The ENUBET project

The knowledge of the initial flux in conventional neutrino beams represents the main limitation for a precision (1\%) measurement of $\nu_e$ and $\nu_\mu$ cross sections. Since 2016, ENUBET has achieved remarkable results in the design of a neutrino beam with superior control of the fluxes and has extended its physics case to cover all the needs of new generation cross section experiments. In this Proposal, we summarize the achievement of ENUBET in the design and simulation of the proton extraction scheme, focusing system, transfer line and instrumentation of the decay tunnel. We present the results on positron reconstruction from the $K_{e3}$ decays and the impact on the determination of the flux, outlining the activities that will be carried on during LS2 and the final validation of the ENUBET demonstrator in 2021. Special emphasis will be given on the activities that will be performed in collaboration with the CERN groups and the use of CERN facilities and infrastructures. In the Proposal, we also discuss the potential of monitored neutrino beams beyond the original aim of ENUBET: the flux measurement at the 1\% level for all $\nu_\mu$ produced in the beam and the determination of the neutrino energy without relying on final state particle reconstruction. In addition, the ENUBET static focusing system paves the way to the construction of a tagged neutrino beam, i.e. a facility where the observation of the lepton in the decay tunnel is associated with the neutrino scattering in the neutrino detector on an event-by-event basis. Perspectives and opportunities offered by tagged neutrino beams are discussed, too.