Mass hierarchy sensitivity of medium baseline reactor neutrino experiments with multiple detectors

TL;DR

This paper investigates how adding a near detector to medium baseline reactor neutrino experiments enhances the sensitivity to neutrino mass hierarchy, emphasizing the importance of detector placement, mass, and energy scale calibration.

Contribution

It introduces an optimized configuration for near detectors in reactor neutrino experiments to improve mass hierarchy sensitivity, considering baseline, mass, and spectral uncertainties.

Findings

Adding a near detector significantly improves hierarchy sensitivity.

Optimal near detector placement is around 13 km with 4 kton mass.

Near detector helps constrain spectral uncertainties.

Abstract

We report the neutrino mass hierarchy (MH) sensitivity of medium baseline reactor neutrino experiments with multiple detectors. Sensitivity of determining the MH can be significantly improved by adding a near detector and combining both the near and far detectors. The size of the sensitivity improvement is related to accuracy of the individual mass-splitting measurements and requires strict control on the relative energy scale uncertainty of the near and far detectors. We study the impact of both baseline and target mass of the near detector on the combined sensitivity. A figure-of-merit is defined to optimize the baseline and target mass of the near detector and the optimal selections are $\sim$13~km and $\sim$4~kton respectively for a far detector with the 20~kton target mass and 52.5~km baseline. As typical examples of future medium baseline reactor neutrino experiments, the optimal location and target mass of the near detector are selected for JUNO and RENO-50. Finally, we discuss distinct effects of the neutrino spectrum uncertainty for setups of a single detector and double detectors, which indicate that the spectrum uncertainty can be well constrained in the presence of the near detector.