Enhanced NeUtrino BEams from kaon Tagging (ENUBET)

ENUBET aims at demonstrating the feasibility of a "monitored" neutrino beam, in which the absolute normalization of the neutrino flux can be constrained at the 1% level. The $\nu_e$ flux is determined by monitoring large-angle $e^+$ from $K_{e3}$ decays in a 40 m long instrumented decay tunnel. The $\nu_\mu$ flux is provided by muons produced by decays of $K$ and $\pi$. Being a narrow band beam ($p=8.5$ GeV/$c$ $\pm$ 10%), the transverse position of the interaction at the detector can be exploited to determine a priori the neutrino energy spectrum without relying on the final state reconstruction ("narrow band off-axis technique"). Lepton monitoring and narrow band off-axis energy reconstruction can be implemented in a single facility based on standard accelerator technologies for a new generation of high precision $\nu_e$ and $\nu_\mu$ cross section measurements at the GeV scale and for precision searches of physics beyond the standard 3$\nu$ paradigm. In 2019-2022 ENUBET has devised the first end-to-end simulation of the facility and demonstrated that the precision goals can be achieved in $\sim$ 3 years of data taking employing neutrino detectors of moderate mass (ICARUS at FNAL, ProtoDUNE at CERN). The technology of a monitored beam has been proven to be feasible and cost-effective, and the complexity does not exceed significantly the one of a conventional short-baseline beam. The Snowmass 2021 DPF Community Planning Exercise is thus timely for the consideration of monitored neutrino beams hosting the next generation of cross section experiments. The ENUBET results will play an important role in the systematic reduction programme of future long baseline experiments, thus enhancing the physics reach of DUNE and Hyper-Kamiokande. In this document, we summarize the ENUBET design, physics performance and opportunities for its implementation in a timescale comparable with DUNE.