Beginning a new phase toward preparation for the International Linear Collider

Pushpa Bhat

At its summer meeting this August, the International Committee for Future Accelerators, known as ICFA, accepted the proposal by its Linear Collider Board and approved the formation of the International Development Team with a mandate to make preparations for the International Linear Collider Pre-Lab in Japan, by the end of 2021. The Pre-Lab, a precursor to the ILC Laboratory, is expected to be launched at the end of the mandate of the International Development Team and after securing the approval from the Japanese government for the ILC project.

The proposed International Linear Collider, about 20 kilometers in total length, will accelerate beams of electrons and their antiparticles, positrons, to 125 billion electronvolts, or GeV, each and collide them at the center of the machine. Each colliding bunch will contain about 20 billion electrons or positrons squeezed to a ribbon-like structure, 300 micrometers long, and about 0.5 micrometers wide and a mere 8 thousandths of a micrometer thick. (For comparison, the thickness of a human hair is about 80 micrometers). The surge of particles produced in the collisions will be tracked and registered in the ILC’s detectors.

The collisions’ center-of-mass energy being 250 GeV, the ILC (referred to as ILC250) will be a “Higgs factory,” providing for copious production of the Higgs boson along with a Z boson via the process e+e→ZH. The design instantaneous luminosity of the collider will be 1.35×1034 cm-2s-1 with plans in place to upgrade to higher luminosity (up to 5×1034 cm-2s-1). The ILC will be upgradeable to higher collision energies in the future.

At the ILC, electrons and positrons will collide with a center-of-mass energy of 250 GeV, producing a large number of Higgs bosons. The ILC will be upgradeable to higher collision energies in the future. Image: International Linear Collider

Experiments at the ILC will enable measurements of the Higgs boson coupling strengths to Standard Model particles with unprecedented precision. It will allow searches for new physics in ways complementary to the LHC physics program. It could shed light on new areas of physics such as dark matter. When upgraded to higher energies (500 GeV and beyond), the ILC will enable precision studies of the top quark and measurements of the top Yukawa coupling and Higgs self-coupling.

ICFA, whose mission is to facilitate international partnerships in the construction and exploitation of large global accelerator facilities, has actively advanced the cause of the ILC for over two decades. In 2005, ICFA set up the Global Design Effort for the ILC, headquartered at Fermilab and directed by Barry Barish of Caltech. The GDE produced the Technical Design Report in 2013 for an ILC with center of mass energy of 500 GeV. Fermilab has been pivotal in the global ILC project for most of its history and will continue to be a significant and influential partner in the ILC project in Japan. The superconducting radio-frequency technology with recent rapid developments at Fermilab is the essential critical technology for the ILC, and this warrants Fermilab’s involvement to ensure the project’s success.

Attendees gather at the ICFA meeting held at SLAC National Accelerator Laboratory from Feb. 20-22. Photo: SLAC

In the past couple of years, the Japanese Ministry of Education, Culture, Sports, Science and Technology, known as MEXT, and the Japanese Federation of Diet members for the ILC have expressed strong interest and support for the ILC project in Japan. They have been negotiating with other ministries in Japan and have continued bilateral discussions with foreign governments, particularly in Europe and with the United States. The 2014 P5 plan in the U.S. explicitly supports the ILC program, in addition to the LHC and LBNF/DUNE. The U.S. Department of Energy has been making a concerted effort working with the State Department and the White House Office of Science and Technology Policy to support the Japanese initiative to move forward to realize the proposed ILC pre-Lab. The recently updated European Strategy for Particle Physics calls for 1) the full exploitation of LHC physics potential, 2) an electron-positron Higgs factory as the next highest-priority collider, 3) investigation of the technical and financial feasibility of a future approximately 100 trillion electronvolt proton-proton collider at CERN in a 100-kilometer ring, with an electron-positron Higgs and electroweak factory as a first stage, and 4) increased R&D on accelerator technologies. The ESPP Update also unequivocally states that the timely realization of the ILC as an electron-positron Higgs factory is compatible with the European Strategy and in that case the European particle physics community would wish to collaborate.

The ILC International Development Team replaces ICFA’s Linear Collider Board (chaired by Tatsuya Nakada, EPFL) and the Linear Collider Collaboration (led by Lyn Evans, CERN), whose mandates ended on June 30 this year. Both organizations had been in place since 2012 and have enabled significant progress on the technical aspects of the ILC, physics and detector studies, and international communications, in coordination with the Compact Linear Collider Community hosted at CERN.

This shows the anticipated timeline for the ILC project.

The ILC International Development Team is being hosted by KEK in Japan and will consist of an executive board and three working groups. The EB, to be chaired by Tatsuya Nakada, will have, as members, representatives from the three regions — the Americas, Europe and Asia — and chairs of the working groups. Details of the ILC-IDT structure and functions and the terms of reference can be found in the ICFA statement and press release from Aug. 4. A rough timeline for the ILC project as per the ILC-IDT is shown above. More details on the status and plans of the ILC-IDT and the ILC project can be found in talks at the recent Americas Workshop on Linear Colliders.

Pushpa Bhat is a Fermilab scientist and has been serving as the secretary for the International Committee for Future Accelerators since 2016.