NaNu Proposal for a Neutrino Experiment at the SPS Collider located at the North Area of CERN Friedemann Neuhaus Matthias Schott Chen Wang and Rainer Wanke

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NaNu: Proposal for a Neutrino Experiment at the SPS Collider located at

the North Area of CERN

Friedemann Neuhaus, Matthias Schott∗, Chen Wang, and Rainer Wanke

∗corresponding author

Abstract. Several experiments have been proposed in the recent years to study the nature of tau

neutrinos, in particular aiming for a ﬁrst observation of tau anti-neutrinos, more stringent upper

limit on its anomalous magnetic moment as well as new constrains on the strange-quark content of

the nucleon. We propose here a new low-cost neutrino experiment at the CERN North area, named

NaNu (North Area NeUtrino), compatible with the realization of the future SHADOWS and HIKE

experiments at the same experimental area.

Contents

1 Introduction...................... 1

2 Detector Concept . . . . . . . . . . . . . . . . . . . 1

3 Neutrino Fluxes . . . . . . . . . . . . . . . . . . . . 3

4 Detector Simulation and Neutrino Identiﬁcation . . 4

5 Expected Physics Reach . . . . . . . . . . . . . . . 5

6 Estimated Costs . . . . . . . . . . . . . . . . . . . 8

7 Summary ....................... 8

1 Introduction

Within the SM, the neutrino sector is still the least un-

derstood and key questions, e.g. on the origin of the

neutrino masses, are still not answered. Several new

neutrino experiments are currently in preparation or

just have started to take data. A particular interest-

ing development are new experiments at colliders, e.g.

FASER [1] and SND@LHC [2], which aim for neutrino

cross-section measurements in a new energy regime. Of

particular interest are tau neutrinos, since the study

of their properties are limited to nine ντevents ob-

served at DONUT [3] and ten ντcandidate events at

Opera [4]. The existence of anti-tau neutrinos has so far

never been experimentally conﬁrmed, making this the

last missing particle within the SM. In fact, a huge num-

ber of tau- and anti-tau neutrinos could be produced

in beam-dump experiments, where energies are high

enough to produce D±

smesons, which subsequently de-

cay via Ds→τ ντwith a branching fraction of about

5% [5]. Originally, the SHiP collaboration suggested a

dedicated neutrino detector to study such tau-neutrino

events, with a convincing physics case [6–8]. In this

work, we propose a cost-eﬃcient alternative neutrino

detector, NaNu (North Area NeUtrino detector), which

can be realized in-between the future SHADOWS and

the HIKE experiments, as schematically shown in Fig-

ure 1. The planned SHADOWS experiment [9] at CERN

is one possible realization of a beam dump experiment,

located at the CERN North Area next to the SPS col-

lider aiming for the search of dark matter and hidden

particles. The SHADOWS detector is about 35 m long

and 2.5 m wide, placed about 1 m oﬀ-axis and 14 m

after the beam dump itself, where a 400 GeV proton

beam provides 5 ×1019 protons on target during a 4-

year data-taking period between 2028 and 2032. The

concept of SHADOWS foresees also the realization of

the HIKE Experiment [10], which will be located about

50 m downstream of SHADOWS and will study ex-

tremely rare kaon decays. A ﬁrst setup towards the full

NaNu detector could be already installed in 2024 and

operated together with the NA62 Experiment.

In this work, we ﬁrst discuss a preliminary NaNu

detector concept, followed by an estimate on the neu-

trino ﬂuxes, the identiﬁcation of neutrino signatures as

well as the physics reach and a cost estimate.

2 Detector Concept

The NaNu detector concept aims for a cost-eﬃcient

design using existing components and well established

technologies. Depending on the available funding and

the eﬀectiveness of the background shielding, it can be

realized in three diﬀerent phases. We start with baseline

concept in the following.

A schematic drawing of the side-view of the stan-

dard NaNu detector is shown in Figure 2 with its four

major components, namely the magnetic system, the

emulsion target, the active trigger system as well as

the muon spectrometer. For the baseline NaNu detec-

tor concept, we foresee to have two same-sized main

detector components, labelled as active- and emulsion-

detector in the following, both with dimensions of 45 ×

45 ×100 cm3. Both detectors are placed partly with

a length of 20cm inside an existing dipole magnet at

CERN with gap dimensions of 50×100×100 cm3and a

magnetic ﬁeld strength of 1.4 T generated by a current

of 2500 A. The transverse plane of the NaNu experi-

arXiv:2210.15532v4 [hep-ex] 9 Apr 2023

0 m 10 m 20 m 30 m 40 m 50 m

SHADOWS Experiment

Decay Volume and Spectrometer

Beam

Dump

NaNu

Fig. 1. Schematic drawing of the top-view on the location of the NaNu

Experiment in the CERN North Area together with the future SHADOWS

Experiment and the beam-line.

0.45m

Beam

0.8m

0.5m

Magnet-System

MM Tracking Layers

Tun gsten/Emuls io n

Air/Emulsion

Scintillator

Support Structures

Led Shield

Fig. 2. Side-view of the NaNu experiment

with its major components: emulsion de-

tector, magnet and tracking stations. The

active detector is behind the emulsion de-

tector and not shown.

ment, facing the interaction point has therefore a total

size of 50 ×100 cm2.

The emulsion detector concept follows largely the

current design of the FASERνExperiment [11]. It con-

sists of silver bromide crystals with diameters of 200 nm

dispersed in gelatin media interleaved with a repeated

structure of absorber plates in which the actual charged

current neutrino interactions occur. Emulsion detectors

have a spatial resolution between 50 to 100 nm and can

be interpreted as detectors with a huge density of ac-

tive channels, i.e. 1014 per cm3. They are therefore per-

fectly suited for the study of short-lived particles with

unique decay vertex structures. We propose to adopt

the emulsion detector design of the successfully run-

ning FASERνexperiment, i.e. use emulsion ﬁlms com-

posed of two layers with a 70 µm thickness which are

separated by a 200 µm thick plastic base. The emul-

sion ﬁlms are interleaved by 1 mm tungsten plates due

to their short radiation length. In total 560 tungsten

plates with a total weight of the ≈2.2 t are foreseen.

Given the signiﬁcant multiple scattering eﬀects within

the tungsten plates, we foresee to place 40 emulsion

tracking layers in the remaining 20 cm within the mag-

netic ﬁeld, allowing for momentum measurements with

higher precision. Those layers will be stabilized by thin

aluminium plates, that are separated by 4 mm air gaps.

Since emulsion detectors cannot record timing informa-

tion, all charged particles leave tracks and lead to signif-

icant pile-up. The emulsion detector is therefore com-

plemented by six micromegas based tracking detectors

with two-dimensional readout as well as a two-gap de-

sign, originally proposed in [12]. The thickness of these

active detectors is about 15 mm and yields a spatial

resolution of ≈150 µm in two spatial dimensions with

1800 readout channels for each detector. The two-gap

design allows in addition the reconstruction of complete

tracklets with angular resolution of 0.03 rad. The emul-

sion detector is designed to identify electron-, muon-

and tau-neutrino interactions.

The active detector has the same dimensions as the

emulsion detector. It consists of 2.6cm thick tungsten

plates, interleaved with 0.9mm thick plastic-scintillators

with a SiPM readout system with ten channels on each

layer. Similar to the emulsion detector, one Micromegas

tracking layer is placed every 15cm. The total weight of

the tungsten is ≈2.5 t. While the Micromegas tracking

layers are foreseen to measure the angle of transvers-

ing muons, the plastic-scintillators are used to measure

hadronic shower energies. The active detector is there-

fore a combination of a tracking detector and a hadronic

sandwich calorimeter. While it cannot identify electron-

and tau-neutrino interactions, it is perfectly suited to

measure interactions of muon neutrinos, in particular

the angle of the muon as well as the energy of the

hadronic recoil system.

Before and after both detector systems as well as on

the side facing the beam-line, a highly eﬃcient muon-

veto system based on plastic-scintillators is foreseen, to

reduce the background of muons for the active compo-

nents of NaNu.

The emulsion detector and active detector are fol-

lowed by a muon spectrometer consisting of four layers

of the same micromegas-based technology as previously

described but with larger dimensions of 100 ×50 cm2

with 3600 readout channels per detector layer. The four

layers are separated by 20 cm each, while the ﬁrst layer

is shielded in addition with a 20 cm iron layer to sup-

press hadronic particles. The active trigger system of

NaNu makes use of plastic scintillators, which are lo-

cated in the front as well as in the back of the emul-

sion layers, where they can be also used to veto muon

signatures. In addition, the self-triggering capabilities

of micromegas detectors can be used to record events

which only leave signatures within the emulsion target.

The location of the NaNu Experiment is foreseen

50 m after the beam dump, i.e. behind the SHAD-

OWS experiment. The distance to the beam axis is

chosen to minimize the expected muon background.

Muons for the SHADOWS experiment are shielded by

dedicated magnetized iron blocks. The expected muon

background for 4 ×1019 proton on target in the trans-

verse plane to the beam line is shown in Figure 4. The

neutrino ﬂux and the neutrino energies increase when

moving towards the beam-line, however, also the muon-

ﬂux increases signiﬁcantly. It is typically assumed that

the reconstruction algorithms of emulsion detectors can

handle ≈106tracks per cm2. The position of NaNu

emulsion detector in the transverse plane was therefore

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NaNu:ProposalforaNeutrinoExperimentattheSPSColliderlocatedattheNorthAreaofCERNFriedemannNeuhaus,MatthiasSchott,ChenWang,andRainerWankecorrespondingauthorAbstract.Severalexperimentshavebeenproposedintherecentyearstostudythenatureoftauneutrinos,inparticularaimingforarstobservationoftauanti-neutrino...

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NaNu Proposal for a Neutrino Experiment at the SPS Collider located at the North Area of CERN Friedemann Neuhaus Matthias Schott Chen Wang and Rainer Wanke

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