Boosting the physics performances of the ENUBET monitored neutrino beam with a redesign of the hadron beamline

The ENUBET project is addressing the possibility of building a facility for high precision neutrino cross-section measurements leveraging on the possibility of ``monitored beams''. These are facilities in which the neutrino flux could be accurately predicted by measuring the rates of charged leptons produced in a fully instrumented decay region. The R&D has demonstrated the possibility of building a beamline based on static focusing elements to perform a nu_e cross section measurement in the DUNE energy range with 1% statistical uncertainty employing 10^20 400 GeV protons on target (pot) and a moderate mass neutrino detector of the size of protoDUNE. The instrumentation of the decay tunnel, based on a cost effective sampling calorimeter solution, has been tested with a large scale prototype achieving the performance required to identify positrons and muons from kaon decays with high signal-to-noise ratio. The systematics budget on the neutrino flux is constrained at the 1% evel by fitting the charged leptons observables. Based on these successful results ENUBET is now pursuing a study for a site dependent implementation at CERN in the framework of the ``Physics Beyond Colliders'' initiative. In this context a new beamline, able to enrich the neutrino flux at the energy of HK and to reduce by more than a factor 3 the needed pot, has been designed and is being optimized. This beamline is currently only simulated with a tool (BDSIM) that limits the analysis needed to assess the achievable systematic uncertainty on the neutrino flux. I this thesis I implemented this improved beamline into the GEANT4 framework. This powerful simulation has been used to fully characterize the potential of the new setup. In particular the study will involve a re-evauation of the performances of the previous beamline for the operation in ``time-tagged'' mode and the possibility to exploit the position of the interacting neutrino to have a prior on its energy (``narrow-band off-axis'' technique). The latter technique will be exploited in particular to estimate the performances for measuring the cross-section of neutral current events with production of neutral pions. The optics of the new beamline have been studied with the possibility of inserting high-granularity and fast silicon detectors (similar to the ``Giga-tracker'' detector used in the NA-62 experiment). GEANT4 data will allow to see which are the requirements for these detectors in terms of geometry and time-resolution to be able to operate in the running conditions of the experiment which characterized by an extremely high particle flux. In addition an optimization of the shielding in the region close to the proton target has been carried out in order to reduce the fraction of neutrinos that are not produced in the instrumented decay region.