The ERC ENUBET Project: high precision neutrino flux measurements in conventional neutrino beams

TL;DR

The ERC ENUBET project aims to develop a novel, instrumented neutrino beamline to measure neutrino fluxes with 1% precision, enabling more accurate cross section measurements crucial for CP violation studies.

Contribution

This work introduces a new technique for monitoring positron production in decay tunnels to precisely determine neutrino fluxes, advancing beyond traditional methods.

Findings

Development of ultra-compact calorimeters for decay tunnel monitoring

Simulation results demonstrating beamline performance improvements

Initial experimental results on calorimeter prototypes

Abstract

The challenges of precision neutrino physics require measurements of absolute neutrino cross sections at the GeV scale with exquisite (1%) precision. This precision is presently limited by the uncertainties on neutrino flux at the source; their reduction by one order of magnitude can be achieved monitoring the positron production in the decay tunnel originating from the $K_{e3}$ decays of charged kaons in a sign and momentum selected narrow band beam. This novel technique enables the measurement of the most relevant cross sections for CP violation ($\nu_e$ and $\overline{\nu}_e$) with a precision of 1% and requires a special instrumented beam-line. Such non-conventional beam-line will be developed in the framework of the ENUBET Horizon-2020 Consolidator Grant, recently approved by the European Research Council. The project, the first experimental results on ultra-compact calorimeters that can be embedded in the instrumented decay tunnel and the advances on the simulation of the beamline are presented. We also discuss the detector and accelerator activities that are planned in 2016-2021.

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Full text

NuPhys2016-Pupilli

The ERC ENUBET Project: high precision neutrino flux measurements in conventional neutrino beams

Fabio Pupilli, on behalf of the ENUBET collaboration

INFN Sezione di Padova, I-35131 Padova, Italy

The challenges of precision neutrino physics require measurements of absolute neutrino cross sections at the GeV scale with exquisite (1%) precision. This precision is presently limited by the uncertainties on neutrino flux at the source; their reduction by one order of magnitude can be achieved monitoring the positron production in the decay tunnel originating from the $K_{e3}$ decays of charged kaons in a sign and momentum selected narrow band beam. This novel technique enables the measurement of the most relevant cross sections for CP violation ($\nu_{e}$ and $\overline{\nu}_{e}$) with a precision of 1% and requires a special instrumented beam-line. Such non-conventional beam-line will be developed in the framework of the ENUBET Horizon-2020 Consolidator Grant, recently approved by the European Research Council. The project, the first experimental results on ultra-compact calorimeters that can be embedded in the instrumented decay tunnel and the advances on the simulation of the beamline are presented. We also discuss the detector and accelerator activities that are planned in 2016-2021.

PRESENTED AT

NuPhys2016, Prospects in Neutrino Physics

Barbican Centre, London, UK, December 12–14, 2016

1 Conventional and monitored neutrino beams

In conventional beamlines the decay tunnel is a passive region and the knowledge of the neutrino flux relies on ab-initio simulations that take into account the proton-target interactions, the reinteraction of secondaries, their tracking and decay. Despite the use of hadro-production data from dedicated experiments and of ancillary measurements (proton intensity, horn currents, beam-target misalignment etc.), the precision on the flux prediction is usually limited to 7-10%.

A very precise measurement of the $\nu_{e}$ ($\overline{\nu}_{e}$) flux can be achieved by directly monitoring in an instrumented decay tunnel the production of large angle positrons (electrons) from $K_{e3}$ decays ($K^{+(-)}\rightarrow\pi^{0}e^{+(-)}\nu_{e}(\overline{\nu}_{e})$) in a sign and momentum-selected narrow band beam [1]. The $e^{+}$ rate gives a direct estimate of the $\nu_{e}$ flux that is not affected by beam related systematics arising from the number of PoT, the hadro-production cross sections and the secondary meson focusing efficiency of the beamline, and this method could reduce the uncertainty on the $\nu$ flux normalization down to 1%.

2 The ENUBET project

The ENUBET (Enhanced NeUtrino BEams from kaon Tagging) project is intended to demonstrate the feasibility of the monitored beam approach, by designing and constructing a detector able to identify positrons from $K_{e3}$ decays in the harsh environment of a neutrino beam decay tunnel [2]. It will also study the accelerator issues and the the precise layout of the kaon/pion focusing and transport system. The project has been approved by the ERC (CoG, P.I. A. Longhin, Host Institution INFN) for a five year duration (since 1 June 2016) and a 2.0 MEUR budget. A controlled neutrino source, like the one proposed by ENUBET, could be exploited by future experiments aiming at $\mathcal{O}(1\%)$ precision in the electron neutrino cross section measurement. It could be also exploited in a phase-II sterile neutrino search, especially in case of a positive signal from the upcoming short baseline experiments. Finally, ENUBET intends to set the first milestone towards a “time-tagged neutrino beam”, where the $\nu$ at the detector is time-correlated with the produced $e^{+}$ in the decay tunnel.

The ENUBET reference design (Figure 1) foresees a secondary beam with an average momentum of 8.5 GeV and a $\pm$20% momentum bite. The tagging detector is a hollow cylinder surrounding a fraction of the decay tunnel and it is instrumented with a calorimeter for e/$\pi$ separation and a photon veto to separate electromagnetic showers from $\pi^{0}$ decays or prompt $e^{+}$ from $K_{e3}$.

The choice of a high secondary energy and of a short tunnel (50 m) reduces the $\nu_{e}$ content of the beam coming from muon decays in flight with respect to the ones from $K_{e3}$ and enhances the $\nu_{e}/\nu_{\mu}$ ratio. Furthermore the resulting positron energy is large enough for an efficient identification through calorimetric techniques and the produced neutrino spectrum matches the one of interest for future long baseline experiments. The emittance of the secondary beam (few mrad over 10$\times$10 cm2) and the radius of the tunnel are optimized in order to prevent undecayed secondaries and muons from pion decay from hitting the calorimeter.

Studies in the preparatory phase of ENUBET have shown that, employing a 500 t neutrino detector (e.g. ICARUS at Fermilab or ProtoDune-SP/DP at CERN) located 100 m from the entrance of the decay tunnel and a 30 GeV (450 GeV) proton driver, 5$\times$1020 (5$\times$1019) PoT would be required to provide a sample of 104 tagged $\nu_{e}^{CC}$ interactions. The tagging detector can be safely operated in terms of pile-up, dose, etc. if local particle rates are below $\sim$1 MHz/cm2; this implies that the proton extraction length cannot be shorter than 1 ms. On the other side, longer extractions significantly exceeding 10 ms are disfavoured if secondary focusing is achieved by magnetic horns. The use of a very efficient focusing system based on DC operated magnets can overcome this limitation and can offer the possibility of reducing particle rates to the level needed to match current detector time resolutions for the operation of the facility also in “time-tagged” mode. Within ENUBET, proton extraction schemes compatible with accelerators at CERN, J-PARC and Fermilab will be investigated.

3 Detector prototyping and simulation

In order to cope with the needs of a high $e/\pi$ separation capability and of a radiation hard, fast-responsive and cost-effective setup, the choice of the tunnel instrumentation has fallen on a shashlik calorimeter with longitudinal segmentation. The basic unit of the calorimeter, the Ultra-Compact Module (UCM), is made of five, 15 mm thick, iron layers interleaved by 5 mm thick plastic scintillator tiles. The total length of the module (10 cm) corresponds to 4.3 $X_{0}$ and its transverse size is of 3$\times$3 cm2. Nine wavelength shifting fibers crossing the UCM are connected directly to 1 mm2 SiPMs through a plastic holder (Figure 2, left). SiPMs are hosted on a PCB and the output signals are summed and routed toward the front-end electronics by copper-kapton lines. 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 greatly improving the homogeneity in the longitudinal sampling.

UCM prototypes were developed by the INFN SCENTT collaboration; they were tested with cosmic rays and characterized with charged particles in the 1-5 GeV range at the CERN-PS East Area facility [3, 4]. The results have proven the linearity of the calorimeter response in the considered energy range without saturation up to 4 GeV and an energy resolution of 18% at 1 GeV (Figure 2, right), well within the requirements of 25%/$\sqrt{E}$ for an efficient e/$\pi$ separation.

The performances of the full instrumented tunnel were simulated with GEANT4 [5]. The particle identification algorithms employed rely on the pattern of energy deposit along the UCMs through a multivariate approach based on a neural network (Figure 3); the information from hits in the photon veto is used for the $\pi^{0}$ rejection [6]. For a positron tagging efficiency of 49%, the $\pi^{\pm}$ and $\pi^{0}$ mis-identification probabilities are 2.9% and 1.2% respectively, confirming that the UCM technique is appropriate for the ENUBET needs.

In november 2016 a module composed of 56 UCMs in 7 longitudinal layers ($\sim$30 $X_{0}$), complemented by an outer module acting as energy catcher, have been exposed to electron and pions with various incidence angles at the CERN-PS; the analysis of these data, currently on going, will allow to test the e/$\pi$ performance under realistic conditions. Further prototype exposures to charged particles (CERN) and neutrons (INFN-LNL CN) are foreseen during 2017-2018, in order to also assess the recovery time and the fulfillment of radiation hardness requirements. R&D prototypes of the photon veto will be $\gamma$-irradiated at the INFN-LNF BTF. Finally a full demonstrator (3 m in length, 180∘ coverage) will be assembled and tested at CERN.

ACKNOWLEDGEMENTS

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 681647).