Machine R&D towards a Neutrino Factory

A. Tonazzo

Dipartimento di Fisica “Edoardo Amaldi”, Universita Roma Tre and INFN Sezione Roma IIIVia della Vasca Navale 84, I-00146 Rome, Italy

The main R&D items toward construction of a Neutrino Factory are presented, with emphasis on a MuonIonization Cooling Experiment.

1. INTRODUCTION

The possibility of having intense neutrinobeams of well-known composition offers a uniquerange of physics opportunities. A Neutrino Fac-tory would be the ultimate tool to study the top-ics of neutrino physics that are most likely to bestill unexplored after the next generation of neu-trino experiments: measurement of CP violationin the leptonic sector and determination of themass hierarchy. The physics motivations for aNeutrino Factory are presented in [1].

The construction of a Neutrino Factory posestechnological challenges, which motivate an ex-tensive R&D program aimed both at proof ofprinciple for innovative technologies and at costreduction. The efforts in progress will be de-scribed in the paper.

This review is largely based on the re-cently published CERN “Yellow Report” onECFA/CERN Studies of a European NeutrinoFactory Complex [2]. Up-to-date information onthe European Neutrino Factory project can befound in [3]; the project, carried out by the Euro-pean Neutrino Group, is supported by the Euro-pean Committee for Future Accelerators (ECFA)and by a new E.U. networking activity for Beamsfor European Neutrino Experiments (BENE) [4]in the framework of FP6 Coordinated Accelera-tor R&D in Europe (CARE). Parallel studies arebeing carried out in the US [5] and in Japan [6].The efforts are joining, raising good hope for im-provement in performance and for reduction ofcosts.

2. THE NEUTRINO FACTORY CON-CEPT

The basic concept of the Neutrino Factory isthe production of neutrinos from the decay ofhigh energy muons.

Figure 1. Possible layout of a Neutrino Factoryat CERN.

An intense proton beam is delivered to a target.The produced pions are collected in a magneticfield, selecting either the positive or the negativecharge sign, and then decay into muons in a decaychannel. The muon beam has to be cooled for in-jection into an accelerating system, which shouldbe able to accelerate the muons within their shortlifetime. The muons are then injected into a stor-

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age ring, with straight sections pointing towardthe detectors. The muons decaying in the straightsections produce intense neutrino beams.

A possible layout of a Neutrino Factory atCERN is shown in Fig. 1.

The parameters of the machine should be op-timised in view of the event rate at the detector.The number of events with neutrino oscillationsat a detector location at distance L is propor-tional to the neutrino flux Φ, the neutrino inter-action cross-section σν and the oscillation prob-ability Posc. The flux at the detector scales asE3ν/L

2, being Eν the neutrino beam energy andtherefore

Nosc ∼ φ · σν · Posc ∼E3ν

L2· sin2 L

Eν∼ Eν :

the rate is proportional to the beam energy, whichshould be as high as possible. However, mattereffects spoil the L/E scaling for long baseline, andthe optimal values turn out to be

L ∼ 1000− 4000 km Eν ∼ 20− 50 GeV.

With 1020-1021 muon decays per year, severaltens of thousands of events of each type per yearwould occur in a detector of 10 kt mass.

The construction of a Neutrino Factory posesseveral technological challenges: numerous R&Dprograms are being devoted to the developmentof the different parts of the complex. Some of thecomponents require proof-of-principle and test ofnew techniques, while in other cases the effort isconcentrated on cost reduction. The most rele-vant R&D projects currently in progress are dis-cussed in the coming sections.

3. TARGET AND COLLECTION

In order to achieve intense muon beams of bothsigns, the production rate of both π+ and π− atthe target should be as high as possible. The op-timisation of the target design will be based onthe soft pion production measurements at exper-iments such as BNL-E910 [7] and HARP [8]. Ahigh-Z material is to be used. Due to the veryhigh power (4 MW) and small size of the protonbeam, the power density in the target exceeds

that of any comparable facility: building a tar-get able to withstand the mechanical and thermalstresses that such a beam will create is a majorchallenge. Water-cooled solid targets, generallyused in the past for beam powers up to 1 MW,become problematic at larger powers: this has ledto consideration of flowing liquid targets, such asmercury or molten lead.

Figure 2. CERN magnetic horn prototype.

The current “Targetry” [9] baseline concept forthe Neutrino Factory consists of continuous flowliquid mercury target, followed by a magnetic sys-tem to focus and capture the produced pions.

The focusing system is a 20 T super-conductingsolenoid in the US design study and a magnetichorn in the CERN design. A prototype of thelatter has already been built [10] and is shown inFig. 2.

Figure 3. Target and collection system.

Sketches of the target concept [9] are shownin Fig. 3. The liquid mercury should preferably

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be flowing in the form of a free jet to avoid thepotential problems caused by a containment pipe.The mercury jet crosses the proton beam at anangle of about 40 mrad, so that the interactionregion is about 2 interaction lengths. To improvethe yield of soft pions, the axes of both the protonbeam and the mercury jet are tilted with respectto the axis of the solenoid magnet by about 100mrad.

Two key issues of the liquid jet target systemdescribed above cannot be addressed with simula-tions or concept studies: one is the jet dispersiondue to rapid energy deposition by an intense pro-ton pulse: the other is how magnetic forces canperturb the jet flow or affect its dispersal.

Tests on free mercury jets have been performedat BNL [11] and at CERN/Grenoble [12]. AtBNL, a free Hg jet, 1 cm in diameter with velocity2.5 m/s, was constructed and exposed to 24 GeVproton pulses. The Hg jet dispersal was observedto vary proportionally to the beam intensity, tohave velocities about one half of those of “con-fined thimble” targets, to be largely transverseto the jet axis, and to occur about 40 µs afterthe proton pulse. In the Grenoble test, studiesof a Hg jet of 4 mm diameter and 12 m/s veloc-ity were carried out in magnetic fields up to 20T. The effect of the field was to stabilise the Hgjet and to reduce its velocity; the deflection anglewas minimal for entry angles up to 100 mrad.

A new proof-of-principle test of a complete tar-get station suitable for a Neutrino Factory hasbeen recently proposed at CERN [13]. The aimis to study a 24 GeV proton beam incident ona free mercury jet target inside a 15 T solenoid.The experiment has been approved; its construc-tion and commissioning are foreseen to be com-pleted by 2006.

4. MUON IONISATION COOLING

The muons from the pion decays have large mo-mentum spread and transverse emittance, whichmust be reduced for an efficient injection into theacceleration system.

The momentum spread will be reduced usingphase rotation: early (high energy) particles arede-accelerated and late (low-energy) particles are

accelerated using, for example, a system of RFcavities.

The transverse emittance must be reduced withcooling. The conventional cooling techniques arenot adequate for muon beams: stochastic cool-ing [14] is too slow for the short muon lifetime,and frictional cooling [15] does not work for neg-ative muons. The most promising technique isionisation cooling.

Figure 4. Layout of the Muon Ionisation CoolingExperiment MICE.

Ionisation cooling was proposed more than twodecades ago [16], and the principle is quite simple.The muons are passed through a material, calledan absorber, where they lose both longitudinaland transverse momentum. The lost longitudinalmomentum is restored by an accelerating system,using RF cavities. The result of this process isa net reduction in transverse momentum spread.Heating due to multiple scattering occurs as well,and the net cooling is a delicate balance betweenthese two effects.

Ionisation cooling has never been demonstratedin practice. An international Muon IonisationCooling Experiment (MICE) [17] has been pro-posed at RAL, with the following aims:

• to show that it is possible to design, engi-neer and build a section of a cooling channelcapable of giving the desired performancefor a Neutrino Factory;

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• to place the cooling channel section in amuon beam and measure its performance invarious modes of operation and beam con-ditions, thereby investigating the limits andpracticality of cooling.

MICE will consist of a cooling channel capa-ble of providing a measurable cooling effect (upto ∼10% reduction in transverse emittance formuons of momenta between 140 and 240 MeV/c).A precision at the level of 1% on the measurementof emittance reduction will be achieved with sin-gle particle measurements, using tracking detec-tors before and after the cooling channel. A parti-cle identification system will reject the non-muoncomponent in the beam, which would bias theemittance measurement. The construction of theexperiment involves a delicate integration of ac-celerator and particle detector techniques, whichhas never been achieved before.

4.1. Layout of the MICE experimentA schematic layout of the MICE experiment is

shown in Fig. 4. The cooling channel is composedof three absorbers, with hydrogen chosen as ab-sorber material to minimise the multiple scatter-ing, and 8 RF cavities operating at 201 MHz [5].

The incoming muon beam first encounters dif-fusers to generate a large tunable input emit-tance. One additional absorber finishes the cool-ing section, both for symmetry and to protect thetrackers against dark currents generated in theRF cavities.

The momentum, position and angles of the par-ticles are measured before and after the cool-ing section by two identical spectrometers, con-sisting of trackers in superconducting solenoids.The baseline option is a scintillating fibre tracker,while a TPC with GEM readout (TPG) is be-ing studied as an alternative; prototypes for bothtypes of detectors have been constructed andtested.

The muon identification system upstream ofthe first spectrometer is used to reject the un-decayed pions: it consists of a C6F14 Cherenkovdetector combined with a scintillator-based time-of-flight system. Downstream of the second spec-trometer, the rejection of electrons from muondecays is performed by an aerogel Cherenkov

detector (n=1.02, blind to muons) and a Pb-scintillating fibre calorimeter.

4.2. MICE measurement techniqueTo allow precision measurement of transmis-

sion and emittance, one muon at a time will betracked through the apparatus and detected usingstandard particle-physics techniques, which aremuch more precise than those typically used inbeam instrumentation. A virtual bunch formedin offline analysis will be used to demonstratehow an actual bunch would have behaved had thebeam intensity been orders of magnitude higher.MICE will measure the change in emittance (thecooling effect) with a relative precision of at most0.1%.

Each of the magnetic spectrometers measures,at given z positions, the coordinates x and y ofevery incident particle and the time. Momen-tum and angles are reconstructed by using severalmeasurement planes. For the experimental reso-lution not to affect the emittance measurementsignificantly, the rms resolution of the measure-ments must be better than about 10 of the rmsbeam size at the equilibrium emittance in each ofthe six dimensions.

The layout described above has one majordrawback: the detectors will be exposed to a largedark current and x-ray background generated bythe nearby high-gradient RF cavities. Dedicatedstudies have demonstrated that the performanceof the detectors will not be affected.

4.3. MICE statusThe MICE Collaboration is composed of about

150 physicists from 37 institutes in Europe, Japanand the US.

The MICE experiment has received scientificapproval. The preparation of its site at RALhas started, and will proceed in several phasesin the next few years. With proper funding andsupports, muon ionisation cooling can be demon-strated by the year 2008.

5. MUON ACCELERATION

A major component of the Neutrino Factoryis the acceleration of muons up to 50 GeV. Thetwo criteria to design the acceleration system are

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speed and effectiveness. A synchrotron would cer-tainly be the most effective machine, but it is tooslow compared to the muon lifetime.

The original designs of Europe and US pro-posed the use of Recirculating linear Accelerators(RLA), in the form of a racetrack with two longacceleration sections and two short arcs. Thissetup would allow for very high RF gradientsover the circumference, with the expensive accel-eration structure being used several times. Thearcs are the most challenging part: since it is notpossible to ramp up the magnets between turns,there have to be separate bends for each energy,with a beam spreader to guide the beam into theappropriate bending section as a function of thebeam energy.

Figure 5. Japan (top) and US (bottom) designwith the use of FFAG accelerators for the Neu-trino Factory.

A possible solution to reduce the cost of theRF system, achieving a higher number of passesthrough it, are the so-called Fixed Field Alter-nation Gradient (FFAG) accelerators, originallyproposed in the Japanese design of the NeutrinoFactory [6] and now considered with interest alsoin the US and in Europe. In this type of acceler-ators, the repetition rate can be raised 10 timesfaster than in ordinary synchrotrons. Possible de-signs of a Neutrino Factory based on FFAG ac-celerators are shown in Fig. 5.

Figure 6. FFAG accelerator prototypes built inJapan.

Much progress has been done in Japan withthe development and proof-of-principle demon-stration of FFAG accelerators at different en-ergies [18]. Two examples of such accelera-tors which have been constructed and tested areshown in Fig. 6.

Latest ideas in US have lead to the invention ofa new type of FFAG accelerator, so called ”non-scaling FFAG” [19], which can be interesting formore than just Neutrino Factories: plans are de-veloping for the demonstration experiment it mayrequire.

FFAG accelerators are also raising considerableinterest in Europe, where R&D activities on thetopic are just starting.

Hopefully the different concepts will eventuallymerge to produce something even better.

6. CONCLUSIONS

The construction of a Neutrino Factory posesseveral stimulating challenges. The main itemshave been presented, describing the R&D activ-ities that are being enthusiastically carried outand the considerable progress that has alreadybeen accomplished.

7. ACKNOWLEDGMENTS

I am indebted to the MICE Collaboration forgiving me the opportunity of this report and forthe help in its preparation.

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