. For instance, it revealed the existence of the longitudinal ridge only for the lowest multiplicity of unlike-sign pairs of particles. Experimental data were also compared with the models’ predictions. The event generators Pythia and Phojet were used. The comparison of the correlation functions indicate that Monte Carlo models do not fully reproduce the data. The main discrep- ancy is observed for like-sign pairs of protons as for simulated data no anticorrelation is observed. For this thesis a toy-model was developed in order to check the influence of conservation laws and minijets production on the angular correlation. The detailed study of this simple model can give insight into the basics mechanisms of particle production.