On 7 November, during its triennial seminar in Ottawa, Canada, the International Committee for Future Accelerators (ICFA) issued a statement of support for the International Linear Collider (ILC) as a Higgs-boson factory operating at a centre-of-mass energy of 250 GeV. That is half the energy set out five years ago in the ILC’s technical design report (TDR), shortening the length of the previous design (31 km) by around a third and slashing its cost by up to 40%.

The statement follows physics studies by the Japanese Association of High Energy Physicists (JAHEP) and Linear Collider Collaboration (LCC) outlining the physics case for a 250 GeV Higgs factory. Following the 2012 discovery of the Higgs boson, the first elementary scalar particle, it is imperative that physicists undertake precision studies of its properties and couplings to further scrutinise the Standard Model. The ILC would produce copious quantities of Higgs bosons in association with Z bosons in a clean electron–positron collision environment, making it complementary to the LHC and its high-luminosity upgrade.

One loss to the ILC physics program would be top-quark physics, which requires a centre-of-mass energy of around 350 GeV. However, ICFA underscored the extendibility of the ILC to higher energies via improving the acceleration technology and/or extending the tunnel length – a unique advantage of linear colliders – and noted the large discovery potential accessible beyond 250 GeV. The committee also reinforced the ILC as an international project led by a Japanese initiative.

Thanks to experience gained from advanced X-ray sources, in particular the European XFEL in Hamburg (CERN Courier July/August 2017 p25), the superconducting radiofrequency (SRF) acceleration technology of the ILC is now well established. Achieving a 40% cost reduction relative to the TDR price tag of $7.8 billion also requires new “nitrogen-infusion” SRF technology recently discovered at Fermilab.

“We have demonstrated that with nitrogen doping a factor-three improvement in the cavity quality-factor is realisable in large scale machines such as LCLS-II, which can bring substantial cost reduction for the ILC and all future SRF machines,” explains Fermilab’s Anna Grassellino, who is leading the SRF R&D. “With nitrogen doping at low temperature, we are now paving the way for simultaneous improvement of efficiency and accelerating gradients of SRF cavities. Fermilab, KEK, Cornell, JLAB and DESY are all working towards higher gradients with higher quality factors that can be realised within the ILC timeline.”

With the ILC having been on the table for more than two decades, the linear-collider community is keen that the machine’s future is decided soon. Results from LHC Run 2 are a key factor in shaping the physics case for the next collider, and important discussions about the post-LHC accelerator landscape will also take place during the update of the European Strategy for Particle Physics in the next two years.

“The Linear Collider Board strongly supports the JAHEP proposal to construct a 250GeV ILC in Japan and encourages the Japanese government to give the proposal serious consideration for a timely decision,” says LCC director Lyn Evans.