Probing the atmospheric mass splitting with reactor antineutrino oscillations at JUNO

Neutrinos, elementary particles that interact only weakly with matter, constitute the first evidence of physics beyond the Standard Model of particle physics. The study of their properties, both on experimental and theoretical grounds, is one of the most active directions within particle physics. The phenomenology of neutrino oscillations provided very elegant solutions to the solar neutrino and atmospheric neutrino anomalies through the transformation of electron and muon neutrinos into other neutrino flavors, respectively. In the last few decades, many experiments provided evidence of neutrinos' existence and revealed peculiar properties, such as their non-zero but extremely small masses and the possibility of oscillation among different flavors. To date, almost all neutrino data collected with accelerator, solar, atmospheric, and reactor neutrinos can be explained within the standard three-neutrino oscillation paradigm, where a total of six parameters are needed to fully describe neutrino oscillations: three mixing angles ($\theta_{12}$, $\theta_{23}$ and $\theta_{13}$), one Dirac CP phase ($\delta_{CP}$), and two independent mass squared differences ($\Delta m_{21}^2$ and $\Delta m_{31}^2$, or equivalently $\Delta m_{32}^2$). Despite significant advancements in neutrino experiments and their precision in recent years, many properties of neutrinos still remain unknown, including the nature of neutrinos (Dirac or Majorana), the existence of CP violation in the leptonic sector, and the scale of the neutrino mass eigenstates, commonly referred to as neutrino Mass Ordering (MO). The Jiangmen Underground Neutrino Observatory (JUNO), situated in South China, is a forthcoming multi-purpose neutrino experiment. With a substantial active mass of 20 kton, it is foreseen to become the World's largest liquid scintillator-based neutrino detector in the next decade. JUNO's primary goal is to determine the neutrino MO using reactor antineutrinos emitted from two adjacent nuclear power plants at a 52.5 km baseline from the experimental site. The oscillation pattern observed in JUNO exhibits subtle variations depending on the neutrino MO, thus providing sensitivity to this parameter. In JUNO's location, the energy spectrum will be distorted by a slow (low frequency) oscillation driven by $\Delta m_{21}^2$ and modulated by $\sin^2(2\theta_{12})$, as well as by a fast (high frequency) oscillation regulated by $\Delta m_{31}^2$ and modulated by $\sin^2(2\theta_{13})$. It will be the first experiment to simultaneously observe neutrino oscillations from two different frequencies, and multiple oscillation cycles of the atmospheric mass splitting $\Delta m_{31}^2$. In addition, JUNO's data-taking period of less than one year is sufficient to establish its dominance in the global precision of three of these parameters. Notably, this is particularly evident in the case of the atmospheric mass splitting $\Delta m_{31}^2$, for which sub-percent precision can be attained within just 100 days of data taking, already exceeding the current state-of-the-art. This thesis focuses on investigating JUNO's sensitivity to oscillation parameters, with a specific emphasis on $\Delta m_{31}^2$. The initial part is dedicated to the entire process involving reactor antineutrinos, spanning from their production at the source to their interaction and detection in the liquid scintillator. Then, the main spectral components are obtained through the official simulation software and the event selection process is described. Subsequently, an Asimov pseudo-dataset is constructed to simulate the nominal energy spectrum at JUNO, thus modeling the antineutrino incoming flux, the oscillation probability, the detector response, and all spectral components. By fitting the pseudo-dataset, the impact of statistical and systematic uncertainties on the estimation of oscillation parameters is assessed.