The European particle physics (PP) community is in the process of updating its strategy with a forward look over the next couple of decades. The last strategy update was in 2013 when the two main scientific recommendations with implications for the accelerator field were:
Europe’s top priority should be the exploitation of the full potential of the LHC, including the high-luminosity upgrade of the machine and detectors with a view to collecting ten times more data than in the initial design, by around 2030. This upgrade programme will also provide further exciting opportunities for the study of flavour physics and the quark-gluon plasma.
CERN should undertake design studies for accelerator projects in a global context, with emphasis on proton-proton and electron-positron high-energy frontier machines. These design studies should be coupled to a vigorous accelerator R&D programme, including high-field magnets and high-gradient accelerating structures, in collaboration with national institutes, laboratories and universities worldwide.
Both of these recommendations have been implemented vigorously, and the PP community met in May 2019 in Granada for an open symposium to review the status of, the fieldand discuss the next steps. A number of large accelerator projects are in consideration.
Based on the views expressed in Granada there seemed to be a strong physics-based consensus that the next global PP project should be an electron-positron collider to serve as a factory for producing Higgs bosons and top-quarks. Such a facility would enable precision measurements that can test the Standard Model (SM) at the (sub-)1% level for the majority of Higgs-boson couplings, and, via Higgs, top-quark and other precision measurements, have sensitivity to beyond-SM effects at mass scales of up to many 10s TeV for some new-physics models. There seemed to be consensus that this could be the best way to make progress on elucidating the mass scale of new particles that could then be investigated via direct production at a possible future high-energy proton-proton collider.
There are several e+e- collider options on the table: the International Linear Collider (ILC) proposed for realisation in Japan; the Compact linear Collider (CLIC) and the Future Circular Collider (FCCee), both proposed for implementation at CERN; the Circular e+e- Collider (CEPC) proposed as an option for realisation in China. Options were also presented for a future hadron collider in Europe (FCCpp) and in China (CppC), involving a 100km-circumference tunnel, which could also be used to house the FCCee and CEPC respectively. The energy and luminosity capabilities of these machines, their corresponding physics potential, and their cost, were discussed with gusto at the Granada symposium.
Looking even further ahead, discussion included the application of plasma-based, high-gradient wakefield-acceleration techniques towards a very high-energy electron-positron collider. There was also discussion of the intriguing idea of a muon collider that could potentially be housed eventually in the LHC tunnel.
The next step in the ESU process is for the production of the ‘Briefing Book’ that summarises the parameters and status of the colliders listed above, as well as other proposed projects, along with their physics potential. Caterina Biscari, Lenny Rivkin, Frank Zimmerman and I have been charged with producing the accelerator section of the Briefing Book, based on the formal submissions to the ESU and the discussions in Granada. The aim is to have this completed by early September for approval by CERN Council. The Strategy Group, comprising representatives from CERN member and observer states, will then use the Briefing Book as input for their deliberations in Bad Honnef in January 2020. A draft ESU should emerge for formal consideration and ratification by CERN Council in Spring 2020. Watch this space …
John Adams Institute Interim Director
Scientific Secretary for Accelerators to the ESU Preparatory Group