A high-intensity proton accelerator operating at a few giga-electron-volts would offer a wide range of opportunities for both particle and nuclear physics.
There are two main frontiers for particle accelerators – high energies and high intensities – and it is the latter that attracted participants to the “Physics with a multi-megawatt proton source” workshop held at CERN on 25-27 May. The meeting was organized by the ECFA Muon Study Group and the European Commission network on “Beams for European Neutrino Experiments” (BENE), in close collaboration with the community involved with the “next-generation” European isotope separation on-line radioactive ion-beam facility, EURISOL. The focus was on physics at the high-intensity frontier and the main aim was to explore the short- and long-term opportunities in Europe for particle and nuclear physics at a multi-MW, few-GeV proton accelerator.
CERN’s director-general Robert Aymar opened the meeting by recalling that CERN has a history and a mission of building and operating accelerators at the high-energy frontier. The latest is the Large Hadron Collider (LHC) and a compact linear collider (CLIC) may follow. CERN also has a successful tradition of exploiting its accelerator complex, so as to address diverse issues in particle physics simultaneously – for example, with fixed-target experiments, neutrinos, radioactive ions and antiprotons. Although the LHC is currently the absolute priority, plans should be made now for future investments. Aymar concluded by asking the workshop to contribute to the optimal evolution of the accelerators at CERN, so as to permit the most ambitious and promising spectrum of physics experiments in future.
In an inspired talk, John Ellis of CERN then summarized the most compelling motivations for physics programmes at the high-intensity frontier. The discovery of neutrino oscillations has opened a new window of exploration, which is unique in several ways. Measured mass splittings and mixings for neutrinos are the first experimental data we have on physics at higher energy scales. The potential discovery of leptonic charge-parity (CP) violation promises an insight into the origin of the most fundamental asymmetries of the universe. The accelerator neutrino community in Europe, in particular the ECFA/ BENE network, will do its utmost to maintain a leading role in accelerator neutrino experiments beyond the CERN Neutrinos to Gran Sasso (CNGS) project. In another direction, the manifestation of the strong and weak interaction in the atomic nucleus can be rigorously investigated by means of radioactive ion beams. Under the leadership of the Nuclear Physics European Collaboration Committee (NuPECC), a large community of European Union (EU) nuclear physicists is advocating the physics potential of the new world-leading facility, EURISOL, for nuclear, astro- and fundamental physics, which will be some 1000 times more intense than present facilities.
Presenting Japanese plans, Shoji Nagamiya, director of the Japan Proton Accelerator Research Complex, J-PARC, described the progress and physics potential at this new facility. In terms of both its physics programme – a joint venture between particle and nuclear physics – and its push for a higher power of 0.75 MW, possibly evolving to a few MW, J-PARC is the natural benchmark for any future high-power facility. Steve Holmes of Fermilab then described plans in the US for new high-power proton drivers. Studies are centred on the Brookhaven National Laboratory, with a 1.2 GeV superconducting linac and an important upgrade of the Alternating Gradient Synchrotron, and Fermilab, with two scenarios: an 8 GeV synchrotron with a 600 MeV linac or an 8 GeV superconducting linac.
Super-beams, beta-beams and factories
In the first of several presentations on current ideas in Europe, CERN’s Roland Garoby introduced the 2.2 GeV Superconducting Proton Linac (SPL), under consideration at CERN, in which cycling at 50 Hz results in a mean beam power of 4 MW. For neutrino physics an accumulator and compressor ring would be added to reduce the beam pulse to 3 microseconds (the so-called “super-beam”) and the length of the bunches to 1 ns rms. For a radioactive ion-beam facility the linac beam would be used directly. The advantage to CERN of the SPL would be as a replacement for the Proton Synchrotron Booster (PSB).
The SPL study benefits from a collaboration between CERN and the Injecteur de Protons de Haute Intensité project (IPHI) and the support of the EU’s Sixth Framework Programme (FP6) and the International Science and Technology Centre for projects in Russia. If a positive decision on the SPL is taken in 2006-2007, the low-energy section could be operational in 2010-2011 and could replace Linac2 to increase the performance of the PSB and the PS; the SPL itself could be ready in 2014-2015.
Rapid cycling synchrotrons (RCSs) also offer interesting possibilities, especially for beam energies beyond a few GeV. Chris Prior from the Rutherford Appleton Laboratory (RAL) illustrated the potential of this alternative by describing RAL’s machine ISIS (~0.2 MW at 800 MeV) in detail. He also presented the plans for future proton drivers with multi-MW beam power envisaged at RAL, FNAL, J-PARC and CERN. Although the experience with existing machines is encouraging, these new projects represent a significant step forward in beam power and there are technical challenges on numerous issues, such as beam loss and the stripping and capture of ions.
Exploiting the beam delivered by such an accelerator is a similarly ambitious goal. Helmut Haseroth of CERN highlighted what this means for neutrino physics. Three different production techniques are envisaged: the “super-beam”, the “beta-beam” and the “neutrino factory”. For the super-beam, neutrinos result from the decay of pions immediately after the target. In the beta-beam case, beta-radioactive ions are generated and accelerated to a γ factor of around 100 and stored in a few bunches inside a ring with long straight sections pointing at remote experiments. Neutrino bursts are generated by the beta decays. In a neutrino factory, muons from the pion decays are collected behind the target, “cooled” and accelerated to 20-50 GeV, and then stored in a ring with long straight sections pointing at remote experiments, where neutrinos result from the muon decays.
Extensive R&D is required for any of these ambitious plans with neutrinos to be realized during the next decade. The work already undertaken by the American and Japanese teams should be complemented by a similar effort in Europe, resulting possibly in a joint target experiment at CERN. The technology of the target and the focusing devices is challenging, but largely common to the super-beam and the neutrino factory. R&D is also still needed for the muon phase rotation and cooling stages of a neutrino factory. Complementary resources are eagerly expected for the international Muon Ionization Cooling Experiment (MICE), which has been approved “scientifically” at RAL. Although the development of fixed-field accelerating-gradient machines for muon acceleration may render cooling superfluous, the highest energy with lepton colliders is obtained with circular muon colliders, which require cooled beams to achieve the desired luminosities. For the beta-beam the main technological issue is the generation of the radioactive ion beam and its acceleration without excessive irradiation of the machine components.
The energy of the primary proton beam is a crucial parameter in the optimization of a neutrino beam. Marco Apollonio of Trieste described the Hadron Production Experiment (HARP) at CERN, which will provide decisive data in that respect. The data necessary for selecting the optimum energy for a proton driver is expected to be available later this year.
The SPL would be an excellent proton driver for a future nuclear-physics facility. The additional installations that would be required were presented by Alex Mueller of IPN Orsay, based on the study carried out for EURISOL. This European project for an accelerated radioactive ion-beam facility uses the isotope separation on-line method for ion generation, with the goal of attaining beam intensities thousands of times higher than at current facilities such as REX-ISOLDE at CERN and SPIRAL at GANIL in France.
CERN’s Mats Lindroos concluded the accelerator session by describing the concept of a beta-beam facility based on the original idea of Piero Zucchelli of CERN. The key feature is that “slow” accelerators can be used because the radioactive ions have a lifetime that is three orders of magnitude longer than muons. Although it will stretch the techniques mastered for nuclear physics well beyond today’s performance, experts are confident that solutions can be found for the production of the required ion beams. The promises of this scheme together with the remarkable synergy between nuclear and neutrino physics justify the necessary R&D, and a feasibility study is included in the EURISOL Design Study that was submitted to the EU in March this year.
From neutrinos to exotic atoms
Opening the particle-physics session, Pilar Hernandez of Valencia presented an in-depth review of the prospects for neutrino-oscillation physics at a megawatt neutrino complex. She reviewed the excitement of recent discoveries and the relative merits of super-beams, beta-beams and neutrino factories. Luigi Mosca of Saclay outlined the status and plans for a new very-large European underground laboratory at the Frejus site. It could host detectors of unprecedented size – up to one megatonne – for the study of proton decay and astrophysical neutrinos (supernovae), as well as of the low-energy neutrinos from super-beams and beta-beams. Chan Kee Jung of Stony Brook reviewed the potential of a megatonne or half-megatonne water Cherenkov detector, as envisaged for the proposed Underground Nucleon Decay and Neutrino Observatory (UNO). This is a proven and well established technique and its extrapolation to larger mass seems feasible. An exiting alternative using liquid argon time-projection chambers was also described by Antonio Ereditato of Naples. In this case the lower detector mass of about 0.1 megatonnes is acceptable, thanks to the superior granularity and pattern-recognition capability.
Steve Geer of Fermilab described the merits of physics at a neutrino factory – the most promising, ultimate neutrino facility, and the natural tool for the final and complete exploration of neutrino mixing and mass splittings, and leptonic CP violation. The higher event rates would allow smaller detectors (around 50 kilotonnes) that would need charge identification, but which could be located in existing labs. Systematic uncertainties in this case are less severe. Much accelerator R&D in this field is in progress or being planned by enthusiastic worldwide collaborations, specifically for the phase rotation and cooling of large muon “clouds” and for the acceleration and storage stages further downstream. Geer stressed that timely R&D is essential, in particular for MICE and the proposed target experiment at CERN.
Pasquale Migliozzi of Naples discussed the absolute necessity of near-detector stations for the study of neutrino oscillations. Only with the precise measurement of neutrino fluxes, interaction cross-sections and detection efficiencies, will we be able to predict reliably the interaction rate in the far-neutrino detectors, prove the existence of oscillation effects and eventually measure their CP (neutrino-antineutrino) asymmetries. Alessandro Baldini of Pisa discussed the potential for discovering leptonic-flavour violation using unprecedented fluxes of slow muons. With the SPL, sensitivity to muon-to-electron conversion may indeed test the rates predicted from supersymmetric loops. Equally fertile and promising is the opportunity at higher energies to study rare kaon decays, as outlined by Augusto Ceccucci of CERN.
The topics introduced by John Ellis on nuclear structure and nuclear astrophysics, in particular understanding nucleosynthesis via the rp- and r-process paths, were expanded by William Gelletly of Surrey and Karl-Ludwig Kratz of Mainz, while Klaus Jungmann of the Kernfysisch Versneller Instituut (KVI), Groningen, presented a menu of different experiments to investigate fundamental symmetries – for example, CP violation, forbidden decays, non V-A terms in beta-decay and unitarity of the Cabibbo_Kobayashi-Maskawa matrix – that are possible with a multi-MW facility. Francesca Gulminelli of LPC Caen explained how the nuclear liquid-gas phase transition could be investigated, and Juha Äystö of Jyväskylä made the case for combining antiprotons or muons with radioactive ions in colliding or trapping experiments so as to provide an unsurpassed probe of the charge and mass distribution of these exotic nuclei.
Yorick Blumenfeld of IPN Orsay described the technical challenges remaining for the development of EURISOL and showed the importance of the EU FP6 Design Study, while Jürgen Kluge of GSI described the laboratory’s proposal for a Facility for Antiprotons and Ion Research (FAIR) at the GSI laboratory. From these presentations it became clear that the physics reach of FAIR (nuclear physics and astrophysics, low-temperature quantum chromodynamics, charmed sector, high-density plasmas, etc.) is complementary to that of EURISOL (nuclear physics and astrophysics, fundamental interactions, solid-state physics, radiobiology, etc.).
Looking to the future
The final session turned to the outlook for the future and began with Wu-Tsung Weng of Brookhaven, who underlined that the idea of using a linac driver like the SPL is realistic and feasible, although work is still needed on a number of issues such as control of beam loss and cost reduction. Development should be vigorously pursued on many technological items, such as the target and pion/muon focusing devices. A broad range of physics is possible at a multi-MW driver, however, intensive discussions among accelerator experts and physicists still have to take place to select the proper accelerator configuration (SPL or RCS). CERN should play an important, if not the leading role, in the international collaboration of R&D efforts and encourage participation from its staff, provided CERN’s core mission is not compromised.
Michel Spiro, director of IN2P3, reviewed the outlook for particle physics, with an emphasis on neutrino oscillations following the “Venice road map”, as formulated at the Neutrino Oscillations in Venice (NO-VE) workshop last December. After the round of experiments at CNGS and European participation in the next round at J-PARC in Japan, a European initiative could resume with a low-energy super-beam/beta-beam complex serving large detectors in a new underground laboratory, and then proceed to the final and complete mapping of neutrino phenomena with a neutrino factory and the smaller magnetic detectors that best match its potential.
The nuclear-physics outlook was provided by Muhsin Harakeh of KVI Groningen. He presented the NuPECC perspective for the future of nuclear physics in Europe. NuPECC, as a representative of the European nuclear-physics community, has declared its highest priority to be the construction of both the FAIR and EURISOL facilities, to serve the need of the estimated 1000 European nuclear scientists.
The concluding remarks came from Jos Engelen, CERN’s chief scientific officer. He acknowledged the unique and compelling nature of the physics programmes proposed by the workshop, encouraged the continuation of the efforts for its definition and promised to give it careful attention. While reminding the audience of the limited resources, he voiced explicitly European pride for some of the most novel ideas and encouraged international collaboration in R&D.
