A new way to determine the neutrino mass hierarchy at reactors

The neutrino mass ordering is one of the fundamental characteristics of the reference 3-neutrino mixing scheme that still remains undetermined experimentally at present. Some of basic neutrino physics observables, which are planned to be measured in currently running and/or upcoming neutrino experiments, depend critically on the neutrino mass ordering. The Neutrino Mass Hierarchy Determination ( MHD) is one of the main goals of the major current and future neutrino experiments. MHD corresponds to the sign of the so called atmospheric neutrino mass, |Delta(m^2_atm)|. The medium baselines reactor anti-neutrino experiments are designed to determine the neutrino mass hierarchy without exploring the matter effect that is used in long baseline accelerator and atmospheric neutrino experiments. The strategy of mass hierarchy study, in medium baselines reactor experiments, can be based on the study of the neutrino vacuum oscillations. A technique was developed, referring to the interference of the two different oscillation frequencies driven by Delta(m^2_31) and Delta(m^2_23), which at first order correspond to Delta(m^2_atm) separated by the much smaller solar neutrino mass, Delta(m^2_sol). Advances in statistical analysis techniques may play a decisive role in the discovery reach at neutrino physics experiments. The statistical analysis for neutrino mass ordering usually proceeds from the standard method based on Delta(Chi^2). This method shows some draw-backs and concerns, together with a debatable strategy. The issues of the standard method on MHD for the neutrino reactor experiments are explained. As a result, a new alternative statistical method was invented. The new method of determining the neutrino mass ordering in medium baseline experiments with reactor anti-neutrino is based on a bi-dimensional statistical estimator. Referring to the JUNO experimental conditions we developed a completely new technique that would provide a robust 5 measurement in less than six years of running. The two orderings could be discriminated at the price of allowing for two different degenerated values of Delta(m^2_atm). This degeneracy on Delta(m^2_atm) (around 12*10^-5 eV^2) can however be tackled at an unprecedented accuracy of much less than 1%, i.e. 10^-5 eV^2, within the same analysis. The sensitivity using the new estimator, F_MO, was obtained assuming that the |Delta(m^2_atm)|, identified as the mass difference between the lightest neutrino mass and the heaviest one, be unique for both hypotheses NH and IH. We will discuss the subtleties of such assumption. On the other hand, also the standard JUNO test statistic depends on some strong assumptions. There are two kinds of sensitivity studies performed in the dissertation. One is that coming from the standard method, a single dimensional (1D) estimator Delta(Chi^2); the other one is obtained using the new alternative method via the bi-dimensional estimator F_MO. The sensitivity is obtained by taking into account the reactor cores’ distribution uncertainty, Daya Bay and Huizhou nuclear power plants contributions, the spectrum shape uncertainty and the detector-related uncertainties, including the energy non-linear response of the detector. The background systematics, especially ^9Li, have minor impact on the sensitivity. Possible results after two, four and six years of running and the foreseen initially-reduced available reactor power have been studied, as well. F_MO estimator gives confidence to reject the false mass hierarchy at more than 5 sigma. These results confirm the very positive perspectives for JUNO to determine the mass ordering in a vacuum oscillation dominated regime if the proper statistical analysis is used.