Determination of mass hierarchy with medium baseline reactor neutrino experiments

TL;DR

This paper evaluates the potential of medium baseline reactor neutrino experiments to determine the neutrino mass hierarchy, emphasizing the importance of baseline length and energy resolution for sensitivity and measurement accuracy.

Contribution

It provides a detailed analysis of optimal experimental parameters and introduces an efficient method to estimate the probability of correctly identifying the neutrino mass hierarchy.

Findings

Optimal baseline length is around 40-55 km.

Three-sigma hierarchy determination requires about 14 years at certain resolutions.

The experiment can measure mixing parameters with high precision within 5 years.

Abstract

We study the sensitivity of future medium baseline reactor antineutrino experiments on the neutrino mass hierarchy. By using the standard chi^2 analysis, we find that the sensitivity depends strongly on the baseline length L and the energy resolution (delta E/E)^2 = (a/sqrt{E/MeV})^2 + b^2, where a and b parameterize the statistical and systematic uncertainties, respectively. The optimal length is found to be L ~ 40-55 km, where a slightly shorter L in the range is preferred for poorer energy resolution. The running time needed to determine the mass hierarchy also depends strongly on the energy resolution; for a 5 kton detector (with 12% weight fraction of free proton) placed at L ~ 50 km away from a 20 GW_{th} reactor, three-sigma determination needs 14 years of running with a = 3% and b = 0.5%, which can be reduced to 5 years if a = 2% and b = 0.5%. On the other hand, the experiment can measure the mixing parameters accurately, achieving delta sin^2(2theta_{12}) ~ 4*10^{-3}, delta (m_2^2-m_1^2) ~ 0.03*10^{-5} eV^2, and delta |m_3^2 -m_1^2| ~ 0.007*10^{-3} eV^2, in 5 years, almost independently of the energy resolution for a < 3% and b < 1%. In order to compare our simple (Delta chi^2)_{min} results with those obtained by simulating many experiments, we develop an efficient method to estimate the uncertainty of (Delta chi^2)_{min}, and the probability for determining the right mass hierarchy by an experiment is presented as a function of the mean (Delta chi^2)_{min}.