T. Eberl: Status of ORCA
The 3-flavor oscillation resonance in the Earth for 3-10 GeV neutrinos is usable to determine the sign of delta_m^2 (see Fig.1, right – the survival probability is shown as a function of energy for atmospheric neutrinos). To do that, neutrino telescopes need to distinguish neutrinos from antineutrinos as they have unequal fluxes and cross sections in the atmosphere, which result in percent-level differences in count rates.
One may measure the zenith angle and the energy of upgoing atmospheric GeV-scale neutrinos precisely, and count muon and electron channel events separately, to give an improved precision on the parameter of atmospheric neutrino oscillation.
The resonance signature below 20 GeV requires a very dense detector. Orca will have a few percent of the volume of the km3net but a much denser array of detector elements.
The detector has been shown in simulations to be able to detect very clearly electron-neutrino charged current interactions, reconstructing the full cherenkov ring of electrons. The figure illustrates the pattern of detected photons for events with a vertex at a distance of 20 and 50 meters. The median zenith angle resolution is of 5 to 10 degrees.
For a calculation of the sensitivity to the mass hierarchy, they have used a likelihood ratio technique, fitting together the oscillation parameters and the flux normailzation. They included in the study a 3-flagour earth matter oscillation, track versus shower event classification, response matrices for detector efficiency obtained from Monte Carlo simulations, and event contaminations to their signals. The result is shown in the figure below, which is computed for delta_cp=0 (NH=normal hierarchy of neutrino masses; IH=inverted hierarchy).
It appears that already for three years of operation a more than three-standard-deviation sensitivity can be obtained.