Letter on intent to build an off-axis detector to study νmu→νe oscillations with the NuMI neutrino beam. Version 6.0

The question of neutrino masses is of fundamental importance. Neutrino oscillations seem to be the only tool available to us to unravel the pattern of neutrino masses and, perhaps, shed some light on the origin of masses in general. The NuMI neutrino beam line and the MINOS experiment represent a major investment of US High Energy Physics in the area of neutrino physics. the forthcoming results could decisively establish neutrino oscillations as the underlying physics mechanism for the atmospheric νμ deficit and provide a precise measurement of the corresponding oscillation parameters, Δm322 and sin2 2θ23. This, however, is just a beginning of a long journey into uncharted territories. The key to these new territories is the detection of νμ → νe oscillations associated with the atmospheric νμ deficit, controlled by the little known mixing angle sin22θ13. A precise measurement of the amplitude of these oscillations will enable a determination of the pattern of the mass hierarchy of neutrinos. If the solar neutrino experiments determine that the value Δm122 is in the range of 10-5-10-4 eV2 then the measurement of the CP violation in the neutrino sector may well be within our reach. The full potential of the NuMI neutrino beam can be exploited by complementing the MINOS detector, under construction, with a new detector(s) placed at some off-axis position and collecting data in parallel with MINOS. The first phase of the proposed program includes a new detector, optimized for νe detection, with a fiducial mass of the order os 20 kton and exposed to neutrino and antineutrino beams. In a five year run its sensitivity to the νμ → νe oscillations will be at least a factor of ten beyond the current limit. The future direction of the program will depend on the results of this first phase, but it is very likely that it will be combination of a significant increase of the neutrino beam intensity via an upgraded proton source and an increase of the detector mass by a factor of five or so. Depending on the circumstances, the goals of Phase II may be a further increase of the sensitivity of a search for νμ → νe oscillations, or, perhaps, a measurement of the CP violating phase (delta) in the lepton sector