Determination of mass hierarchy with medium baseline reactor neutrino experiments

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/root E/MeV)(2) +b(2), where a and b parameterize the statistical and systematic uncertainties, respectively. The optimal length is found to be L similar to 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 similar to 50 km away from a 20 GWth reactor, an experiment would determine the mass hierarchy with (Delta chi(2))(min) similar to 9 on average after 5 (15) or more years of running if the energy resolution (a, b) = (2, 0.5)%((3, 0.5)%) is achieved. The probability that an experiment with the expectation of <((Delta chi(2))(min))over bar> = 9 resolves the mass hierarchy is estimated to be similar to 90% by taking into account statistical fluctuation in the data. On the other hand, the experiment can measure the mixing parameters accurately, achieving delta sin(2) 2 theta(12) similar to 4 x 10(-3), delta(m(2)(2) - m(1)(2)) similar to 0.03 x 10(-5)eV(2), and delta vertical bar m(3)(2) - m(1)(2)vertical bar similar to 0.007 x 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)