Studies on neutrino oscillation parameters $\Delta m^2_{21}$ and $\sin^2 \theta_{12}$ at nuclear reactors with the JUNO experiment

Neutrino oscillations are a well-established quantum phenomenon and have been experimentally verified by many different experiments in the last decades. There are still open questions on the topic; one of them is the determination of the neutrino mass ordering (NMO). To address this question, the Jiangmen Underground Neutrino Observatory (JUNO) has been proposed and is currently under construction in China. Furthermore, JUNO is expected to reach an unprecedented sub-percent precision in the measurement of three of the oscillation parameters that are used to model neutrino oscillations. JUNO will detect electron antineutrinos produced inside nuclear reactors as a by-product of the fission processes happening inside the cores. A good understanding and modelling of the source are of paramount importance, even though it constitutes a great challenge. Furthermore, recent measurements from short-baseline reactor experiments show discrepancies with respect to the various models available in the literature. The neutrino oscillation analysis relies on the calorimetric measurement of the energy of the electron antineutrinos. The interaction of an electron antineutrino with the liquid scintillator target mass is followed by the emission of scintillation and Cherenkov light; the light is detected by a system of PMTs generating a signal which is finally processed by the front-end and readout electronics. Any non-linear behavior in this chain of processes could distort the spectrum and lead to a wrong determination of the NMO and to biases in the measurement of the oscillation parameters. In particular, there are two known sources of non-linearity: the intrinsic non-linearity in the emission of light by the liquid scintillator, which is mainly due to the quenching effect; and the instrumental non-linearity of the PMT system and of the readout electronics, thus a thorough characterization of the two hardware systems is of paramount importance. In this work, we present a model of the reactor spectrum and flux which accounts for all recent measurements from short-baseline reactor experiments. We also propose a model, based on Geant4, to describe the non-linear relation between the energy deposited in the detector by the antineutrino and the emitted light. The test protocol used to thoroughly characterize the readout electronics during mass production is also presented in detail. Finally, we provide JUNO expected sensitivity to the measurement of the solar oscillation parameters, $\Delta m^2_{21}$ and $\sin^2 \theta_{12}$, after the first year of data-taking; the impact of the main sources of systematic uncertainties is also investigated.