Determination of the mass hierarchy with medium-baseline reactor-neutrino experiments

I discuss 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/ 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, the larger resolution the shorter optimal L. For a 5 kton detector (with 12% weight fraction of free proton) placed at L ~ 50 km away from a 20 GW_th reactor, an experiment would determine the mass hierarchy with (Delta chi^2)_min ~ 9 on average after 5 (15) or more years of running with the (a, b) = (2, 0.5)% ((3, 0.5)%) energy resolution. This type of experiment can also measure the relevant mixing parameters with the accuracy of ~ 0.5%.