Résumé

Le projet de collisionneur linéaire international a toujours le vent en poupe

Cela fait trois ans que la communauté internationale qui planifie le Collisionneur linéaire international a publié son rapport de conception technique. Le Collisionneur linéaire international est une proposition de nouvel accélérateur de particules, qui ferait entrer en collision des électrons et leurs antiparticules correspondantes, des positons, à une énergie de 500 GeV. Ce projet est inscrit dans toutes les feuilles de route pour la physique des particules, mais il n’a pas encore été décidé, jusqu’à présent, s’il doit être construit ou non. En attendant, la R&D se poursuit sur des éléments essentiels des détecteurs et accélérateurs de pointe, notamment sur les aspects de la conception qui seraient fonction de l’endroit où la machine serait construite.

It’s been three years since the worldwide community of the International Linear Collider (ILC) published its Technical Design Report (TDR). The proposed new particle accelerator would smash electrons and their antiparticles, positrons, into each other at energies of 500 GeV. However, even though the ILC features on all particle-physics road maps worldwide, no decision has been taken so far as to whether or not it should be built. In the meantime, R&D continues on key aspects of the state-of the-art accelerator and detectors, with particular focus on those aspects of the design that depend on where the machine would be built. A proposed site exists, and, if it goes ahead, the machine would be built underneath the lush hills of a region in northern Japan called Kitakami, in Iwate province, some three hours north of Tokyo. The green light depends on commitments from and negotiations between many governments, notably the Japanese, which hasn’t yet confirmed its willingness to host the world’s next big particle-physics adventure.

The ILC is said to complement results from the LHC because of the different nature of its collisions. Whereas the LHC collides protons with protons, the ILC would collide electrons with their antiparticles, positrons, with the option of starting out as a Higgs factory at 250 GeV and upgrading to 1 TeV in other stages. The physics case has recently been summed up in a paper published in the European Physical Journal C: "Due to the collision of point-like particles the physics processes take place at the precisely and well-defined initial energy √s, both stable and measurable up to the per-mille level," the paper states. The energy at the ILC is tunable, which allows precise energy scans to be carried out and permits kinematic conditions for the different physics processes to be optimised. In addition, the beams can be polarised: the electron beam up to about 80%, the positron beam up to about 30%. Due to all of these circumstances, it is possible to fully reconstruct the final states so that numerous observables such as mass and total cross-sections, but also differential energy and angular distributions, are available for data analyses. For more information, see Eur. Phys. J. C 2015 75 371 doi:10.1140/epjc/s10052-015-3511-9.

Precise, efficient and novel systems

The ILC would use superconducting radiofrequency technology to accelerate its particles. Some 16,000 1 m-long accelerating cavities made of pure niobium with an accelerating gradient of up to 35 MV/m are needed to get electrons and positrons up to speed. The final-focus system needs to be extremely precise and efficient if collisions at the design luminosity of 2 × 1034 cm–2 s–1 are to occur in the two detectors. The detectors – after planning, design and testing by universities from around the world, involving many students – will take turns to sit in the interaction point. A novel system called "push–pull", where one detector is pushed into the interaction point while the other is pulled out so that one can take data while the other is being serviced, was devised in the course of the R&D work for the project’s TDR, published in 2013. Compared with the option of switching the beam between two separate interaction regions, this option managed to cut the estimated cost by a significant amount because it eliminated several kilometres of tunnel and some cubic metres of cavern digging in one go.

The TDR sets the estimated cost of the project at $7.8 billion plus 23 million man-hours. This includes all civil engineering, technology production, construction, the accelerator components, etc, but it does not include detectors, contingency, escalation or operation costs. "The basis of the final design and the future construction for the ILC project has been completed, and we’re basically ready to push the green button," said then-ILC-director Barry Barish, who led the team of physicists and engineers from around the world who formed the Global Design Effort (GDE) from 2005 to 2013, and who took the project to a construction-ready stage. Three previous regional projects (NLC, JLC and TESLA) needed to be combined into the best and most cost-effective option. People were busy evaluating one option against others, coming up with new ones, checking compatibilities and keeping an eye on the cost. Despite some major setbacks along the way, the R&D work culminated in the TDR. But even though the maturity of the technologies would allow for the machine to be built tomorrow, tunnel-boring machines have to wait for the official green light.

With the publication of the TDR, the mandate of the GDE ended, and a new organisation was put in place: the Linear Collider collaboration, or LCC. Barry Barish returned to LIGO to find gravitational waves and Lyn Evans took over and united the friendly competitors, the ILC and the Compact Linear Collider (CLIC) study, under one organisational roof. Even though the two linear colliders have very different designs, there are still synergies to be exploited between them. Detector developers, for example, work closely together on such state-of-the-art parts like high-granularity calorimeters as part of the CALICE collaboration. These high-granularity calorimeters have, in fact, spun off to the LHC, and will be used in the CMS detector’s calorimeters for the high-luminosity upgrade.

Move from technology to diplomacy

Lyn Evans, former LHC project leader and director of the Linear Collider collaboration, founded in 2013, calls the process a move from technology to diplomacy. Together with his team of project and regional directors, he is busy facilitating negotiations between state officials from various countries to get the approval process under way. The process is slow, and requires many small steps and a large number of study groups and committees; while Japan needs reassurance from governments and funding agencies of potential future member-state countries, to take a decision to host, partner states would prefer to hear "Let’s go!" from Japan, before committing themselves to vast amounts of money and manpower. To break an impasse, discussions between political leaders from relevant countries and proactive approaches by scientists to the governments of their countries are under way. A decision is expected sometime around 2018.

Research and design work hasn’t stopped, though. The main focus is now on adapting the generic collider design to the specifications of the future site. For example, access shafts and tunnels have been adapted to the geology that exists at the site. With the help of a civil-engineering tool originally developed for the Future Circular Collider study, the interaction region has now shifted by a few kilometres so that detector parts can be lowered into the cavern vertically, rather than needing to be driven in on an inclined slope. Engineers are looking at the nearest port that would receive most of the huge accelerator and detector parts from around the world, and at the bridges that these components would need to cross. One might expect doubts, even fear, from the local community, but the contrary is the case: pro-ILC banners, drawings, bumper stickers and flags along roads are visible proof of the region’s support. Local governments have set up ILC promotion offices manned by international residents of Japan, who make sure that everybody in Kitakami not only knows about but also gives their blessing to the ILC. Hitoshi Yamamoto, professor at Tohoku University, tells of the support that he witnessed during a recent site visit of the civil-engineering group: a grandfather and granddaughter saw the group of researchers standing on a field and walked up to them. The group expected to be told to go away, but instead the grandfather pointed at his granddaughter, saying "Please try your best to build the ILC – for this child."

A new international science project would undoubtedly bring benefits to the region, even though the global nature of the ILC would mean that components and parts would be built and tested in labs and universities around the world, and then shipped to their final destination, mirroring what was done for the LHC at CERN. Industrialisation is therefore a high-priority topic for the ILC community: getting 16,000 high-tech cavities built, tested and shipped halfway around the world isn’t obvious, and researchers are learning a lot from the European X-Ray Free-Electron Laser (European XFEL) currently under construction at DESY in Hamburg, Germany. This uses the same technology as the ILC over a length of some 2 km, providing a neat model for cavity and cryomodule serial production. The European XFEL, which started its life as a spin-off from the TESLA accelerator once planned at DESY, also using SCRF technology, employed two companies for cavity production and devised a complicated (but functioning) ballet of component production, transport, testing and integration between various production places, the companies, DESY, the French CEA laboratory Irfu in Saclay and CNRS lab LAL in Orsay. For the ILC, an order of magnitude more parts will have to be shipped around the world.

Redoubled international efforts

The International Committee for Future Accelerators (ICFA) has decided to continue the linear-collider organisation, and has extended the mandate of the LCC by a year. At the February meeting, ICFA reached a consensus that the international effort, led by ICFA, for an ILC in Japan should continue, and a subgroup has been formed to study the future of the linear-collider organisation and make a proposal for a new structure to be in place from 2017.

ICFA has traditionally been the committee to which the ILC effort reported its progress, the body that set up committees and boards, gave them their mandates and monitored developments. Its partner organisation, the Asian Committee for Future Accelerators (ACFA), met with the Asia-Pacific High Energy Physics Panel (AsiaHEP) in February, and decided to issue a statement about the ILC and the potential circular Higgs factory to be built in China, CEPC. About the ILC, the statement says: "AsiaHEP and ACFA reassert their strong endorsement of the ILC, which is in a mature state of technical development…In continuation of decades of worldwide co-ordination, we encourage redoubled international efforts at this critical time to make the ILC a reality in Japan." About CEPC, it states: "We encourage the effort led by China in this direction, and look forward to the completion of the technical design in a timely manner."

These statements mirror what the strategic road maps for the future of particle physics in the different regions have said: that the physics case for the ILC is "extremely strong" and that the "interest expressed in Japan in hosting the ILC is an exciting development" (P5, US). "There is a strong scientific case for an electron–positron collider, complementary to the LHC, that can study the properties of the Higgs boson and other particles with unprecedented precision and whose energy can be upgraded," states the European Strategy in its fifth recommendation. "Europe looks forward to a proposal from Japan to discuss a possible participation." Obviously all strategies give top priority to the continued operation of the LHC and its future upgrade for operation at higher luminosities, to ensure the exploitation of its full scientific potential, and recommend competitive neutrino programmes and priorities and the development of a post-LHC accelerator project at CERN with global contribution.

• For further details, visit www.linearcollider.org.