Future neutrino oscillation facilities: Physics priorities and open issues

The recent discovery that neutrinos have masses opens a wide new field of experimentation, for which one has to be ambitious, ingenious... and patient. Accelerator-made neutrinos are essential in this program. Ideas for future facilities include high intensity muon neutrino beams from pion decay ('Superbeam'), electron neutrino beams from nuclei decays ('Betabeam'), or muon and electron neutrino beams from muon decay ('Neutrino Factory'), each associated with one or several options for detector systems. There are different opinions in the community on how to proceed through this rich choice of questions and possibilities. We now begin a "scoping study" aimed at determining a set of key R&D projects enabling the community to propose an ambitious accelerator neutrino program at the turn of this decade. As an introduction to this study, a set of physics priorities, a summary of the perceived virtues and shortcomings of the various options, and a number of open questions are presented.