ar X iv : h ep - p h / 07 03 02 9 v 1 4 M ar 2 00 7 Which long - baseline neutrino experiments ?

We discuss the physics of superbeam upgrades, where we focus on T2KK, a NuMI beam line based experiment NOνA*, and a wide band beam (WBB) experiment independent of the NuMI beam line. For T2KK, we find that the Japan-Korea baseline helps resolve parameter degeneracies, but the improvement due to correlated systematics between the two detectors (using identical detectors) is only moderate. For an upgrade of NOνA with a liquid argon detector, we demonstrate that the Ash River site is preferred compared to alternatives, such as at the second oscillation maximum, and is the optimal site within the U.S. For a WBB experiment, we find that high proton energies and long decay tunnels are preferable. We compare water Cherenkov and liquid argon technologies, and find the break-even point in detector cost at about 4:1. In order to compare the physics potential of the different experimental configurations, we use the concept of exposure to normalize the performance. We find that experiments with WBBs are the best experimental concept. NOνA* could be competitive with sufficient luminosity. If sin 2 2θ 13 > 0.01, a WBB experiment can perform better than a neutrino factory.