  The ENUBET project is aimed at designing and experimentally demonstrating the
concept of monitored neutrino beams. These novel beams are enhanced by an
instrumented decay tunnel, whose detectors reconstruct large-angle charged
leptons produced in the tunnel and give a direct estimate of the neutrino flux
at the source. These facilities are thus the ideal tool for high-precision
neutrino cross-section measurements at the GeV scale because they offer
superior control of beam systematics with respect to existing facilities. In
this paper, we present the first end-to-end design of a monitored neutrino beam
capable of monitoring lepton production at the single particle level. This goal
is achieved by a new focusing system without magnetic horns, a 20 m
normal-conducting transfer line for charge and momentum selection, and a 40 m
tunnel instrumented with cost-effective particle detectors. Employing such a
design, we show that percent precision in cross-section measurements can be
achieved at the CERN SPS complex with existing neutrino detectors.
