Strategies for Future Accelerator Neutrino Physics

Accelerator neutrino (ν) physics has come back to the forefront, with the discovery of ν transitions, our first and unique window beyond the standard model. The experimental program to provide a complete map of the ν mixing matrix, including its far reaching CP violation sector, and test its unitarity constraints is likely to extend over several future decades, as it has been for quark mixing. So far, conventional ν beams based on pion (π) decay have been used and more are already being planned, at higher power (superbeams), in Japan and the US, in conjunction with larger or novel detectors. Superbeams have limited potential, however. Novel very intense beams of ν parents, longer lived than πʼs, accelerated and then coasted in a decay storage ring replacing the π decay tunnel, promise the ultimate reach. R&D for muon decay ring (ν factory) and ion decay ring (betabeam) experiments is thus a decisive task today.