Observing the behaviour of elementary particles at an even finer level

The International Linear Collider (ILC) will be a linear accelerator roughly 20 km long that accelerates and collides electrons and positrons (positively charged electrons) to create high energy states and explore the properties of elementary particles and the laws of physics. The ILC will be a next-generation core project in particle physics, and researchers around the world are currently making preparations so that it can start operating around 2030.

Under the latest plan, announced in 2017 by the International Committee for Future Accelerator (ICFA), the ILC will achieve an energy of 250 GeV for electron-positron collisions. In the future, advances in accelerator technologies are expected to further increase the energy. This may appear to pale in comparison to the 13 TeV achieved by the second phase LHC experiments at CERN, but the ILC has salient features that set it apart from the LHC.

While the LHC collides protons, which are composite particles (hadrons), the ILC will collide electrons and positrons, which are both elementary particles. Composite particles are like cherry pies, while elementary particles are like the cherries. Colliding two cherry pies causes the filling to scatter, and it is hard to determine exactly what happened during the collision. When colliding two cherries, on the other hand, the collision can be seen with great clarity. This is why the ILC is well-suited for detailed investigations of the properties of elementary particles. It holds promise for discoveries that could launch the next stage of particle physics research, such as close study of the Higgs boson, the elucidation of the origins of matter and the search for supersymmetric particles, the grand unification of forces, and the discovery of the true nature of dark matter.

After many years of research through international collaboration, the ILC Technical Design Report (TDR) was completed in June 2013. Researchers from the University of Tokyo and ICEPP hold key positions in organisations promoting the project. Hitoshi Murayama (Professor at Kavli IPMU, The University of Tokyo) is the Deputy Director of the Linear Collider Collaboration (LCC), which coordinates linear collider research activities around the globe, and ICEPP Professor Toshinori Mori is a member of ICFA as the Japanese representative, which has a great deal of influence over the entire project.

Research and development aimed at the realization of the project is being carried out in earnest, and progress is being made in the development of accelerators and ultra-precise detectors based on new concepts that leverage state-of-the-art technologies. Furthermore, ICEPP Associate Professor Wataru Ootani was selected for the speaker bureau of the calorimeter development group and is contributing to the international publishing of research findings. Efforts aimed at realizing the ILC include intensified coordination and close information sharing between countries planning to participate in ILC experiments.

Japan's Kitakami mountains are a strong candidate site for the construction of the ILC. If the decision is made to construct the ILC in Japan, it will create a major international scientific city that brings together people and industries from around the world. ICEPP seeks to gather wisdom and technologies, not only from Japan, but from the world at large, to make this next-generation core project, which has drawn the attention of the world, the best it can possibly be. ICEPP researchers and students are combining their strengths to bring the project to Japan.

Why are there different accelerator shapes?

Conceptual image of the completed ILC (distant view). The yellow tubular objects are vacuum containers called "cryomodules" that contain superconductive accelerating cavities, superconductive electromagnets, and liquid helium. ©Rey.Hori

Accelerators can be broadly divided into circular and linear accelerators, and into hadron and lepton accelerators. The LHC, which collides protons with each other, is a circular hadron accelerator. The ILC, which collides electrons with positrons, is a linear lepton accelerator. Hadrons are composite particles composed of multiple quarks tied together by gluons (elementary particles mediating the strong force).
Accelerators have developed from linear to circular forms, growing larger and reaching higher energies, but circular accelerators have one restriction. Electrons and positrons have a small mass, and when they curve they radiate light, losing energy. This is why large circular accelerators are primarily hadron accelerators.
However, in hadron accelerators, a variety of phenomena other than those being studied also occur at the same time, which is why they are not necessarily suited to studying the properties of elementary particles in great detail. It has long been the dream of particle physicists to achieve high collision energies at the TeV (teraelectronvolt) level with linear lepton accelerators such as the ILC.

Differences in the qualities of accelerators used in the LHC and the ILC. The ILC collides individual elementary particles (electrons and positrons), so it is well-suited for the close study of the properties of elementary particles.