Solid Target R&D Programme Colliding the proton beam with a dense target is currently the only known way to produce enough muons for the neutrino factory. This deposits 1MW of heat into the small volume of the target, requiring engineering beyond the current state of the art. The UK is investigating the use of a simpler solid target to complement liquid target studies at CERN and elsewhere. Solid targets may reach temperatures of 2000°C, at which the response to thermal shocks induced by the proton pulses is largely unknown. To test the candidate materials’ resilience over the target lifetime, scientists at RAL are producing thermal shocks by passing a fast current pulse through a wire of the material. The wire may be heated to the temperature of interest and the pulses repeated at 50Hz to put it under similar conditions to the target of a real proton driver. This diagram shows the main systems that constitute the neutrino factory. Four areas are highlighted in which the UK will perform key technology demonstrations within the next five years. FFAG Electron Model of Muon Acceleration (EMMA) An new kind of accelerator called a non-scaling FFAG has been devised for muon acceleration before the storage ring. FFAGs, or Fixed Field Alternating Gradient accelerators, achieve similar goals to synchrotrons but with fixed magnetic fields. This removes a major limitation for the neutrino factory as muons, which decay in roughly 2.2  s, must be accelerated quickly and powerful magnets can only be varied slowly. The term ‘non-scaling’ here means that cheaper dipole and quadrupole magnets can be used instead of the original, custom-shaped ‘scaling’ FFAG magnets. Despite these advantages, a non-scaling FFAG has never been built before. A scaled-down machine called EMMA, using electrons instead of muons, could soon demonstrate the technology at Daresbury Laboratory, where an existing electron accelerator will be able to provide its input beam. Other Applications FFAGs have the potential to lower the cost or increase the performance of a variety of accelerators. Proton and ion versions of the machines have been suggested as compact sources for next-generation cancer radiotherapy. Larger versions could be competitive with synchrotrons as proton drivers. H − Ion Source LEBT: Low Energy Beam Transport RFQ: Radio Frequency Quadrupole Beam Chopper 180MeV H − Linac Stripping Foil (H − to H + /protons) Achromat for removing beam halo Two Stacked Proton Synchrotrons (boosters) 1.2GeV 39m mean radius Both operating at 50Hz Two Stacked Proton Synchrotrons (full energy) 6GeV 78m mean radius Each operating at 25Hz, alternating for 50Hz total Proton bunches compressed to 1ns duration at extraction Mean power 5MW Pulsed power 16TW FFAG I (3-8GeV) FFAG II (8-20GeV) FFAG III (20-50GeV) Absorber Accelerating cavity Entire MICE beamline Cut-away absorber showing the liquid hydrogen containment Muon Ionisation Cooling Experiment (MICE) The muon beam must be ‘cooled’, or reduced in size, to fit inside the accelerators downstream. This can be achieved by a technique known as ionisation cooling. The principle of ionisation cooling is to pass muons travelling in a range of directions through a material or absorber whose constituent atoms they ionise, losing energy in the process. Momentum is lost in the direction of travel, but then replaced only in the forward direction by the electric field in an accelerating cavity placed after the absorber. Thus transverse momentum is consistently removed, producing a well- collimated beam of muons that can be focussed to a smaller size downstream. While muon cooling is theoretically possible, it has not been tried in practice and the technical obstacles are considerable: for instance, the best absorber material is liquid hydrogen, which must be contained and cooled to cryogenic temperatures. Therefore the MICE Collaboration will build a short section of an ionisation cooling channel at RAL, using muons from the ISIS accelerator to measure the cooling effect to an accuracy of 0.1%, in order to predict the performance of the full neutrino factory cooling channel. (produces pions from protons) Proton Beam Dump Solenoidal Decay Channel (in which pions decay to muons) RF Phase Rotation Muon Cooling Ring Solenoidal Muon Linac to 3GeV (other technologies possible) The UK Neutrino Factory Design Present2010s2020s2030s Proton Power150kW1MW  2.5MW  5MW Neutron Science ISISISIS MW Upgrade ESS-class Machine Neutrino Physics Neutrino Factory Fundamental & Higgs Physics Muon Collider Far Detector 1 Far Detector 2 Neutrino Factory Proton Driver Front-End Test Stand (FETS) The basis of a neutrino factory is a powerful (4–5MW) pulsed proton accelerator, known as a proton driver. It starts with an H − ion source followed by components for low-energy beam transport and acceleration (LEBT, RFQ). An important operation known as beam chopping must also be performed at low energies: it produces clean gaps in the beam that allow injection into a circular accelerator without any beam loss, which at these power levels would produce unacceptable radioactivity. The LEBT and RFQ are being designed at Warwick and Imperial College respectively, while RAL has an active ion source research programme and chopper development project. Together, these will make FETS: a full-scale prototype of the initial segment of the proton driver, soon to begin construction at RAL. Other Applications The proton driver for a next-generation neutron spallation source is very similar to that for the neutrino factory and FETS meets the specification for both projects. The technique of beam chopping will find applications in nearly all high-power proton accelerators, including those for nuclear waste transmutation, materials testing (for example IFMIF, which supports the ITER fusion experiment) and safe subcritical nuclear power plants known as energy amplifiers. FETS chopper Fast chopping section Slow chopper and beam dump FETS RFQ Beam Target enclosed in 20Tesla superconducting solenoid Near Detector R109 To Far Detector 2 To Far Detector 1 Muon Decay Ring (muons decay to neutrinos) Physics Motivation The recent discovery that neutrinos have mass has invalidated the current standard model of physics. A new theory is required, with the potential to explain on a deeper level why the particles of nature are as they are, but the only experimental data that can inform such a theory is data that contradicts the standard model. Hence neutrino mass measurements are one of the few windows onto these new laws of nature. When neutrinos were believed to be massless, their interaction with other leptons (via the weak nuclear force) was predicted to occur in a straightforward manner, where e interacted with e,  with  and  with . However, with the introduction of mass, the states previously believed to be fundamental turned out each to be a mixture of new states 1, 2 and 3. As these parts have different masses, their quantum wavefunctions will become out of synchronisation over time, making the mixed neutrino appear to oscillate so that it interacts with types of lepton other than the one it was formed with. This oscillation occurs as a function of L/E, the distance to the detector divided by the neutrino energy. As higher-energy neutrinos are much easier to detect, the neutrino factory places detectors as far away as practical from the source to let these oscillate fully. The oscillation wavelength measured will be a direct indicator of the neutrino masses. The neutrino factory complex could be built up in stages: an outline plan exists to incrementally upgrade the ISIS accelerator at RAL, supporting both the neutrino factory and enhancements to the current neutron and muon science (aligning with the UK Neutron Strategy technology case). The neutrino factory itself can be upgraded to a high-energy machine known as the muon collider, which would extend capabilities across the whole of particle physics, surpassing the LHC at CERN in several areas. [ 900–1000 m below ground ] Left: staging scenario leading to a UK muon collider. Right: plan view of the new accelerators superimposed on the RAL and Harwell site.

THE UK NEUTRINO FACTORY: CONCEPTUAL DESIGN AND DEVELOPMENT S J Brooks, on behalf of the UKNF Collaboration ( Tel: ASTeC Intense Beams Group, Building R2, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX In the last decade, one of the most surprising discoveries in particle physics was that neutrinos are not massless particles, as previously believed, but instead have very small masses. A powerful and high- quality source of neutrinos is needed to make accurate measurements that will provide the deciding factor between alternative unified theories of physics, currently indistinguishable by experiment. The Neutrino Factory is a particle accelerator complex that would produce neutrinos beams able to travel through the Earth’s interior and be detectable thousands of kilometres away. The neutrinos in this beam are produced by the decay of another particle called the muon. Muons need to be accelerated to high energies in order to produce neutrinos in sufficiently focussed beams, which is a significant challenge as the acceleration must happen before they decay (muons last on average 2.2 microseconds). Muons also need to be manufactured with another accelerator since such unstable particles are not readily obtainable in nature. This is achieved by colliding a proton beam of power 4- 5MW with a stationary target, producing muons as reaction products. This proton accelerator is also suitable for use as an intense pulsed neutron source, providing an order of magnitude improvement over the current record-holding ISIS source. The UK has a diverse programme of Neutrino Factory R&D under the UKNF Collaboration. Research activities focus on design and computer modelling for the various subsystems, several of which use techniques never used before in accelerators. Development projects such as MICE, EMMA and FETS will demonstrate in reality the key technologies required for a neutrino factory. Web Links UK Neutrino Factory website (links to all related projects) International Scoping Study of a Neutrino Factory and Super-Beam Facility Muon1: a public distributed computing project to optimise the neutrino factory