The NP06/ENUBET project:towards a monitored neutrino beam
Marta Torti (Univ. and INFN Milano Bicocca)
on behalf of the ENUBET Collaboration

Lomonosov Conference 2021 - 19th August 2021

Enhanced NeUtrino BEams from kaon Tagging
ENUBET goal: demonstrate the technical feasibility and physics performance of a neutrino beam
where lepton production at large angle is monitored at single particle level→ Monitored neutrino beams
ENUBET: ERC Consolidator Grant, June 2016 – May 2021 (now extended to 2022 to overcome
COVID difficulties). PI: A. Longhin.
Since April 2019: ENUBET also a CERN Neutrino Platform Experiment – NP06/ENUBET.
ENUBET Collaboration: 62 physicists & 13 institutions; Spokespersons: A. Longhin,F.Terranova; Technical Coordinator: V. Mascagna.

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The NP06/ENUBET project●●●

The uncertainty on the neutrino flux is the main source of systematic error for cross section
measurements.
Need high-precision determination of νe and νμ cross section at the energy of interest for DUNE andHyperK to reduce substantially the systematics of long-baseline experiments → Increase the sensitivity
to oscillation parameters, in particular, the CP violating phase δ .
The other source of systematic uncertainty for cross section measurements is the reconstruction of theneutrino energy, biased by the inaccurate reconstruction of the final state particles.

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Conventional facility where we monitor the decays in which neutrinos are produced
event-by-event.
“By-pass” uncertainties from hadro-production, beamline effciency, POT.
Reduce the uncertainty on the flux of νe and, possibly, νμ below 1%.
ENUBET is a very narrow band beam (5-10% momentum bite) → Strong correlationbetween the energy of each ν and its interaction vertex due to kinematics.
Narrow band off axis technique method → Reconstruction of the energy in theneutrino detector without relying on final state particles.
Neutrino energy known at 10-20% level → Ideal tool to study neutrino interactions in3nuclei.

NP06/ENUBET - Monitored neutrino beam
Monitored ν beammeasure rate of leptons <-> monitor ν flux

measure positrons
from Ke3 → flux νeA. Longhin, L. Ludovici, F. Terranova, EPJ C75 (2015) 155

Muon identification andmonitoring from Kμν → flux νμK+ → π0 e+ νe

K+ → μ + νμins

protonstargetdec
tednetrum

protondump
Transfer Line

unay tnel

hadrondump

Muon monitoring atsingle particle level
replacing the hadron dumpwith a range meter fromπμν → flux νμπ+ → μ + νμMany systematics are bypassed monitoring the leptons in the decay tunnel at single particle level
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Slide# : 4

Beamline and accelerator studies●

Conventional beamline where the pions and kaons are produced by protons on a fixed target.
Mean energy of the hadrons selected = 8.5 GeV.

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Selected particles are transported to the decay tunnel that is located off the axis of the proton beam.
40 m long decay tunnel instrumented with calorimeters along its wall to monitor the leptons → K e3
decays become the only source of νe : ∼ 97% of the overall νe flux.
Ke3 positrons emitted at large angles → hit the walls of the instrumented tunnel.

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Two possible focusing design are pursuing:●●a purely static system with quadrupoles placed directly downstream the ENUBET target → works
with a proton “slow extraction” method;horn-based beamline with a focusing magnetic horn between the target and the transferline → needs
a proton “fast extraction” method.
5Slide# : 5

The static ENUBET beamline

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Proton drivers: CERN (400 GeV), FNAL (120 GeV), J-PARC (30 GeV) .
Target: Graphite, 70 cm long and with a radius of 3 cm → selected after dedicated studies.
Positron screen: 10 mm Tungsten foil downstream the target. Get rid of the beam e + background in the tagger.
Transfer line
●Kept short to minimize early K decays;●
Small beam size: non-decaying particles must exit the decay tunnel without hitting the tunnel walls;●Optics: optimized with TRANSPORT to a 10% momentum bite centered at 8.5 GeV/c.
Focusing system: normal-conducting magnets ( quadrupoles and two bending dipoles). Total bending of thebeam w.r.t the primary proton line of 14.8°.
Particle transport and interaction: full simulation with G4Beamline.
Hadron dump and Proton dump with cilindrical structure. Engineering studies needed to define them.
6Slide# : 6Decay tunnel instrumentation - schematics
Calorimeter with e/π/μ separation capabilities:●
Lateral readout Compact Module (LCM):●
sandwich of 5 steel tiles (3x3x1.5 cm2 ) interleaved with 5
plastic scintillator tiles (3x3x0.5 cm2 );●longitudinal segmentation;●
SiPM active area: 4x4mm2 , Cell size: 40 μm ;●three radial layers of LCM.
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Each LCM has 10 WLS (1mm) fibers coupled with SiPM.

Photon-Veto (t0-layer) allows π0 rejection and timing:●
plastic scintillator tiles arranged in doublets forming innerrings (3x3x0.5 cm2 mounted below the LCM);●
time resolution of ~400 ps.

beamExploit event topology for PID

Slide# : 7

Decay tunnel instrumentation – prototype test results
Prototype of 84 LCM tested in 2018 @ CERN PS-T9Large SiPM area for
10 WLS readout(1 LCM)●●●

7 planes on a 3x4 matrix (transverse dim 12x9 cm2) → 30 X0 , 3.15 λ0. Containment of em
showers up to 5 GeV ;Tests with beam of e- , μ- and π- , with momentum [1-5] GeV ;Angles tested w.r.t beam direction (mimic Ke3 positron): 0, 50, 100, 200 mrad.
Reconstructed Energy: Data/MC comparison
at 100 mrad
SiPM saturation effect (Pcross-talk ~44%)
Dedicated measurment campaign at INFN lab→ MC corrected for the effect, accounts for nonlinearities
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Decay tunnel instrumentation – prototype test results
Energy Resolution at 0 mrad●●●●Fit σE /E = S/√E(GeV) ⊕ C ;17% energy resolution at 1 GeV ;impact point of the particle affects contribution to saturation at
higher energies: particles near the edge of the tile are shared
between adjacent LCM → lower contribution to saturation thanthose in the center;mean energy deposited by π- in each plane of the calorimeterfrom data evaluated and compared to simulation: discrepancy
below 10% and comparable to uncertainty due to low-energy
hadronic shower simulation.

Photon veto detectort0-layer needs: γ ID capability and precise timinig→ Positrons of K decays in ENUBET cross 5 tiles on average●●●

1-mip/2-mip separation: 1-mip signal with ε =87%
Background rejection ε =89% (2-mip like), 95% Purity
Time resolution ~ 400 ps
F. Acerbi et al, JINST (2020), 15(8), P08001

2 mip

1 mip
Slide# : 9The demonstrator●●neutronshielding

10cm

30cm

SiPMs &electronics

Build the prototype (1.65 m long and 90° anglecoverage) to demonstrate performance, scalability
and cost-effectiveness;
Will be tested at the CERN on 2022.

WLS fibers
3 radiallayers
sampling
calorimeter

200cm

photon-veto

10The demonstrator
Several activities currently on-going towards the test of the demonstrator.
●●●●Large scale production of the scintillators (UNIPLAST Moscow & INR). Total number of scintillator
tiles for the demonstrator will be ~10000.
Improved light readout scheme completely validated by GEANT4 optical simulation → distance
between fibers optimized to achieve best possible light collection and uniformity.
Effciency map measurement of tiles with similar final shape at INFN-Bologna with a cosmic ray
tracer.
ENUBINO: pre-demonstrator small
prototype = 3 LCMs is being assembled
and will be soon characterized withcosmics at INFN-LNL.

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Lepton reconstruction and identification
The PID is performed by the energy pattern in the modules and by the photon veto. The event selection isbased on 19 variables employed by a Neural Network.
Full GEANT4 simulation reproducing the detectors in the decay tunnel:
Signal instations afterthe hadrondumpcalorimeter+photon veto
Monitoring of●●positrons from Ke3 → νemuons from K μ2 and K μ3 → νμ●muons from π decay → low-E νμ12

Lepton reconstruction and identification
Ke3 positron reconstruction to constrain νe
Full GEANT4 simulation of the detector: validated by prototype tests at CERN in 2016-2018; hitlevel detector response; pile-up effects included (waveform treatment in progress); event building andPID algorithms (2016-2020).
Analysis chain:
1. Event builder: start from event seed and cluster
energy deposits compatible in space and time;

Visible energy -Builder

Visible energy - NN

2. e/μ/π/γ separation: multivariate analysis (MLPNN from TMVA) exploiting 19 variables (energy
pattern in calorimeter, event topology, photonveto).
Analysis performance
S/N = 2.1
Efficiency = 22% (~half of efficiency loss
is geometrical)
F. Pupilli et al., PoS NEUTEL2017 (2018) 07813Lepton reconstruction and identification
K μ2 and K μ3 muon reconstruction to constrain high energy ν μHigh angle muons: reconstruction of track in tagger with dedicated event builder and multi variate
analysis. Main background from halo muons is identified and can be used as control sample.
Analysis chain:
1. Event builder: start from event seed and cluster energy deposits compatible, in space and time, with atrack from a muon;
2. μ-like background separation: multivariate analysis (MLP-NN from TMVA) exploiting 13 variables(energy pattern, track isolation and topology);
Analysis performance
S/N = 6
Efficiency = 34% (~half of efficiency loss
is geometrical)

Tagger impact point Builder

Tagger impact point - NN

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Lepton reconstruction and identification
π μ2 muon reconstruction to constrain low energy ν μLow-E νμ from π decays can be constrained by monitoring associated μ emitted at low angle and go
through the tunnel and the hadron dump.
Instrumented hadron dump with detector layers interleaved by absorberhadrondump

absorber
a1Exploit:●correlation between number of traversed stations(muon energy from range-out) and neutrino energy;●
difference in distribution to disentangle signal fromhalo- muons.
●Detector technology: constrained by muon andneutron rates.

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Physics performance – νe●

●At nominal SPS 4.5 1019 POT/year 104 νe CC at a 500 ton neutrino detector located 50 m from thetunnel exit in about 2 years of data taking.
The neutrinos coming from the decay tunnel are clearly separated in energy from those generated in
the proton dump and in the first section of the beamline.

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73.5% of the total νe flux generated inside
the tunnel and more than 80% above 1GeV.
Below 1 GeV main component is producedin the proton-dump region → further
improve the separation against it by
optimizing the proton dump position.
12% given by the straight section in front of
the tagger → corrected for by relying on thesimulation.

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Physics performance – νμNarrow momentum widthof the beam (O(5-10%))and small neutrino
detector distance.
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Strong correlation between E ν inthe detector and the radial
distance (R) of the interaction
vertex from the beam axis.

Narrow-band off-axis
technique where the E ν isprovided event-by-event basis
without relying on a final stateparticles in νμ CC.

By selecting interactions in radial windows of ± 10 cm, we collect respective samples of ν μ CC events.
Loose energy cut enough to separate π/K component.
Width of pion peak at different R → estimator of the precision on E ν .
8% to 25% at DUNE energy.
From πFrom KF. Acerbi et al., CERN-SPSC-2018-034
17Conclusions●●The ENUBET project is an ERC Consolidator Grant, extended to 2022, and part of the Neutrino
Platform experiments at CERN (NP06) that is aiming at the realization of the first monitored
neutrino beam.
ENUBET is on schedule: the design phase is over, the simulation are nearly completed, and we aregoing to build the demonstrator, that will be tested in 2022 at CERN.

●The physics performance are appealing, but we have to go through for complete studies.

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Conceptual Design Report at the end of the project (2022).

We look forward to seeing ENUBET up and runningin the DUNE/HyperK era!

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