On June 19th CERN Council adopted the update of the "European Strategy for Particle Physics 2020". This document updates an earlier strategy paper that was adopted by the Council in 2013, just after the Higgs discovery in 2012. It identifies the medium and long-term priorities of fundamental research in particle physics and thus forms an important basis for the construction of the European research infrastructure. The implications of this strategy update for Swiss particle physics are answered by Rainer Wallny, Professor of Particle Physics at ETH Zurich and President of the Swiss Institute of Particle Physics (CHIPP), the umbrella organisation of Swiss particle physics.
Prof. Wallny, how do you feel about the strategy update that the CERN Council recently adopted?
The update of the „European Strategy for Particle Physics“ gives me great confidence. CERN Council adopted a vision for particle physics that ensures the future progress of our field and in which CERN, as the European center for particle physics, will continue to play a central role.
What is the key message of the updated research strategy from the Swiss perspective?
Firstly, there is a clear commitment to a precision machine, the "Higgs Factory", which will produce Higgs bosons on a large scale. Secondly, the feasibility studies that have been decided upon represent a clear path towards a proton accelerator that will have at least 100 TeV of center of mass energy. Both machines are needed to precisely measure the properties of the Higgs boson and maximise the discovery potential of physics beyond the Standard Model. For example, we hope to find a particle that could explain the dark matter that we find in the Universe. Another key point is to intensify research and development work on accelerator technology, as is already being done in Switzerland via the CHART initiative. This is an initiative of the State Secretariat for Education, Research and Innovation (SERI), EPFL, ETH Zurich, the Paul Scherrer Institute (PSI), the ETH Board and the University of Geneva, in collaboration with CERN. The decision of the CERN Council supports this research strategy and I hope that CHART will be further strengthened.
The strategy update covers the three major sub-areas of particle physics in which Swiss physicists are involved in various ways, i.e. research with intense particle beams at low and high energies, neutrino physics and, last but not least, astroparticle physics. Has Swiss particle physics distributed its "resources" correctly, or does it need new emphasis in the light of the strategy update?
Swiss physicists are very well positioned with their diverse projects within CHIPP, also in comparison with other European countries. For particle physics, as for all other scientific disciplines, freedom of research is central. It would be counterproductive if the distribution of resources were coordinated "top-down". After all, the "European Strategy" was itself a bottom-up process in which the particle physicists of all member countries could participate.
According to the strategy update, the "top priority" is the construction of a "Higgs Factory", understood as an electron-positron collider with which Higgs particles can be produced in large numbers and which forms the basis for the detailed investigation of this particle. In the longer term, the "Higgs Factory" is to be followed by a proton-proton collider. To this end, a study is now to be carried out to examine "the technical and financial feasibility" of a proton-proton collider with energies of at least 100 TeV, possibly including a preliminary stage with an electron-positron accelerator for the production of Higgs bosons and gauge bosons of the electroweak interaction. This is precisely the concept that has been developed over the last ten years with significant Swiss participation under the name "Future Circular Collider" (FCC). Where do you see the challenges of the feasibility study?
A central point is the technical feasibility that needs to be assured before a final decision can be made. Among the many technological challenges that such a new machine will bring and that will require a wide portfolio of expertise, magnet technology is of particular interest for Switzerland, an area that we have been working on for some time with the Swiss CHART initiative. Furthermore, the geology of the subsoil where the new accelerator is to be built is another area where I could imagine that Switzerland can bring in its expertise. No less important are the financial feasibility and sustainability, i.e. how we can develop energy-efficient methods for particle accelerators such that the ecological footprint of the new accelerator remains justifiable. We are breaking new ground on all these points, but I also expect the development of solutions to give us a boost to innovation.
CERN management has now been tasked by the CERN council to implement a feasibility study on the FCC. Does that mean the Compact Linear Collider (CLIC) - the linear collider at CERN, which was previously considered an alternative concept to the FCC - out of the race?
As far as I know, the R&D studies on CLIC acceleration technology will be continued, since the "drive beam" concept is a promising innovation in accelerator technology. However, I read the CERN Council's strategy update in such a way that the FCC option for the "Higgs Factory" (FCC e+e−) is given preference over the CLIC concept. But I would not go so far as to say that CLIC is completely "out of the race".
The CERN particle accelerator LHC will continue to be used until 2038. According to the roadmap of the strategy update, a decision on the successor is to be taken around 2026/27, and this would then probably be commissioned in 2048 at the latest, i.e. ten years after the end of the LHC. Is that fast enough?
The LHC took 15 years from the approval of the project to its commissioning. In this respect, this time scale seems realistic because we will still need some time to carry out the feasibility studies and this time is well invested. Of course, I would be happy if things could proceed a little faster. It would be fantastic if we could put a new particle accelerator into operation before the middle of the century, even though I myself will be emeritus by then. Research in particle physics and answering deep questions about the Universe is a business that requires stamina and patience.
CERN Director Fabiola Gianotti says that the decision on the successor should also be made in the light of the results of the high-luminosity LHC, i.e. the further upgraded, more powerful LHC that is scheduled to go into operation in 2026. Can you explain this consideration from a particle physics perspective?
So far the LHC has only provided about 6% of the total data expected by 2038. New discoveries are not expected to lie behind the corner but searches for new physics have still a large potential at the LHC. The analysis strategies now need higher level of sophistication and a deeper understanding of the theoretical predictions. Apart from the searches for new particles, we also measure fundamental parameters of the Standard Model with high precision and we are looking for deviations from the predictions the Standard Model , e.g. those concerning the properties of the Higgs boson. Such high precision analyses of the data from the LHC have the potential to provide clues that might be relevant for the construction of the successor.
One does not betray secrets when one points out that the construction of large-scale research facilities happens in international competition, involving global strategic thinking and collaboration. In the Strategy Update, the European particle physicists explicitely offer their cooperation on the International Linear Collider (ILC), if it is realised in time. This means that a Higgs factory would be built in Japan with European support. Do you believe that a second Higgs factory can be built at CERN, provided that the ILC is built in Japan first?
In particle physics we have the term "coopetition", i.e. the interplay of cooperation and competition: both belong together, because nowadays large accelerators can only be built on a global scale with cooperation, and competition also serves to safeguard scientific knowledge. I personally believe, however, that if the construction of a "Higgs Factory" were to begin immediately in Japan, with European support, efforts round here would then concentrate on building the proton accelerator. Even then, of course, there would still be a possibility of pursuing the option of an FCC e+e−. The strategy document formulates this in such a way that a timely ILC is "compatible" with the European strategy.
Back to the short-term perspective: Will the strategy update for the coming years result in new work priorities for Swiss particle physics?
I see the European and Swiss strategies as being very well aligned. I am confident that Swiss particle physics will continue to make a significant contribution to this European vision, for example in accelerator and detector development.
Interview: Benedikt Vogel