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Poster Summary: ENUBET – Enabling High Precision Flux Measurements in Conventional Neutrino Beams

March 16, 2017

(by Fabio Pupilli) The poster presents a novel approach to reduce neutrino beam flux systematics, presently of the order of 7-10%, by building a monitored electron (anti-)neutrino source that employs conventional technologies.

In the ENUBET (Enhanced NeUtrino BEams from kaon Tagging) ERC program this is achieved by directly monitoring in an instrumented decay tunnel positrons (electrons) emitted in Ke3 decays in a sign and momentum-selected narrow band beam. This method allows to get rid of beam related systematics arising from the number of PoT, the hadro-production cross sections and the secondary meson focusing efficiency of the beamline, and promises to reduce the uncertainty on the neutrino flux normalization down to 1%.

sterile fig1

Figure 1

A monitored neutrino source like the one proposed by ENUBET will constitute an unprecedented opportunity for future experiments aiming at a O(1%) precision in the electron neutrino cross section measurement; in Fig. 1 (right) the impact of the ENUBET technology on this field, assuming a 1% syst. + 1% stat. (10k nu_e CC interactions) combined error, and the neutrino energy spectrum in the present beamline configuration are reported. Such a breakthrough would be highly beneficial for the high precision oscillation experiments searching for CP violation. Furthermore it could be exploited in a phase-II sterile neutrino search, especially in case of a positive signal from the upcoming SBL experiments. By a proper setup of the hadron beamline, with a slow proton extraction and static focusing devices to maximize the meson collection efficiency, ENUBET intends to set the first milestone toward a time-tagged neutrino beam, where the neutrino at the detector is directly correlated to the produced lepton in the detector.

sterile fig2

Figure 2

In order to cope with the need of a high positron/pion separation capability to suppress the background from other kaons decay modes and of a radiation hard, fast-responsive and cost-effective setup, the choice of the tunnel instrumentation has fallen on a calorimeter with longitudinal segmentation all around the decay tunnel, complemented with rings of plastic scintillator doublets acting as gamma-veto for pi0 rejection (Fig. 2, top part). The basic unit of the calorimeter, the UltraCompact Module (fig. 2, bottom part), made of iron layers interleaved by plastic scintillator tiles, has a thickness of 4.3 radiation lengths and a transverse size comparable to a Molière radius. It is readout by 9 WLS fibers directly coupled to 1 mm^2 SiPMs; unlike conventional shashlik calorimeters, this scheme avoids the occurrence of large passive regions usually needed to bundle the fibers and route them to a common photo-sensor, thus improving the homogeneity in the
longitudinal sampling.

The poster also reports the ongoing R&D and test beam activities at CERN-PS that led to the validation of prototype UCMs in terms of energy resolution and linearity; furthermore the results of a full GEANT4 simulation of the full tagger in a realistic setup demonstrate a
positron tagging efficiency o  about 49% and a charged pion and neutral pion rejection of 97% and 99% respectively.

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