An executive summary of the panel discussion at CEPC Rome Workshop (24 - 26 May 2018)
On the 26th of May 2018, as a conclusive session of the 2018 Workshop on the Circular Electron-Positron Collider - EU edition, a discussion among members belonging to different european institution and the workshop attenders was organised in order to share opinions about next steps in particle physics. The panel was formed by the following members:
Daniela Bortoletto (University of Oxford, Oxford, UK)
Franco Bedeschi, (Istituto Nazionale di Fisica Nucleare, Pisa, Italy)
Alain Blondel (University of Geneva, Switzerland)
Imad Laketineh (Institute de Physique Nucléaire de Lyon, France)
Werner Riegler (CERN, Geneva, Switzerland)
Marcel Vos (University of Valencia, and CSIC Valencia, Spain)
Yifang Wang (Institute of High Energy Physics, Beijing, China)
and convened by:
Biagio Di Micco (Università Roma Tre, Roma , Italy)
Xinchou Lou,(IHEP, Beijing, China)
the scientific secretary was:
Manqi Ruan (IHEP, Beijing, China)
the report of the discussion follows.
The panel emphasised the physics importance of an electron positron Higgs factory. The precise measurement of the Higgs boson properties is vital to progress in understanding the fundamental laws of nature. Even after the HL-LHC programs will be fully deployed, the Higgs coupling to charm quarks will have remained most likely untested and no direct measurement of the Higgs total width will have been possible. The total width measurement is important to determine the Higgs partial decay width to new invisible particles, that could shed light on unexpected new physics phenomena. Nowadays the Higgs sector of the SM Lagrangian (SML) is largely unconstrained by the existing measurements, and each term of the SML is a fundamental element of nature, that cannot be left untested. Moreover electron -positron machines have a strong capability to find indication of beyond standard model signals through precision measurements at the Z, WW and t-tbar energy threshold and, in addition, the possibility to discover very rare processes in Z decays. This applies mainly to the circular machines where a more accurate beam energy calibration can be performed and a higher integrated luminosity is achievable.
In the past several years, the conceptual design study of circular e+e- colliders has demonstrated their feasibility and technological readiness. They offer high performance and extremely good experimental conditions, including beam energy calibration. They also provide a very strong physics case for the construction of a large tunnel infrastructure which could host, once the required magnet technology will be available at an affordable cost, a hadron collider of unprecedented energy. The e+e- circular collider and the pp collider offer almost perfect complementarity for Higgs physics as well as in the search for new physics.
Concerning the high energy pp options, the technologies for high-energy proton colliders, especially the next generation of high field magnet (i.e., 20 Tesla), still takes time to develop. The 16T technology, based on Nb3Sn superconductor, discovered in 1954, still needs developments to reach a large scale industrial production, while the high temperature superconducting magnets, that can reach 20 Tesla and beyond, are far to be ready for first tests.
An electron-positron collider is the best option for the next big facility setup for discoveries and test in fundamental physics. While Japan is considering the linear collider option, China is pursuing the circular option and CERN is investigating both the linear collider option (through CLIC studies) and the circular option (through FCC-ee studies). The panel has underlined the necessity to work towards a running machine to be built as soon as possible, possibly running at the same time of the High Luminosity LHC to reduce delays in the development of the future high-energy physics program.
The panel discussed the situation of manpower/resource distribution in different countries and the future international collaborations. CERN has a full program to run the High Luminosity LHC collider until 2038, nevertheless the option to build an FCC-ee collider seems to gain more and more favour in the CERN community. The current High Energy physics collider program in China may be concluded at the beginning of the 2020 decade, the CEPC facility could then be built without conflicting with other ongoing operations. If CERN plans to build an electron machine first, China would be happy to collaborate to this project. It is important that in the world a Higgs factory using a circular electron-positron collider is built. Worldwide a clear vision and a roadmap is needed, to stimulate people to start working on the next generation collider physics. It’s crucial for the high energy circular electron-positron collider option to be included in the European Strategy program. The panel agreed that a coordination at European level is needed to setup the proposal for a circular electron-positron collider option to be built anywhere in the world (CERN and China are the natural candidates), and this should be prepared collecting the effort of people working on both FCC-ee and CepC in order push for a circular electron-positron collider. This effort should not exclude people working on the linear collider option, that could be interested in particular in the detector design and construction. In fact the circular electron-positron colliders, allowing to have more than one interaction point, are extremely compelling for detector technology studies. The panel encourages synergies with existing projects and the explorations of new paths to deliver the best possible technologies able to maximise the physics output of such machine.