Proton-proton femtoscopy in Υ(nS) decays at Belle

The study of hadron production in high-energy collisions provides access to the nonperturbative regime of Quantum Chromodynamics, where confinement and hadronization govern the formation of final-state particles. Among the various tools developed to probe this regime, two-particle femtoscopy has proven particularly powerful, enabling the extraction of spatial and temporal scales associated with particle emission through correlations at low relative momentum. While femtoscopic techniques have been extensively applied in heavy-ion and proton–proton collisions, their use in electron–positron annihilation remains comparatively unexplored. The clean experimental environment provided by e+e− colliders, together with the well-constrained nature of the nucleon–nucleon interaction, offers an ideal setting in which to study the space–time structure of hadronization without the complicating influence of collective effects or final-state rescattering in the medium. This thesis presents a detailed femtoscopic analysis of proton–proton pairs produced in e+e− annihilation at the Υ(1S) and Υ(2S) resonances, making use of the high-statistics datasets collected by the Belle experiment at KEK. The proton–proton system is particularly advantageous for such studies: its final-state interaction, governed by wellestablished strong and Coulomb potentials and modified by Fermi–Dirac statistics, acts as a calibrated probe of the emission source. This allows the correlation function to be interpreted primarily in terms of the geometry of hadron production. By exploiting a combination of event-mixing techniques, background suppression strategies, and stateof- the-art theoretical modeling of the interaction kernel, the analysis extracts effective source radii as a function of the pair momentum and event category. The results provide new experimental constraints on the space–time structure of hadronization in Υ decays. The measured correlation functions exhibit clear signatures of the proton–proton final-state interaction, and their comparison with theoretical calculations enables the determination of source sizes in the femtometer range. The dependence of the extracted radii on the kinematics and production environment sheds light on the mechanism by which baryons are formed in e+e− collisions, offering insights complementary to those obtained from meson femtoscopy and from studies in hadronic and nuclear collisions. In addition, the analysis contributes to the broader program of baryon–baryon correlation studies, providing a benchmark against which less precisely known interaction channels may be calibrated. Overall, this work demonstrates the potential of femtoscopic techniques in e+e− annihilation and establishes proton–proton correlations as a sensitive probe of the hadron emission source in the decay of heavy quarkonium states. The methods and results presented here lay the groundwork for future measurements at Belle II, where significantly larger datasets will enable more differential and higher-precision studies of baryon production and strong interactions at the femtometer scale