Towards an update of the European Particle Physics Strategy
The European Strategy for Particle Physics is the cornerstone of Europe’s decision-making process for the long-term future of the field. Mandated by the CERN Council, an update of the Strategy is currently under way. A major step in this process was the Open Symposium, attended by some 600 physicists, held in Granada, Spain from 13-16 May 2019.
Following the discovery of the Higgs boson in 2012, the measurement of the parameters of the Standard Model (SM) of particle physics with high precision is a major imperative. While the energy scale at which new physics may emerge is presently unknown, there is general consensus that the search for possible deviations to the SM’s predictions is a reliable road towards breakthroughs in the field. Determining precisely the properties of the Higgs boson is a major objective of this strategy. With the High-Luminosity LHC (HL-LHC) many of the Higgs couplings will be measured with a precision of a few percent but this may not suffice to probe the SM at the level required to reveal new phenomena. Understandably, the main debate at the Open Symposium was about the future colliders that could bring a substantial improvement in our understanding.
Despite broad consensus on the central physics goals, the community is presently confronted with choices among potential future colliders. Although the HL-LHC will collide protons and ions up to the late 2030s, it is already essential to look beyond. Projected advances in accelerator technology, policy, time-scale and cost must be carefully considered. Advanced high-field RF systems are key to lepton colliders. The Compact Linear Collider (CLIC) would use room-temperature RF structures with a two-beam acceleration scheme. The circular FCC-ee would require superconducting RF to achieve its target performance at several interesting energies, including the Z boson, Higgs, W-pair threshold and the top quark. Either collider would yield a major improvement, probing some Higgs couplings to below the percent level. In combination with other precise electroweak observables at the Z pole and WW threshold, these measurements may lead the way towards answering some of the important questions about the Universe, e.g., the nature of the electroweak phase transition in the early Universe, the matter-antimatter asymmetry, dark matter or compositeness. The merits of the various electron-positron colliders were discussed in depth.
High-energy hadron colliders beyond the LHC rely on larger tunnels and higher-field superconducting magnets. A major contender is the Future Circular Collider (FCC-hh), built from 16 T magnets, in the same 100 km tunnel attached to the present CERN complex as the FCC-ee. After the FCC-ee, this could provide exploratory p-p collisions at 100 TeV collision energy, a comprehensive programme, spreading the cost and operation over decades. Muon colliders or plasma-wakefield accelerators may eventually take us further.
Extensive discussion of global collaborations focused on possible participation in new collider facilities in Japan (ILC) and China (CEPC), as well as two major accelerator neutrino projects, DUNE in the United States and Hyper-Kamiokande in Japan.
The presentations made at the Open Symposium can be reviewed at https://indico.cern.ch/event/808335/timetable/#20190513.detailed
