Shinichiro Michizono describes the International Linear Collider, a proposed 250 GeV electron–positron collider using superconducting radiofrequency technology.
The International Linear Collider (ILC) is a proposed electron–positron collider for elementary particle physics experiments with a total length of approximately 20 km. The design study for the ILC started in 2004 and the Technical Design Report1, the outcome of many years of research and development (R&D) conducted by the Global Design Effort (GDE) international team, was published in 2013. After the publication, R&D activities regarding linear colliders were organized by the Linear Collider Collaboration.
With the discovery of the Higgs boson at CERN’s Large Hadron Collider (LHC) experiment in 2012, particle physics entered a new era and the scientific significance of the ILC became clearer as a Higgs factory. In November 2017, the International Committee for Future Accelerators released a statement2 in support of the construction of the ILC for experiments at a centre-of-mass energy of 250 GeV. Whereas LHC is a 14 TeV circular proton–proton collider, protons being composite particles, ILC is a linear collider of elementary particles: electrons and positrons. The ILC adopted superconducting radiofrequency (SRF) technology, whose level of maturity was proven by the operation of the European X-ray Free Electron Laser (X-FEL) in Hamburg.
The discovery of the Higgs boson completed the standard model (SM) of particle physics. As successful as the SM is, it leaves many important questions open, such as what is the nature of dark matter and dark energy, what is the origin of the matter–antimatter asymmetry and why did the Higgs field fill the entire Universe approximately one-trillionth of a second after the Big Bang. There must be physics beyond the SM (BSM) that we need to understand. Since the latter question is directly connected to the Higgs boson, BSM physics naturally requires an extended Higgs sector (the particles and phenomena involved in the Higgs mechanism), which might also hold answers to the other questions. However, since the discovery of the Higgs boson, the LHC has so far seen no manifestations of BSM physics. This makes the precise study of the Higgs boson extremely important.
In the SM, the Higgs boson’s couplings to various SM particles are proportional to their masses. BSM physics is expected to modify this proportionality, leaving its nature imprinted in the deviation pattern from the SM. Well-motivated BSM scenarios such as supersymmetry, extra dimensions and composite Higgs fields predict different deviation patterns. To observe such patterns and identify the corresponding BSM physics, we need improved measurement methods with less systematic errors yielding better measurement precision for various Higgs couplings. The Higgs boson could be produced in large amounts at the 250 GeV ILC through simple collisions of elementary particles, electrons and positrons. At the LHC, the Higgs boson production is more complicated as the colliding protons are composite objects consisting of quarks and gluons. This difference would allow the ILC to reach the required precision to measure the Higgs couplings.
The ILC has the potential to discern between the need for an extension of the space–time concept, as in the case of supersymmetry or extra dimensions, or if there is a deeper layer of matter, as in the composite Higgs scenario. If the Higgs boson is found to decay invisibly or if dark matter particles pair production is observed, the ILC could become a dark matter laboratory. If the Higgs boson is found to violate the matter–antimatter symmetry, the ILC may also open up a new avenue to investigate the matter–antimatter asymmetry of the Universe.
A linear accelerator is more advantageous for accelerating electron and/or positron beams to higher energies than a ring accelerator, in which the beam loses its energy owing to synchrotron radiation. A unique feature of linear colliders is their capability to increase the collision energy by improving the acceleration technology and/or extending the accelerator length. Another advantage of linear colliders over ring colliders is that the spin of the electron and/or positron beam can be maintained during the acceleration and collision. This can help significantly improve measurement precision, since particles with left-handed and right-handed polarizations in general interact differently and hence might introduce unwanted systematic errors unless polarized beams are used.
The ILC would include electron and positron sources, damping rings to reduce the emittance (to a value corresponding to the spread of the beam) of the electron and/or positron beams, beam transportation from the damping rings to the main linear accelerators, the main linear accelerators to accelerate the electron and/or positron beams, beam delivery system to focus and adjust the final beam to maximize the luminosity (the intensity of beam collision at the collision point corresponding to the number of generated Higgs particles) and the interaction region where detectors are installed and beam dumps absorb the energy of the beams. These components are schematically illustrated in Fig. 1. The ILC would adopt SRF technology for the beam acceleration at the main linear accelerators.
There are two main advantages of using the SRF technology for the ILC. The first is the efficient power transfer from the AC power source to the beam owing to the small surface resistance of the accelerating structure (cavity) made of Nb. Further energy efficiency improvements are considered as part of the of Green ILC concept, which aims to establish a sustainable laboratory. The second is the large beam aperture (with an approximately 70 mm diameter at 1.3 GHz), which relaxes the alignment accuracy of the cavities and suppresses the beam instability, both of which contribute to the larger tolerance of the beam transportation. The ILC would require approximately 8,000 superconducting Nb cavities. The mass production capability of high-quality cavities has been confirmed by the construction of the European X-FEL.
As the cost of the superconducting accelerator part is half the total construction budget, the cost reduction in this SRF technology is extremely important. R&D activities to lower the price of superconducting materials by utilizing low-cost Nb and improve the cavity performance by using a new surface treatment method3 are ongoing as part of a United States–Japan collaboration.
A key technology alongside the SRF is the nanobeam technology at the beam collision point. Each accelerated beam (electron and/or positron) is focused to a width of 500 nm and height of 8 nm at the interaction point where the detector is located. Beam focusing experiments have been conducted by an international team at KEK’s Accelerator Test Facility, which uses a beam of 1.3 GeV and 37 nm, corresponding to the beam size of 8 nm at the ILC.
In 2013, the Tohoku area in Japan was endorsed by the Linear Collider Collaboration as a strong candidate for the ILC site, and, since then, further studies have been conducted. The ILC-project implementation planning4 describes the organization, governance and plans of the international laboratory. If approved, before the actual construction, a 4-year period of preparation would be necessary. After that the construction would take 9 years. After completion, accelerator commissioning would take 1 year and the experiments could be conducted for 20 years after that.
The author declares no competing interests.
ILC Global Design Effort (GDE). ILC technical design report. Linear Collider http://www.linearcollider.org/ILC/Publications/Technical-Design-Report (2013).
The International Committee for Future Accelerators (ICFA). Statements. ICFA https://icfa.fnal.gov/statements (2019).
Grassellino, A. et al. Unprecedented quality factors at accelerating gradients up to 45 MV/m in niobium superconducting resonators via low temperature nitrogen infusion. Preprint at arXiv https://arxiv.org/abs/1701.06077 (2017).
Foster, B. et al. Revised ILC project implementation planning. Linear Collider http://ilcdoc.linearcollider.org/record/62116/files/PIP_complete_IssueC.pdf (2015).
The author would like to thank Keisuke Fujii for holding fruitful discussions and providing helpful information.