The Higgs particle, cornerstone of the Standard Model (SM) and predicted 50 years ago was discovered by the Large Hadron Collider at CERN on 4 July 2012. In fact, this extremely important particle has been searched intensively by circular electron positron colliders such as LEP-II at the end of the nineties at CERN, after SLC at Stanford at the end of the eighties. The year 2012 was an important milestone year not only for particle physics itself, but also for future lepton and hadron collider facilities — both linear and circular, in energy frontiers exploring within and beyond the SM (like dark matter search for example).
As for linear colliders, in August 2004 the International Committee for Future Accelerators (ICFA) decided that the future linear collider should be based on superconducting technology based, and baptised it the International Linear Collider (ILC, 500 GeV). From 2005, the ILC Global Design Effort was established and led by Barry Barish. In 2007, the ILC Reference Design Report (RDR) was released with a reference ILC cost. After a series of efforts on design optimisation and technology R&D, the ILC Technical Design Report (TDR) was released in 2013, one year after the discovery of the Higgs particle. The Linear Collider Collaboration (LCC) then took the baton including the Compact Linear Collider (CLIC) study. It is led by Lyn Evans with efforts of further R&D, cost reduction and ILC/CLIC synergies.
Taking into account LHC Run 2 results, the Japanese HEP community (JapHEP) made an important statement on 22 July 2017 concludes that “in light of the recent outcomes of IHC Run2, JAHEP proposes to promptly construct ILC as Higgs factories with the center-of mass energy of 250 GeV in Japan”.
250-GeV ILC layout
With this new input from JapHEP, the Linear Collider Board (LCB), a panel of ICFA led by Tatsuya Nakada, dedicated to supervise the development of LCC, has requested the collaboration to conduct machine studies (based on ILC TDR) and physics studies for a 250-GeV ILC to assess the JapHEP proposal. According to LCC studies results, LCB concluded that this option shows not only a compelling physics case as a Higgs factory but also substantial 40 percent cost reduction compared to the original 500-GeV ILC.
On November 2017, LCB gave strong support to the JAHEP proposal to construct a 250-GeV-ILC in Japan and encouraged the Japanese government to give the proposal a serious consideration for a timely decision. At the same time and with the input from LCB, ICFA (led by Joachim Mnich) declared its support LCB conclusions and encouraged also strongly Japan to realise the 250-GeV ILC in a timely fashion as Higgs boson factory as an international project led by Japanese initiative. To my mind, this recent ICFA statement is as important as its statement on superconducting technology for ILC in August 2004, after 13 years great efforts from the ILC community. Now, the world high energy physics community is waiting for a potential positive declaration from the Japanese government, by the end of 2018,on their intention to host ILC in Japan.
As for circular colliders, just two months after the Higgs boson was found , in September 2012 Chinese scientists proposed a Circular Electron Positron Collider (CEPC) in China at 240 GeV centre of mass for Higgs studies with two detectors situated in a very long tunnel more than twice the size of the LHC at CERN. It could later be used to host a Super Proton Proton Collider (SppC) well beyond LHC energy potential to reach a new energy frontier in the same channel.
CEPC-SppC schematic layout
After the ICFA Higgs Factory Workshop in November 2012, CERN proposed also a similar one, the Future Circular Collider (FCC) with a much longer tunnel than that of LHC.
In June 2013, the 464th Fragrant Hill Meeting was held in Beijing on the strategy of Chinese high energy physics development after Higgs discovery, and the following consensuses were reached: 1) support ILC and participate to ILC construction with in kind contributions, and request R&D fund from Chinese government; 2) as the next collider after BEPCII in China, a circular electron positron Higgs factory (CEPC) and a Super proton-proton Collier (SppC) afterwards in the same tunnel is an important option as a historical opportunity, and corresponding R&D is needed.
In February and July 2014, ICFA issued statements that ICFA supports studies of energy-frontier circular colliders and encourages global coordination, and that it continues to encourage international studies of circular colliders, with the ultimate goal of proton-proton collisions at energies much higher than those of the LHC. In April 2016, during the AsiaHEP and ACFA meeting in Kyoto, a positive statement of AsiaHEP/ACFA Statement on ILC+CEPC/SppC has been made with strong endorsement of the ILC and encouraging the effort led by China on CEPC/SppC. In September 2016, in the meeting of the Chinese High Energy Physics of Chinese Physics Society, a statement on the future Chinese high-energy physics based on accelerator said that CEPC is the first option for a future high-energy accelerator project in China as a strategic action with the aim of making CEPC as a large international scientific project proposed by China.
In October 2016, the 572th Fragrant Hill Meeting was dedicated to CEPC and it concluded that CEPC has a solid physics reason to be built, with big physics potential in SppC. In the beginning of 2015, Pre-Conceptual Design Reports (Pre-CDR) of CEPC-SppC have been completed with international review, where a single-ring-based pretzel orbit scheme was studied. The International Advisory Committee of CEPC was also established in 2015. In 2016, Chinese Ministry of Science and Technology has allocated several tens of millions RMB on CEPC R&D to start with. Since mid-2015, based on crab-waist collision at two interaction points, Partial Double Ring (PDA) , Advanced Partial Double Ring (APDR) and the Fully Partial Double Ring (FPDA) schemes have been studied systematically with the aim of comparing the luminosity potentials and proposing a baseline and an alternative options for CDR studies. On January 2017, CEPC-SppC baseline and alternative designs for Conceptual Design Report (CDR) have been decided by the Steering Committee of CEPC-SppC, which laid an important basis for the completion of CEPC CDR at the end of 2017.
The CEPC baseline design is a 100-kilometre Fully Partial Double Ring (FPDR) scheme as shown with 30MW radiation power of single beam at Higgs energy, and with the same SCRF accelerator system for both electron and positron beams, and Double Ring (DR) scheme at the production energies of the Z and W particles. In April 2017, CEPC-SppC Progress Report was released, where four collider ring scheme studies and a baseline/alternative choice making process were well recorded.On 7 November CEPC-SppC Industrial Consortium (CIPC) was established, with CIPC members participating R&D and industrialisation process. At the end of 2017, CEPC CDR was accomplished and the final publication will be finished before June 2018.
2018 is the starting year for CEPC Technical Design Report, which is scheduled to be completed in 2022, the year of CEPC construction starting as scheduled. On 23 January the China Reform and Development Committee (led by President Xi Jinping) met and passed the plan called “Chinese Initiated International Large Scientific Plan and Large Scientific Project”, which has paved the way for large projects like CEPC, with project proposal, evaluation, R&D, project approval, and construction.
With the above historical recall, in summary, 2018 is indeed a milestone year for both 250-GeV ILC and CEPC, two potential Higgs factories in Asia in complementarity. While the first one one has big potential to reach electron positron colliding energy frontier up to 1 TeV in pushing superconducting accelerator technology to a new limit, the other has the great potential to develop into a hadron hadron collider of 100 TeV (centre of mass energy) in pushing superconducting magnet technology to an unprecedent high level. Both kinds of technologies will surely find wide applications in all aspects of fields including our everyday life.
We wish 250-GeV ILC and CEPC, two complementing Higgs factories in Asia, move forwards to their final successes.
Jie Gao, IHEP, CAS, China, is member of the Linear Collider Board and chair of the Asian Linear Collider Steering Committee, CEPC-SppC IB Vice Chair and CEPC-SppC Accelerator Convener.
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