Neutrino beams produced by the decay of muons have been the subject of increasing interest in the last twenty years. They provide a unique op-portunity to reduce the main source of systematic error encountered in accelerator-based long baseline neutrino experiments. A Neutrino Factory, based on a high energy muon storage ring, has been shown to outperform all realistic alternatives and to be capable of making oscillation measurements at the percent level. The mixing parameters, the CP-violating phase and the mass hierarchy could be measured with incomparable resolution. The unprecedented level of purity of the muon decay neutrino beams allows one to probe the unitarity of the PMNS matrix as well as to make a definite measurement of neutrino interaction cross-sections.
A Neutrino Factory could be built using accessible technologies, with a performance matching the requirements of an exciting physics program.
Cost estimates are quite high, and several techniques considered have never been applied in practise. A sizeable R&D program is required to lower the costs and investigate new technologies. The appeal of the Neutrino Factory physics and its conceptual designs are presented below.
2.3.1 Facility design
All accelerator-based long-baseline neutrino experiments currently use the same approach to produce neutrino beams. So-called super beams are pro-duced using intense proton beams dumped on high-power targets to drive the production of unstable mesons. Pions and kaons produced in the interactions are collected in a magnetic horn and decay in a pipe to produce tertiary neutrinos. The combination of both meson signs and multiple decay modes produce a complex mixture of mostlyνµ and their antiparticle, with a small contamination of νe and anti-νe. The uncertainty on the neutrino flux is entangled with the uncertainty on the neutrino cross-section and remains a dominant systematic uncertainty in both T2K and NOνA [93,115]. Hadron production experiments such as NA61/SHINE have been designed to reduce these systematics but they remain significant [116]. Recent work has also gone into developing a cleaner neutrino super beam by implementing a series of dipoles in the beam line to select a single meson charge [117]. Future super beam experiments are likely still to suffer from the lack of understanding in hadroproduction and neutrino cross-section.
An attractive way of reducing flux and cross-section systematics is to use a different neutrino production method. The Neutrino Factory uses muons stored in a ring to produce beams of well-defined flavour content [118]. The separation of neutrino and anti-neutrino, made possible by the selection of magnet polarity, removes wrong-sign identification background. The Michel
energy spectrum of muon decays has been thoroughly studied in the past and provides an exquisite understanding of the neutrino beam energy distribution.
Alternatively, Beta Beams facilities propose to use boosted unstable isotopes as a alternative clean source of neutrinos [119]. This method yields mono-energetic neutrino beams given isotopes that decay via electron capture.
The Neutrino Factory is a project currently in its conceptual design stage.
The idea and physics potential of such a facility have been advertised since the late nineties and has drawn the interest of a large international collaboration.
The International Design Study for the Neutrino Factory (IDS-NF) was established by a group of high energy physicists the ninth International Workshop on Neutrino Factories, super-beams and beta-beams in Okayama in August 2007 [120]. The main components of the facility are presented in figure2.12. Minor adjustments to the original design were necessary after the recent measurement of a largeθ13. The changes mostly pertained to the beam energy and baseline. A recent study by the European Commission Framework Programme 7 EUROν proposed a low-energy Neutrino Factory (LENF) adapted to the largeθ13 [121]. Several existing sites are considered for the implementation of the facility, including CERN, FNAL, RAL and JPARC [15]. The US Feasibility Study II quotes a price tag in the vicinity of $1.9 billion [122].
Figure 2.12: Schematic layout of the main components of the International Design Study for the Neutrino Factory [120].
The deployment of a Neutrino Factory requires an ambitious R&D program that has been under way for more than a decade. The neutrino beams are produced from the decay of muons circulating in a storage ring.
The primary aim of the accelerator complex is to achieve an optimal muon intensity. The muon production starts with a high power proton source to create intense bunches of protons fired into a target. Linear and circular options are under consideration to drive a 4 MW proton beam. Building a target that can withstand the mechanical and thermal stresses that such a beam will create is a major challenge. Mercury jet targets have been studied and validated as a viable option by the MERIT experiment [123].
The produced pions are then magnetically captured and focused by a powerful magnet and transported to a 30–40 m decay pipe where they decay to muons. The large momentum spread of the decay muons will be reduced using phase rotation in which early (high energy) particles are de-accelerated and late (low energy) particles are accelerated using a system of RF cavities.
The resulting muon bunches have a large size and spread in the longitudinal and transverse momentum, i.e. a large emittance. The beam must becooled to fit the acceptance of the accelerating section. The reduction of transverse emittance is achieved using an ionization cooling channel. Muons are passed through a series of absorbers in which they loose momentum by ionizing the material. Radio Frequency (RF) cavities restore the longitudinal momentum, effectively reducing the angular spread of muons. The Muon Ionization Cooling Experiment has been designed to demonstrate the feasibility of the technique and is thoroughly described in chapter3.
The muons are then accelerated in a series of sections, before being injected into the storage ring. Several technologies have been considered, including a Fixed-Field Alternating Gradient (FFAG) accelerator. The tech-nique was developed in the fifties but garnered little interest at the time as an electron accelerator. The evolution in magnet design and RF cavities has prompted a resurgence of such devices. FFAGs combine the cyclotron advantage of continuous operation with the inexpensive compactness of the synchrotron. A proof-of-principle non-scaling FFAG, called EMMA – Electron Model for Many Applications – has been constructed at the STFC Daresbury Laboratory in the UK [124].
2.3.2 Physics
The potential of a Neutrino Factory is unprecedented in neutrino physics [125].
It provides measurements of the oscillation parameters with an unchallenged precision due to controlled systematics. Numerous recent studies still demon-strate the superiority of a muon-based facility over any realistic upcoming super-beam experiment [121, 126, 127]. The quadrant in whichθ23 lies is expected to be determined at 2σifθ23>48° or θ23<43°. The precision on
θ13 is expected to reach the percent level for the current world average of
∼9°. The left panel of figure2.13shows the fractional resolution on the small mixing angleθ13 as a function of its true value. The proposed low-energy neutrino factory (LENF) achieves the highest resolution, regardless of the true oscillation parameters. The large mass splitting ∆m213 is expected to be resolved with a precision of 0.5 %.
÷
÷ õ õ
3 4 5 6 7 8 9 10
0.02 0.04 0.06 0.08 0.10
Θ13H°L
DΘ13Θ13
C2P BB350 LENF T2HK
-150-100-50 0 50 100 150 5
10 15 20 25
∆H°L
D∆H°L
Figure 2.13:Precision onθ13(left panel) andδ(right panel). The normal hierarchy has been assumed. The EUROν baseline neutrino factory is labelled LENF. For comparison, the performance of the C2P super-beam experiment, T2HK and the γ= 350 beta-beam are shown. The width of the bands in each panel represent the dependence of the uncertainty ofθ13onδ(left panel) and the dependence of the uncertainty onδonθ13when it is varied in the range 5.7–10° (right panel) [121].
The Neutrino Factory would enable the scientific community to take the measurement of the CP-violating (CPV) phaseδ to the precision physics era. It is the only instrument that can measureδwith a precision similar to that of its quark sector counterpart. Figure2.14 shows the fraction of the CPV phase range that can be tested by a variety of proposed machines. The Neutrino Factory outperforms every super-beam and beta-beam experiment, even in its low-energy configuration (NF5). It is also guaranteed to determine the mass hierarchy at 5σ[127].
A Neutrino Factory is the ultimate instrument to study the unitarity of the mixing matrix,UPMNS, as it offers a well defined energy spectrum as well as a high purity neutrino beam. The flavour composition of the beam is well known and the beam is focused and intense. The production of very high energyνe, above theτ production threshold, allows one to study the νe→ντ mixing channel. Recent studies show that the Neutrino Factory is a superior candidate to test the three-neutrino paradigm [128].
NF10 NF5 BB350 BB+SPL WBB T2HK LBNEmini
NOvA+ 2020
CKM2011
GLoBES 2012
D∆at 1Σ
0 10 20 30 40
0.0 0.2 0.4 0.6 0.8 1.0
D∆@°D
Fractionof∆
GLoBES 2012
CPV at 3Σ
0.06 0.08 0.10 0.12 0.14
0.0 0.2 0.4 0.6 0.8 1.0
sin22Θ13
Fractionof∆
Figure 2.14:Comparison between the different current and proposed accelerator neutrino experiments. The results that would be obtained by 2020 through the combination of T2K, NOνA and reactors is also included. (Left) Fraction ofδas a function of the precision at 1σfor sin22θ13. (Right) Fraction ofδfor which CPV can be established at 3σas a function of sin22θ13in the currently allowed range. A true normal hierarchy has been assumed. The vertical dotted line in the right panel corresponds to sin22θ13= 0.1. In the left panel, the vertical grey band depicts the current precision for the CPV phase in the quark sector [126].
The cross sections for neutrino scattering are known at the 2–3% level at energies above 30 GeV but, as the energy decreases, the uncertainty increases considerably [129]. Because muon decay is well understood, the flux and hence the total cross section should be measurable across the full energy spectrum to the 1% level [125]. Even in a minimal configuration, i.e.
νSTORM that does not rely on muon cooling, it would deliver an interesting measurement of the cross section that would drastically reduce the statistical uncertainty of all future accelerator-based neutrino experiments [130].
The electroweak sector of the Standard Model, in particular the de-termination of sin2θW, could be tested from the measurements of both electron and muon neutrino cross sections off electrons instead of quarks.
The current measurement could be improved by an order of magnitude [131].
Non-neutrino science is also possible; intense beams of muons with momenta of order 100 MeV/c and a variety of time structures can be provided for slow muon physics studies. Both muon lifetime high precision measurements and magnetic muon studies would allow many parameters of the SM to be determined with unprecedented precision [132].