Investigation of the Sensitivity of WATCHMAN to Measure the Neutrino Mass Hierarchy

WATCHMAN is a gadolinium-doped water-Cherenkov reactor-monitoring antineutrino detector currently under development for nuclear nonproliferation purposes. Experimental sites under consideration lie 13 km and 20-25 km away, respectively, from the nearest nuclear reactor. We simulate the response of a WATCHMAN-type detector receiving a 100 GW$\cdot$kt$\cdot$yr exposure from a reactor 13 km away. We transform the detected electron-antineutrino disappearance oscillation spectrum from $L/E$ space into the $|{\Delta}m^2|$ frequency domain. There, we perform a shape analysis on the Fourier peak geometry in the hierarchy-dependent region around $|{\Delta}\mathrm{m}^2_{31}|$ to attempt a mass hierarchy reconstruction. We find that the WATCHMAN detector at 13 km lies in a previously undiscovered region of sensitivity to the ordering of the neutrino masses, and that for some regions of the oscillation parameter space, a mass hierarchy determination is achievable.