  The narrow band beam of ENUBET is the first implementation of the "monitored
neutrino beam" technique proposed in 2015. ENUBET has been designed to monitor
lepton production in the decay tunnel of neutrino beams and to provide a 1%
measurement of the neutrino flux at source. In particular, the three body
semi-leptonic decay of kaons monitored by large angle positron production
offers a fully controlled $\nu_{e}$ source at the GeV scale for a new
generation of short baseline experiments. In this contribution the performances
of the positron tagger prototypes tested at CERN beamlines in 2016-2018 are
presented.
