Strategies for Future Accelerator Neutrino Physics

Vittorio Palladino

University Federico II and INFN Napoli, Italy

Abstract

Accelerator neutrino (ν) physics has come back to the forefront, with the discovery of ν transitions, our first andunique window beyond the standard model. The experimental program to provide a complete map of the ν mixingmatrix, including its far reaching CP violation sector, and test its unitarity constraints is likely to extend over severalfuture decades, as it has been for quark mixing. So far, conventional ν beams based on pion (π) decay have been usedand more are already being planned, at higher power (superbeams), in Japan and the US, in conjunction with largeror novel detectors. Superbeams have limited potential, however. Novel very intense beams of ν parents, longer livedthan π’s, accelerated and then coasted in a decay storage ring replacing the π decay tunnel, promise the ultimate reach.R&D for muon decay ring (ν factory) and ion decay ring (betabeam) experiments is thus a decisive task today.

Keywords: Neutrino, Neutrino Oscillations, Neutrino beams, Neutrino Detectors, Neutrino R&D

1. Introduction

This talk is an introduction to those following it inthis morning session, that will in turn introduce the fewmost promising future projects under study. SomewhatEuropean in perspective, it will try to keep an interna-tional breath.

In the summer of 2006, the CERN Council StrategyDocument suggested that Europe should be ”in positionto define the optimal accelerator ν program · · · in around2012”. To match this challenge, the EUROnu DesignStudy (DS) is preparing design reports for three possiblefuture ν beam facilities (a superbeam, betabeam and ν-factory) and the LAGUNA DS is preparing feasibilityreports of underground sites capable to host new, verylarge far ν detectors. On the basis of these studies andof the international context, the NEu2012 Network ispreparing to propose a strategy road map for the nextrevision of the CERN Council Strategy in 2012 or so.

A Eu ν strategy must inevitably be a global strategy.Internationally, Japan is defining [1] its plans for the fu-ture of JPARC, where large US and Eu teams are active.In the US, around Fermilab, plans are being made fora new conventional ν beam [2] from the Main Injector

while, simultaneously, the US muon (μ) accelerator pro-gram [3] is supporting the International Design Study ofa ν-Factory (IDS-NF) at least as much as European andother partners.

A long term strategy is needed. Quark mixing firstemerged in 1953, with the discovery of strangeness, al-most 60 years ago. The CKM mixing matrix has beenthe object of intense studies since and we are today stillbuilding strangeness, charm and beauty factories. Thestudy of the ν PNMS mixing matrix just began and hasa long way to go [4]. Its 3 by 3 nature has still to beestablished. Its CP violating (CPV) phase has to bemeasured. Its unitarity and invariance properties (CP,T, CPT) have to be mapped. Many of these answers canand will only come from high energy neutrinos. Novel,superior ν beams appear to be, eventually, inevitablyneeded.

Anticipating it, the natural conclusion of this talk isthat the accelerator ν communities face today two si-multaneous tasks:

1) do the experiments we can do or propose today,progressing as much as possible with conventional νbeams and the ν detectors presently possible. Larger

Available online at www.sciencedirect.com

Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 217–222

0920-5632/$ – see front matter © 2012 Elsevier B.V. All rights reserved.

www.elsevier.com/locate/npbps

doi:10.1016/j.nuclphysbps.2012.09.035

detector mass and beam power, in this order of prioritytoday, are the weapons of this next attack.

2) prototype the ultimate experiments, i.e. design,prototype and build the third possible weapon, novel νbeams, betabeam [5] or/and ν-factories [3], storage anddecay rings instead of a decay tunnel, capable of moreν per unit time and unit beam power.

Such a long term program, second generation conven-tional (super)beams first and ultimate neutrino produc-tion rings later, can be sustained only if the indispens-able R&D effort that it implies is strengthened today andkept at adequate levels in the years ahead.

2. The study of neutrino transitions

It is one of the frontier subjects of particle physicstoday. At this conference, the highlights are the firstappearance ντ candidate in OPERA [7] and first ν eventin ICARUS [8]. About 50% of the sessions had talks onthese studies or on studies ancillary to them.

The independent existence of two ν flavor transitionsis today very solidly established [4]. They both indi-cate a large flavor mixing and an oscillatory modulationin L/E, respectively with wavelength of 500 Km/GeV(atmospheric) and 15000 Km/GeV (solar). The favoredexplanation today of all available data is that:

1) the known 3 ν flavor eigenstates νe, νμ, ντ are infacteach a linear mix of three ν mass eigenstates ν1, ν2, ν3of mass m1, m2, m3

2) a 3 by 3 complex matrix governs the strength ofthe flavor mixing. It contains four physical quantities,three mixing angles θ12, θ23, θ13 and one CP violatingphase δ.

The two detected transitions are then part of a largerset of phenomena still to be observed.

The measurements of the solar and atmosphericwavelengths respectively imply a best value of 7.6 10−5

eV2 for the squared mass difference δm212 and a common

value of 2.4 10−3 eV2, 30 times greater, for the othertwo differences, δm2

13 and δm223. The measured mixing

strength is large for both transitions, very much unlikequark transitions. The solar mixing sin2θ12 turns out tobe about 1/3, while the atmospheric mixing sin2θ23 iseven larger, intriguingly close to 1/2, its maximal value.

Atmospheric transitions, that behave much as if theywere mostly νμ → ντ, fit well terrestrial experimentsaiming a ν beam at a massive far ν detector. These ex-periments can be tightly controlled and precise, we ex-pect now from them the measurement of the three deci-sive physical quantities that are still unknown.

The third mixing parameter sin2θ13 has not yet beenmeasured. It is not large, an experimental upper bound

of about 3% follows from the failure to detect, so far,νμ → νe transition over the flight path (baseline) of theatmospheric transitions. Detection of that transition orof its inverse transition νe → νμ is essential to establishthe 3 by 3 nature of ν mixing.

The subdominant νμ ↔ νe transition is also the onewhere the largest effects from the CP violating phase δare expected. Its experimental signature would be anasymmetry between ν and ν transition rates and/or anasymmetry between direct and inverse transition rates(time reversal).

This subdominant direct or inverse channel is ex-pected to provide also the third and last piece of infor-mation still missing, the sign of δm2

atm, that is the signof δm2

13 and δm223 that determines whether m3 is much

larger (normal νe mass hierarchy) or much smaller (in-verted hierarchy) than m1 and m2. An additional asym-metry between ν and ν transition rates on top of the onedue to a non zero CPV phase must exist, due to the dif-ferent interaction of ν and ν with electrons when propa-gating through matter over large distances. The sign ofthis asymmetry can tell us what is the correct hierarchy.

If νμ ↔ νe promises the largest rewards, all ninepossible transitions among ν flavors should be studied.The history of quark mixing shows us that unitarity andinvariance properties (CP, T, CPT) of all rows of theν mixing matrix will ultimately have to be preciselymapped. In practice, the three possible transitions ofνe and the three of νμ are accessible to measurements,while transitions of ντ presently appear unfortunatelyout of experimental reach.

Conventional beams have so far provided us with thebest tool to study νμ transitions over atmospheric base-lines. The limitation of their low rates can be miti-gated by assembling larger far detector masses and us-ing higher power proton drivers (superbeams). Up toa point, however. The rate limitation, their irreduciblecontamination of νe’s and other experimental complica-tions have lead us to consider novel ν beams.

The novel idea common to these novel ν beams isstoring and coasting intense beams of longer lived νparents, beta emitting ions (betabeam) or/and muons (ν-factories), accelerated to high energies, inside storageand decay rings that replace the traditional π decay tun-nel.

In the case of betabeams, long lived beta emittingions produce pure νe or νe beams for thorough studiesof νe transitions. One can envisage to study all the ex-perimentally accessible transitions by a combination ofsuperbeam and betabeam experiments.

In the case of ν-factories, the ν parents are muons. μ+

beams produce simultaneous beams of νμ and νe, while

V. Palladino / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 217–222218

μ− beams do the opposite. As for betabeam, the mostrewarding ”golden” channel will be νe → νμ, but ν-factories promise eventually the best map of all accessi-ble transitions in a single unique well controlled experi-mental setup. The price to pay is magnetization of largefar detector volumes, necessary to separate experimen-tally the ν and ν interaction samples.

3. Current generation accelerator studies of neu-

trino transitions

To date, K2K and MINOS [6] have confirmed theoriginal atmospheric result. OPERA may be soon con-firming νμ → ντ dominance. Emphasis is therefore nowshifting to the subdominant νμ → νe transitions, still us-ing conventional beam coupled with far detectors withgood detection efficiency and low background for singleelectrons.

One new experiment exploiting a new conventionalbeam, T2K [9], is already taking data at JPARC. It stilluses the 22 kton fiducial volume of SuperKamioka, 295Km away. Present beam power (100 Kwatts) alreadyexceeds that available from the K2K beam but it is ex-pected to be steadily improving up to 700 Kwatt. It willbe also the first neutrino beam pointing at the far de-tector at a small off axis angle. This provides a betterdefined and lower neutrino energy that matches betterthe 500 Km/GeV wavelength.

A second experiment, NOνA [10], using off-axis theFermilab existing MuMI conventional beam, is in ad-vanced state of preparation. It is assembling a 20 Ktonstotally active scintillator detector (TASD) in a new offaxis far (800 Km) detector location in Minnesota andwill be soon taking data with an increasing fraction ofthe total detector. Plans do exist to make the beampower available to NOνA increasingly larger than whatNuMI has so far being delivering to MINOS, up to 700KW or so.

By 2018 or so, these two experiments will either dis-cover the νμ → νe transition and measure a non zerothird mixing parameter sin2(2θ13), thus proving the 3 by3 nature of the mixing matrix, or will impose on it a 1%or slightly lower upper limit on it.

Figure 1 illustrates the 3 σ discovery potential of thethird mixing parameter expected from these two accel-erator experiments and a number of reactor experimentsthat are also in preparation.

4. Next generation of studies of neutrino transitions

Beyond T2K and NOνA, plans are being made to startprobing the ν mass hierarchy and CP violation, still ex-

ploiting conventional beams. The key experimental fac-tor for further progress is sizeably larger detector mass,hundreds of KiloTons (KT) of instrumented water orequivalent, and larger, MegaWatts (MW) scale, protonbeam power. They must be inevitably the weapons offurther attack.

Both are quite a challenge. Assembling larger de-tector mass demands new larger underground sites. In-strumented Water Cerenkov tanks much larger than Su-perKamiokande will be a highly non trivial extrapola-tion. The Liquid Argon technique, that has larger effi-ciency and lower backgrounds, is only approaching ma-turity and we do not know yet how and when tanks re-ally larger than ICARUS will become important con-tributors.

It is somewhat obvious but still important, in this con-text, to remind the relevance of these detectors for theequally fundamental measurement of nucleon lifetime.

An upgraded T2K program is being planned, and afew possible candidate sites for new water tanks upto one Megaton are being envisaged. Baselines up toslightly above 1000 Km are being considered. Higherproton beam power is also a clear objective, the newT2K beam was designed to evolve to withstand up to4 MW. In the last few years, however, concern hasemerged about the possibility to go beyond 1.7 MWproton beam power with the JPARC synchrotron.

An entirely new NuMI beam, pointing to the new un-precedentedly large DUSEL underground detector lo-cation is next flagship project at Fermilab. An essentialfeature is its longer (1300 Km) baseline. It would be up-gradable to operate at 2 or more MW, thanks to a newproton driver, the X Project that is at the heart of Fer-milab future strategies. The Deep Underground Scienceand Engineering Laboratory at Homestake, S. Dakota(DUSEL) lab is now established as the site and for a300 KTs water tank and/or an equivalent mass of Li-Ar.The new NuMI project plans to go back to a wide bandbeam so to use (facing the novel challenge that are im-plied) also the distorsion of the νμ energy spectrum tomeasure the mixing parameters.

Less defined plans do exist in Europe too, where, justas in all regions, the relative merits of large water tanks(MEMPHYS) and of smaller, but more efficient, Li-Ar(GLACIER) tanks are being assessed.

5. Longer term prospects for neutrino transitions

The physics reach expected by a few variants of allmentioned future ν beam facilities, including the novelneutrino facilities that will be further detailed later, are

V. Palladino / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 217–222 219

���������� ��� ���� � ��

Figure 1: Expected 3 σ discovery potential of the third mixingparameter from T2K, NOνA and a number of reactor experimentsthat are also in preparation, for the normal ν mass hierarchy (NH).

ππππ beams

ββββ beams

NF μμμμ beams

sin22θθθθ13 discovery at 3σσσσ CL

EUROnu physics WP

limits of π beamsratebeam+detect bkgnds

s13eiδcoupling

Figure 2: Expected 3 σ discovery potential of the third mixingparameter of a few variants of all the future ν beam facilities men-tioned in the text.

summarized in the next three figures 2, 3, 4. Producedby the joint physics subgroups of IDS-NF and EUROnu,these figures were included in a recent report [11] fromthe Neutrino Panel of the CERN Scientific Policy Com-mittee (SPC).

Figure 2 shows the 3 σ discovery potential ofsin2(2θ13). It depends on the fraction of values of theCPV phase that can be simultaneously accessed. Thelimits of pion decay beams, due to their low rates andtheir irreducible νe content, are evident.

Figure 3 shows the 3 σ discovery potential of the signof δm2

atm, ie of the correct νmass hierarchy. A long base-line (LBL) is essential here, modest baseline superbeamexperiments have only limited or no reach.

Figure 4 shows the 3 σ discovery potential of a CPviolating phase δ.

The SPC Panel concluded that it is unrealistic to ex-pect a high intensity ν source of any kind in Europe be-fore 2020. On that time scale, Europe should rather payclose attention to the mentioned superbeam programsin Japan and USA. To be competitive in the 2020’s, Eu-rope should concentrate on the R&D for a new intensesource, ν factory or betabeam, decay ring facilities capa-ble to bring about neutrino beams of substantially higherneutrino flux and free of wrong flavor backgrounds.

An outstanding proton driver must inevitably be themotor of such a scientific program. At CERN a 4 MW(or more) high power superconducting proton linac (HPSPL) has been and is the subject of extensive and todaynow rather advanced study. It is the natural continuationof the so called LINAC-4, presently under construction.It promises even more potential than the present designof the Project X proton linac at Fermilab that in turnappears quite superior to the potential of the JPARC up-

grade program.ν factory or betabeam are two different, possibly

complementary, longer term approaches. Betabeamneutrinos are of lower energy and should be coupledwith detectors hundreds of Km away. Neutrino factoryneutrinos are of higher energies and are best exploitedby detectors thousands of Km away. Design reports forboth facilities are being prepared for 2012 by the EU-ROnu design study, that, in the neutrino factory case,operates within the larger international context of theIDS. Design will include the engineering work neces-sary to evaluate safety infrastructures and, last but notleast, costs. We will soon have more realistic ideas ofthe feasibility of these options and of the global effortimplied by their realization.

6. Betabeams

Details will be plentiful in E. Wildner’s talk. Thereare many variants of betabeam envisaged, only two arepresently seriously being studied. Very large WaterCerenkov and Li-argon detectors are again the naturalpartners of such betabeams.

The so called baseline betabeam envisages to produceν (ν) from 6He (18Ne) radioactive ions accelerated to γ= 100. Neutrinos have a few hundred MeV of energy,suited to CERN-Frejus like baselines. This is the mostmature and solid option that has now been object of sev-eral years of studies and can now be seen as almost es-tablished.

The second variant being studied envisages to pro-duce ν (ν) from higher Q longer lived 8Be (8B) radioac-tive ions still accelerated to γ = 100, with 1 GeV or so

V. Palladino / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 217–222220

Modest BLπ and β

LBLπ beam

EUROnu physics WP

NuFact’s

sign Δ Δ Δ Δm2atm discovery at 3σσσσ CL

Figure 3: Expected 3 σ discovery potential of the sign of δm2atm, ie of the correct νmass hierarchy, of a few variants of all the future ν beam facilities

mentioned in the text.

energy suited to CERN-Gran Sasso like baselines. EU-ROnu is its first exploratory study.

The physics potential of other very very promisingbetabeam options has been pointed out. It has beenshown that higher γ can be extremely rewarding. Butthe additional technical challenges involved will not yetbe seriously studied.

7. Neutrino Factories

Again, details will be plentiful in K. Long’s talk. Awell defined baseline neutrino factory design has beentaking shape over more than 10 years. First feasibilitystudies in the US at the turn of the century have been fol-lowed by an International Scoping Study first and nowthe International Design Study (IDS).

That includes the large magnetic detector indispens-able to exploit the factory. The concept of a 50-100Kton magnetized iron neutrino detector (MIND) hasbeen developed, a large version of MINOS or, if oneprefers, a very large version of the glorious CDHS de-tector.

The European contributions have been important, inparticular those of the UK component. It is a pleasureto recognize, however, the driving push coming from

the US, first via the NFMCC1, then also via the MTF2

at Fermilab where a dedicated Muon Test Area has beenbuilt and operates now. NFMCC anf MTF have recentlymerged in the US Muon Accelerator Program (MAP),even more solidly based and better funded. A muonbased view of Fermilab future is the real drive behind it,a neutrino factory being a possible step on the way to amuon collider that could really be the long term futureof the Fermilab laboratory.

8. Conclusions

The long term accelerator neutrino program outlined,second generation conventional (super)beams first andultimate neutrino production rings later, can be sus-tained only by a strengthened, very enthusiastic andwell supported R&D program. The several R&Ddemonstration projects in progress are largely inter-national, in particular for the ν factory, and Europeshould definitely do more in this sector. We will notbe ready, otherwise, when superior neutrino beams willbe needed.

1Neutrino Factory and Muon Collider Collaboration2Muon Task Force

V. Palladino / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 217–222 221

���� �� �������� � ���������������� ��������ν ��� �� ������ ������

NB IDS ←ISS ←BENE

Figure 4: Expected 3 σ discovery potential of a non zero CP violating phase δ of a few variants of all the future ν beam facilities mentioned in thetext. It includes the caption adopted for the SPC Panel report, detailing the meaning of each line.

References

[1] K. Sakashita’s contribution to these Proceedings.[2] R. Svoboda’s contribution to these Proceedings.[3] K. Long’s contribution to these Proceedings.[4] S. Parke’s contribution to these Proceedings.[5] E. Wildner’s contribution to these Proceedings.[6] B. Vahle’s contribution to these Proceedings.[7] O. Sato’s contribution to these Proceedings.[8] A. Guglielmi’s contribution to these Proceedings.[9] T. Kobayashi’s contribution to these Proceedings.

[10] K. Heller’s contribution to these Proceedings.[11] CERN Yellow Report 2010-003, ISSN 0007-8328, ISBN 978-

92-9083-354-3

V. Palladino / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 217–222222