# Uncertainty in the Reactor Neutrino Spectrum and Mass Hierarchy Determination

## TL;DR

This paper investigates how uncertainties in reactor neutrino spectra and fuel composition affect the determination of neutrino mass hierarchy, proposing methods to mitigate these issues with near detectors.

## Contribution

It introduces a model-independent approach to handle spectrum uncertainties and analyzes the impact of fuel composition evolution on hierarchy sensitivity.

## Key findings

- Spectrum uncertainty impacts hierarchy determination.
- Large near detectors can minimize spectrum difference effects.
- Fuel composition evolution can be reconstructed from near detector data.

## Abstract

One of the challenges that must be overcome in order to determine the neutrino mass hierarchy using reactor neutrinos is the theoretical uncertainty in the unoscillated reactor neutrino spectrum: this is one of the reasons why, recently, it was proposed to add a near detector to the JUNO experiment. A model-independent treatment of the spectrum uncertainty will be discussed, as well as the effect that it will have on the final result. Moreover, since the neutrino spectrum depends on the chemical composition of the fuel, the spectra at the near and far detectors will be different, because they will receive neutrinos from different cores. Taking into account the time evolution of the chemical composition of the fuel in the reactor core, it is possible to reconstruct the far detector spectrum from the near detector data. We will show that the method used to reconstruct the spectrum can affect sensitivity to the mass hierarchy, however if the near detector is large enough the difference will be negligible.