Superbeams and Neutrino Factories - Two Paths to Intense ... · at NuMI (MINERvA, NOvA) and DUNE in the future, as well as the xed target experiments SeaQuest and test beam facility

February 26, 2020 16:49 WSPC/INSTRUCTION FILE output

Modern Physics Letters Ac© World Scientific Publishing Company

Superbeams and Neutrino Factories - Two Paths to Intense

Accelerator-Based Neutrino Beams

VLADIMIR SHILTSEV

Fermi National Accelerator Laboratory

Batavia, IL 60510, [email protected]

Received (Day Month Year)

Revised (Day Month Year)

High energy and high beam power accelerators are extensively used for the neutrino

physics research. At present, the leading operational facilities are the J-PARC facility

in Japan which recently approached 0.5 MW of the 30 GeV proton beam power andthe Fermilab Main Injector complex that delivers over 0.75 MW of 120 GeV protons

on the neutrino target. Besides such type of neutrino superbeams, the concept of neu-

trino factories offers a great promise for particle physics research. In this brief reviewwe present the status and planned upgrades of the J-PARC and Fermilab accelerators

and leading proposals for the next generation accelerator-based facilities of both types,their challenges and required and ongoing accelerator R&D programs aimed to address

corresponding performance and cost risks.

Keywords: Accelerators; neutrinos; neutrino factories.

PACS Nos.: 13.15.+g, 14.16.Pq, 29.20.-c, 29.20.Lq, 29.25.-t, 29.27.Bd

1. Introduction: Accelerators for Neutrino Research

Neutrinos are amazing particles being among the most abundant in the universe but

rarely interacting with matter, deeply connected with outstanding scientific myster-

ies of modern days, such as matter–antimatter asymmetry in the universe, but full of

unexpected surprises and discoveries as attested by Nobel Prizes in Physics in 1988,

1995, 2002 and 2015. They are produced naturally in big numbers in thermonuclear

reactions in the Sun and nuclear power plans on Earth, in cosmic, atmospheric and

geological processes, but production of high-intensity neutrino beams using pro-

ton accelerators offers arguably the most opportunities for studies as such beams

are much better controlled in terms of neutrinos energy, initial type, distance to a

detector, timing, etc.1 Such beams have been instrumental discovery tools in par-

ticle physics and offer hints of physics beyond the Standard Model.2,3 Two main

schemes, usually called superbeams and neutrino factory beams, have many things

in common, but also have many differences in their setups. We therefore highlight

the most important features and differences in this section.

1

FERMILAB-PUB-20-087-AD-APC

This manuscript has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy, Office of Science, Office of High Energy Physics.

DOI: 10.1142/S0217732320300050

(accepted)


2 Authors’ Names

Fig. 1. J-PARC neutrino beamline. 30 GeV protons out of the Main Ring synchrotron are directedinto 90 cm long graphite target composed of graphite rods and produce secondary particles. Among

those, π+ mesons converge in the forward direction under the focusing effect of magnetic horns,

which powered by a few hundred kA current pulses synchronized with each beam shot. On a flightin a 100-m-long tunnel the pions decay into µ+νµ pairs. The muon neutrino (and a small fraction

of muons) leave the facility, whereas all the other particles such as the remaining protons and

undecayed π-mesons are absorbed by a beam dump composed of large graphite blocks. The profileof the muons that penetrate the beam dump is used as an indirect monitor of the νµ beam direction.

Neutrino detectors located 280 m downstream of the target can measure the profile, purity, and

energy distribution of the muon neutrinos. The neutrinos further travel 295 km through the Earthtoward large Super-Kamiokande water Cherenkov detector (adapted from7).

Superbeams4–6 are based on conventional beam dump techniques for producing

neutrino beams. An intense high energy proton beam is directed onto a thick nuclear

target producing mostly pions and kaons, which are captured by an optical system of

magnetic horns in order to obtain well directed beam of same charge secondaries (see

Fig.1. High-energy neutrino beams are products of the decays of charged pions and

kaons in a long decay channel. Shielding downstream of the decay channel removes

the undecayed secondary mesons and remaining protons, but transmits the weakly

interacting neutrinos. If positively charged pions and kaons have been selected for

the decay channel, the resulting beam downstream of the shielding will contain

mostly muon neutrinos produced in π+,K+ → µ+νµ decays and some 0.5% of

electron neutrinos produced in the three-body decays K+ → e+π0νe. At sufficiently

high primary proton energy, the beam will contain a small ντ component coming

predominantly from prompt tauonic decays of Ds mesons. Such mixed neutrino

beam composition is, in general, not ideal for neutrino experiments. Antineutrino

beams can be made by using a negatively charged meson beam. Superbeams strive

to operate with a proton beam intensity closer to the mechanical stability limit of

the primary target which is O(1 MW). Placing the far detector few degrees off the

neutrino beam axis reduces the overall neutrino flux, but leads to an important

shrinkage of the neutrino energy spectra and relative suppression of the electron

neutrino contamination.

At neutrino factories, 20 to 50 GeV muons, generated in pion decays same way

as in superbeams, are stored in the long straight sections of a storage ring.8 This

would produce a well-directed beam with a precisely known mixture of neutrino

types; namely, 50% νµ and 50% ν̄e if a µ− beam is stored, and 50% ν̄µ and 50% νeif a µ+ beam is used. Besides the benefits of the neutrino type knowledge, produc-


Instructions for Typing Manuscripts (Paper’s Title) 3

tion of a neutrino beam using muon decays rather than meson decays allows precise

and easy calculation of the absolute neutrino fluxes from the measurements of the

stored muon beam current and momentum. Another great advantage of the neutrino

factory comes from the νe and ν̄e in the beams since the flavor oscillations νe ↔ νµand νe ↔ ν̄µ play a special role in neutrino physics measurements. In a conventional

beam the initial flavor is νµ and experiments must search for νµ → νe transitions.

The experimental sensitivity is eventually limited by a small but annoying νe com-

ponent in the initial beam that makes it difficult to probe oscillation probabilities

below O(10−2). However, in a neutrino factory experiment it is straightforward to

suppress the backgrounds down to ≤ O(104) of the total event rate by looking for

a “wrong-sign muon” in the detector, i.e. a muon of opposite sign to the muons

stored in the neutrino factory, that would be an indication of νµ appearance from

νe → νµ transitions. Comparative analysis of physics potentials of both methods

can be found in.9

Of course, the neutrino factories are more technically complex as the muons

need to be not only produced and collected, as in the superbeams, but also, af-

terwards, cooled and very quickly re-accelerated.10 While corresponding techniques

have been separately demonstrated, the integrative design and construction of a

neutrino factories is the task for the future - see discussion below.

2. J-PARC and Fermilab Facilities

Currently, the most powerful accelerators for the neutrino research are under oper-

ation at J-PARC (Tokai, Japan) and at Fermilab (Batavia, IL, USA). Construction

of the Japan Proton Accelerator Research Complex (J-PARC) was started at the

Tokai site of JAEA in 2001 and finished in 2009.15 The facility consists of three

accelerators: 400 MeV linac, 350m circumference 3 GeV proton rapid cycling syn-

chrotron (RCS) and a 1.6 km circumference 30 GeV main ring (MR) synchrotron.

The design beam power goal for the RCS and MR are 1 MW. The particle research

facilities constructed at J-PARC are the Materials and Life Science Facility (MLF)

for usage of muon and neutron beams, and the Nuclear and Particle Physics Facility

(Hadron Facility) for K-meson (Kaon) beam and the Neutrino Facility for neutrino

beam which is now sent 295 km away to the Super-Kamiokande experiment (see

Fig.1).

Fermilab accelerator complex - see Fig.2 - includes 400 MeV H- pulsed normal-

conducting RF linac, 8 GeV proton Booster synchrotron, 8 GeV Recycler storage

ring that shares a 3.3 km tunnel with the 120 GeV proton Main Injector (MI)

synchrotron, and a 3.1 GeV muon Delivery Ring. A number of beamlines connect

the accelerators, bring the beams to fixed targets and to various high energy physics

experiments. The most notable future additions (dotted lines) include the LBNF

beam line for DUNE and 0.8 GeV CW-capable SRF PIP-II linac located inside the

Tevatron ring and the corresponding beamline for injection into the Booster.11,12

The complex supports a number of experiments – e.g., the 400 MeV Linac beam


4 Authors’ Names

Fig. 2. Fermilab accelerator complex (from11).

is sent to the Mucool Test Area, 8 GeV protons from the Booster are supplied

to the 8 GeV Booster Neutrino Beam (BNB), ANNIE, MicroBooNE, MiniBooNE,

MITPC, ICARUS, and SBND, and to the “muon g-2” and “Mu2e” (near future)

muon experiments. The 120 GeV MI proton beam supports neutrino experiments

at NuMI (MINERvA, NOvA) and DUNE in the future, as well as the fixed target

experiments SeaQuest and test beam facility. See Refs.13,14 for detailed information

on these experiments. The Fermilab accelerator complex has delivered neutrino

beam with over 85% uptime on average over the past 5 years.12

Fig.3 presents to date progress of these high-energy high- power accelerators for

neutrino research: the J-PARC facility in Japan has achieved 475 kW of the 30 GeV

proton beam power,16 the Fermilab Main Injector delivers record high 760 kW of

120 GeV protons.12

3. Paths to Beam Power Increase

Average proton beam power on the neutrino target is scales with the beam energy

Eb, number of particle per pulse (PPP) Nppp and cycle time Tcycle as :

Pb =EbNpppTcycle

. (1)



Correspondingly, there are several ways to increase Pb: a) the ”brute force” approach

- to increase the beam energy, that would require either better or new magnets and

RF acceleration system, and/or to decrease the cycle time (again, new magnets and

RF might be needed); the key challenges along that path will be the cost of the

upgrade (for the reference, the total project cost of the J-PARC facility is about

$1.7B) and significant increase of the facility AC power consumption; b) another

approach is to increase the number of protons per pulse (PPP) - where the key

challenges mostly associated with numerous beam dynamics issues and, sometimes,

with the cost. In both cases one would need reliable horns and targets to shape the

secondary beams. There the challenge is that the operational lifetime of the targets

gets worse (shorter) with the power increase and frequent replacements of them

may pose serious impediment on facility’s uptime.

Modern high power proton RCSs are quite expensive, their cost is second only

to that of colliders and the magnets and associated power supplies constitute 40%

to 50 % of the cost while the tunable frequency RF cavities and RF power sources

are another 15-25%. Also, the magnets and RF cavities are major consumers of the

AC power: e.g. FNAL MI 1.7 T 3 T/s magnets use 9 MW of power, while the RF

system uses another 2.5 MW. The RCS overall power efficiency is a serious issue -

e.g. production of some 0.5 MW beams at J-PARC comes with about 40 MW site

AC power. Still, the upgrade of the J-PARC main magnet power supply will allow

reduction of the cycle time from 2.48 s to 1.32 s and proportional increase in Pb -

see Fig.3.

Fig. 3. Fermilab and J-PARC proton beam power history and upgrade plans.

There are several approaches actively pursed by the RCS machine designers


6 Authors’ Names

and engineers for the power upgrades and future machines such as more efficient

SC magnets, like 4 T/s ones for the FAIR project in Darmstadt (Germany) built

with NbTi SC cables17 or 12 T/s HTS-based magnet prototype recently tested at

FNAL;18 FFAG(fixed field alternating gradient) accelerators;19 also, the focus is on

more efficient power supplies with capacitive energy storage and recovery, and on

more economical RF power sources such as 80% efficient klystrons, magnetrons, and

solid-state ones (compare to current ∼ 55% ).20

The utmost PPP challenge is about how to lower the beam losses while increasing

intensity Nppp. The tolerable uncontrolled radiation level in accelerator enclosures

is typically about W ∼1 W/m, so the fractional beam loss must be kept under

∆NpppNppp

≤ W

Pb∼ W

Npppγ, (2)

i.e., the limit should go down with increase of beam intensity, energy and power. On

the contrary, repelling forces of the proton beam’s own space-charge lead to increase

of beam sizes and particle losses at higher beam intensities. In circular accelerators,

the empirical space-charge parameter limit is observed at21

∆QSC =NprpBf4πεnβγ2

≤ 0.3− 0.4. (3)

Here Np is the single bunch intensity (Nppp devided by the number of bunches in

the ruing), rp is classical proton radius, Bf is the bunching factor (ratio of the peak

to average bunch current), εn is the normalized beam emittance proportional to the

square of the beam size σ2. Beyond the limit, the losses grow unacceptably with

increase of the beam intensity that violates the condition Eq.(2).

There are several lines of attack on the problem: i) to increase the injection

energy into the RCS - that allows to gain the intensity Np ∼ βγ2, but requires

(often - costly) injection linacs (linacs have an anvantage of much faster average

acceleration of about 5-20 MeV/m compared to 0.002-0.01 MeV/m in the rings,

thus, protons get through lower energies faster and not being blown up by the space

charge forces; one of the limitations of the high intensity linacs is the intrabeam

stripping of electrons off H− particles); ii) to employ larger aperture magnets which

would allow certain blowup of the transverse rms size σ without the beam halo

hitting the aperture and resulting in the losses - this method can also be expensive;

iii) to flatten the beam current pulse shape by using the 2nd harmonics RF and

reduce Bf , sometimes by as much as a factor of 2; iv) to use the transverse painting

(shaping) of the beams via charge-exchange injection to linearize the SC forces; v) to

design better efficiency collimation system, say, from the current ∼60-80% to 95% -

that will not change the total loss but will result in a lower uncontrolled irradiation

due to the particles which might avoid dedicated collimators or dumps and scattered

around the ring; vi) to make the beam focusing lattice perfectly periodic, e.g., like

in the Fermilab Booster RCS with periodicity of P=24, or in the J-PARC MR

with P=3 ; vii) to introduce Non-Linear Integrable Optics elements to reduce the

losses;22 viii) to employ electron lenses for the space-charge compensation.23 The



last two approaches are novel and are being tested at the dedicated IOTA ring at

Fermilab.25 There are other intensity dependent issues in the RCSs for HEP, such

as efficient injection of high power beams (that requires stripping electrons off H-

particles by foils or lasers), electron cloud effects and coherent beam instabilities.

In general, RCSs for the neutrino research employ several of these techniques in

their quest for higher beam power on target. E.g., the J-PARC accelerators follow

the above items ii, iii, iv, vi, while the Fermilab’s next complex upgrade, called

the Proton Improvement Plan-II (PIP-II) project, is to replace the existing 400

MeV normal-conducting RF pulsed linear accelerator with a new 800 MeV machine

based on superconducting RF cavities, capable of CW operation. The PIP-II linac

construction is started in the Spring of 2019. The linac will allow an increase of the

120 GeV proton beam power available to the new LBNF beamline to 1.2 MW. In

addition, the 8 GeV Booster RCS rate will be increased from 15 to 20 Hz, allowing

full Main Injector beam power to be achieved at the lower energy of 60 GeV (vs

current 120 GeV), and 80 kW of beam for the 8 GeV neutrino program.26 PIP-II

linac will be part of eventual extension of beam power to LBNF/DUNE to more

than 2 MW (PIP-III - either a SRF linac or an RCS27) and will also provide a flexible

platform for long-range development of the Fermilab complex; in particular, provide

an upgrade path for a factor of 10 increase in beam power to the Mu2e experiment,

and for extension of accelerator capabilities to include flexible high-bandwidth pulse

formatting/high beam power operations.12

The J-PARC team also explores possibilities for the multi-MW beam power for

neutrino experiments, such as construction of a new 8-GeV Booster in addition

to their existing 3 GeV RCS to attain 3.2 MW out of the MR and even a new 9

MW power machine with a 9-GeV proton driver consisting of three SRF linacs (1.2

GeV, 3.3 GeV and 6.2 GeV) in the straight sections of the KEKB tunnel which can

operate after the conclusion of the Super KEK B-factory project.16

As mentioned above, issues associated with high power targetry, such as the

radiation damage and thermal shock-waves, are critical and depend on pulse struc-

ture. Existing neutrino targets and horns are good to about 0.8 MW beam power,

and MW and multi-MW targets are under active development and prototyping.

Ongoing R&D program includes studies of material properties, new forms (foams,

fibers), new target designs (e.g., rotating or liquid targets).24 In addressing these

issues we learn from lower energy but record 1.4 MW beam power machines such

as PSI in Switzerland and the SNS in the US.

4. Future Neutrino Superbeams Proposals

Four proposals of accelerator-based facilities for neutrino physics are presented be-

low. Below, I will reference only the corresponding brief inputs to the 2019 European

Particle Physics Strategy Update symposium (EPPSU, May 2019, Granada, Spain)

where all of them were presented. More details on each of the proposals can be

found therein. Table 1 summarizes main parameters of these experiments.


8 Authors’ Names

4.1. Protvino-to-ORKA

The Protvino accelerator facility is located some 100 km south of Moscow in Russia.

It currently consists of a 30 MeV linear accelerator, and a 1.5 GeV booster syn-

chrotron and the 1.5 km circumference U-70 synchrotron, which accelerates protons

up to 70 GeV. Typical proton intensity of up to 1.5·1013 protons per 10 s cycle

results in the average beam power up to 15 kW. The proposed upgrade of the in-

jection scheme will make it possible to increase beam intensity to 5·1013 protons or

more to provide some 75-90 kW of the beam power 75 kW with a 7 s cycle. With a

proper proton beamline and neutrino target to be built, the will allow to generate

and direct a neutrino beam from Protvino towards the KM3NeT/ORCA detector

which is currently under construction in the Mediterranean sea 40 km off shore

Toulon, France. The Protvino-to-ORCA experiment, would yield an unparalleled

sensitivity to matter effects in the Earth, allowing to determine the neutrino mass

ordering with a high level of certainty due to its long baseline of 2595 km after

only 5 years of operation after a possible start in 2026.28 The second phase of the

experiment comprizing of a further intensity upgrade of the accelerator complex

to 450 kW operation and a significant modification of the ORCA detector would

start ca. 2035 and allow for a competitive and complementary measurement of the

leptonic CP-violating Dirac phase with a megaton detector.

4.2. ESS Neutrino Super Beams (ESSνSB)

The 1.83E Euro European Spallation Source (ESS) is currently under construction

in Lunbd (Sweden). It will start high power operation in early 2020’s and employ a

600 m long SRF 2 GeV proton linac. At the average beam current of 62.5 mA and

4% duty factor it will deliver 2.8 ms beam pulses with average power of 5 MW on a

spallation target. The total site AC power requirement is about 32 MW. The beam

power can be raised to 10 MW by increasing the accelerator duty cycle from 4% to

8% and the additional 5 MW used to generate a uniquely intense neutrino Super

Beam (ESSνSB) for measurement of leptonic CP violation.29 Tentative schedule

calls for the project’s CDR in 2021, and TDR 2024 and construction period of

2026-2029. Besides the upgrade of the linac repetition rate from 14 Hz to 28 Hz,

it should switch from operation with protons to operation with H- particles. An

accumulator ring with 400 m circumference will need to be built to compress to

the beam pulse to µs - see Fig.4. Due to very short beam pulse, the required 5MW

neutrino target station will be much more challenging than the 5 MW ESS neutron

spallation target. One should also expect - and address - very strong space charge

effects both in the linac and in the accumulator ring.

ESSνSB is expected to be complementary to other proposed Super Beam exper-

iments by the fact that the resulting high intensity O(0.3 GeV) neutrino-beam is

directed towards the north in the direction of the Garpenberg mine, 540 km away,

which could host the far 1 megaton water Cerenkov detector, at the location of

the second neutrino oscillation maximum, making the performance of ESSνSB for



Fig. 4. Scheme of the ESS Super Beam (ESSνSB) accelerator complex that includes 2 GeV H-

linac, a 400-m circumference accumulator ring, 5 MW target station, and neutrino near detector.30

leptonic CP violation precision measurements highly competitive. The total cost of

the ESSνSB is estimated to be 1.3 B Euros.

4.3. ENUBET Short Baseline Facility at CERN

The ENUBET collaboration has proposed a short baseline experiement based on

the CERN’s SPS proton synchtron to carry out high precision measurements of the

neutrino cross sections as function of energy.31 The simplest implementation of this

facility can be based on a conventional fast extraction of the SPS 400 GeV protons,

a µs horn and a ∼ 40 m narrow band transfer line. The ultimate average beam

power out of the SPS can be as high as about 0.5MW achieved during the CNGS

operation. The central energy of secondaries (pions, kaons) will be 8.5 GeV that

will result in 0.5-3.5 GeV neutrino’s.

5. Neutrino Factory Proposals

The neutrino factory (NF) provides very high intensity neutrino and antineutrino

beams which are exact CP conjugates - a total of O(1021) muon decays per year. The


10 Authors’ Names

flavor content and energy spectrum as well as the total flux can be determined to

better than 1%, which, combined with the great flexibility in neutrino energy, makes

NF the ideal source for precision neutrino physics. Moreover, the beam contains

equal numbers of muon and electron flavors and therefore, it is possible to directly

measure the relevant cross sections, including nuclear effects, in the near detector.

Fig. 5. Schematic diagram of the 25 GeV Neutrino Factory accelerator facility.32

The functional elements of a Neutrino Factory, illustrated schematically in Fig.5,

are: i) a proton source producing a high-power (1-4 MW) multi-GeV bunched proton

beam; ii) a pion production target that operates within a high-field solenoid. The

solenoid confines the pions radially, guiding them into a decay channel; iii) a solenoid

decay channel; iv) a phase rotation system of RF cavities that captures the muons

longitudinally into a bunch train, and then applies a time-dependent acceleration

that increases the energy of the slower (low-energy) bunches and decreases the

energy of the faster (high-energy) bunches; v) a cooling channel that uses ionization

cooling to reduce the transverse phase space occupied by the beam, so that it fits

within the acceptance of the first acceleration stage; vi) an acceleration scheme that

accelerates the muons to the final energy of 5 – 25 GeV; vii) 5-25 GeV “racetrack”

storage ring(s) with long straight sections pointed towards neutrino detectors. The



cost of NF depends on the proton driver power, total RF acceleration required,

strength and total length of bending magnets and the scale of the civil construction

and is estimated to be in the range of 4.7-6.5 BEuro.33

Performance of the NF depends on the power of the proton driver and the

degree or absence of the muon cooling. The ionization cooling method though rel-

atively straightforward in principle, has some practical implementation challenges

such as RF breakdown suppression and attainment of high accelerating gradients

in relatively low frequency normal-conducting RF cavities immersed in strong mag-

netic fields. The International Muon Ionization Cooling Experiment (MICE) at RAL

(UK) has recently demonstrated effective O(10%) reduction of transverse emittance

of initially dispersed 140 MeV/c muons passing through an ionization cooling chan-

nel cell consisting of a sequence of LiH or liquid Hydrogen absorbers within a lattice

of up to 3.5 T solenoids that provide the required particle focusing.34 The MERIT

experiment at CERN has demonstrated that the liquid Mercury target system can

sustain an intense primary proton beam of 4MW power and more.35 While key NF

beam physics challenges have be addressed, several important accelerator technol-

ogy and cost feasibility issues are subject of ongoing R&D and conceptual design

optimization.10,36

Fig. 6. Scheme of the νSTORM accelerators : proton beam from the CERN SPS (or other high

power proton accelerator) hits the target, the resulting muons are collected and injected into a

585 m racetrack storage ring where they circulate for hundred turns decaying into the very welldirected neutrino beam.

5.1. νSTORM

The basic idea for νSTORM facility (the production of neutrino beams from the

decay of muons in a racetrack-like decay ring) can be considered as the first step on

the path toward the realization of a neutrino factory or a future µ+µ− collider.37

The Neutrinos from Stored Muons (νSTORM) facility has been designed to deliver

a definitive neutrino-nucleus scattering programme using beams of electron and

muon antineutrinos from the decay of muons confined within a storage ring.38 Pro-

tons from the CERN’s SPS are sent onto a target and converted into muons. The

µ± beams with a central momentum of between 1 GeV/c and 6 GeV/c and a mo-

mentum spread of 16% are injected and stored in a racetrack-shaped ring, oriented

toward neutrino near-detector some 50 m away and a far-detector 2 km away - see


12 Authors’ Names

Fig.6. At νSTORM, the flavour composition of the beam and the neutrino-energy

spectrum are both precisely known and the storage-ring instrumentation will allow

the neutrino flux to be determined to a precision of 1% or better.

The facility does not call for the record power out of the SPS - it requires only

156kW of 100 GeV protons (4·1013 protons per pulse in two 10 µs fast extractions

with Tcycle=3.6 s). The major challenges of the proposal is necessity to have a large

diameter (0.5 m) magnets to accept most of the secondary muons and a sophisticated

focusing lattice which should assure survival of about 60% of muons after 100 turns

with the 10%rms beam momentum spread. Cost estimate for such a facility if built

at CERN is 160 MCHF.

Conceptually close to νSTORM is the idea of Beta-beams39 which exploits the

β-decay of relatively short-lived radio-isotopes such as 18Ne or 6He with half-lives

of around 1 s which are ionised, accelerated to relativistic speeds and put into a

racetrack storage ring, where they eventually decay. The maximum neutrino energy

that is available is given by the end-point of the β-spectrum Qβ , typically in O(10

MeV), and the Lorenz boost γ ∼ 50-500, so Emax = γQβ can be as high 5 GeV.

All suitable isotopes are too short-lived and need to be produced artificially and

the achievable production rates O(1018/yr) ultimately limit the luminosity of the

Beta-beams. Another drawback of the scheme is its estimated cost range of 1.4-2.3

BEuro.33

Table 1. Modern and future proton accelerators for high energy neutrino research.

Facility Eb, GeV Tcycle, s Nppp, 1013 Pb, MW Year

FNAL MI 120 1.33 5.2 0.76 2019

(PIP-II) 120 1.2 7.6 1.2 2026

(PIP-III) 120 1.2 15 2.4 2030’sJ-PARC 30 2.48 25 0.475 2019

30 1.16 43 1.3 2028

ESSνSB 2 0.071 110 5 ca.2030

ORKA/P 70 7 5 0.09 ca.202670 7 25 0.45 ca.2035

ENUBET 400 5.8 2.25 0.51 (tbd)

νSTORM 100 3.6 4 0.16 (tbd)

25 GeV ν-Factory 5 0.02 10 4 (tbd)

6. Summary

Over the past decade we witnessed impressive progress of the high-energy high-

power accelerators for neutrino research - J-PARC facility in Japan has approached

0.5MW of the 30 GeV proton beam power, the Fermilab Main Injector delivers over

0.75MW of 120 GeV protons. The needs of neutrino physics call for the next gener-

ation, higher-power, megawatt and multi-MW-class superbeams facilities. Some of

them are already under construction, such as, e.g., Fermilab’s PIP-II linac upgrade.



In general, it is expected that MW-class and, especially, multi-MW beam facili-

ties will face many challenges. Hence, comprehensive accelerator R&D is required

and, in many cases - is started, such as on the cost saving technologies for efficient

rapid cycling magnets and RF sources, on the control of space-charge effects, insta-

bilities and beam losses (some counter-measures can be tested at the operational

machines and at the IOTA ring at Fermilab), on the multi-MW neutrino targets

and horns, and on the muon ionization cooling. There is also a number of proposals

which show significant scientific promise and should be further studied - such as of

ESSvSB, Protvino/ORKA, ENUBET, and the νSTORM which will represent the

first step towards future neutrino factories. The latter is synergistic with world-wide

muon collider accelerator physics R&D activities which were recently re-energized

by experimental demonstration of the muon ionization cooling.

Acknowledgments

I would like to acknowledge very fruitful discussions on the subject of this presen-

tation and thank Yuri Alexahin, Alan Bross, Paul Czarapata, Paul Derwent, Jeff

Eldred, Steve Holmes, Valeri Lebedev, Sergei Nagaitsev, Bill Pellico, Eric Stern,

Cheng-Yan Tan, Alexander Valishev, Bob Zwaska (all – Fermilab), Eric Prebys

(UCD), Frank Schmidt (CERN), and David Bruhwiler (RadiaSoft). Some of this

material has appeared in other forms written by the author.

Fermi National Accelerator Laboratory is operated by Fermi Research Alliance,

LLC under Contract No. DE-AC02-07CH11359 with the United States Department

of Energy.

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14 Authors’ Names

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Documents

Superbeams and Neutrino Factories - Two Paths to Intense ... · at NuMI (MINERvA, NOvA) and DUNE in the future, as well as the xed target experiments SeaQuest and test beam facility