Accelerator Neutrino Physics: Status and Perspectives in Europe

Abstract

The occurrence of oscillations among three active neutrino flavors during their propagation in free space and in matter is extremely sold from the experimental point of view [1]. In spite of this, the most straightforward test of such phenomenon is still lacking. In particular, the direct appearance of a new flavor b (b ≡ ν e , ν μ , ν τ ) from a pure source of a given flavor a ≠ b has never been observed. This is due to a very special conspiracy of theoretical and experimental facts. All sources that we have in our disposal and that could exhibit oscillations at the solar scale are made up of electronic neutrinos whose energy is well below the kinematic threshold for muon production. Hence a direct test of new flavor appearance through the observation of charged current events is unfeasible. On the other hand, oscillations at the atmospheric scale for baseline O(103)km occurs for energies of about 1GeV. Still, the peculiar structure of the leptonic mixing matrix suppress ν μ → ν e oscillations at this scale while the observation of ν τ CC events in ν μ → ν τ transitions is extremely challenging from the experimental point of view. Such observation, together with the determination of the overall size of the subdominant ν μ → ν e oscillations at the atmospheric scale (size of the θ 13 angle) are the particularly well suited for long-baseline (LBL) accelerator neutrino experiments and are the main aim of the the current and, probably, next generation of LBL programs in Europe [2].