Nearly seventy years ago, before the CERN Laboratory was established, two models for European collaboration in fundamental physics were on the table: one envisaged opening up national facilities to researchers from across the continent, the other the creation of a new, international, research centre with world-leading facilities. Discussions were lively, until one delegate pointed out that researchers would go to wherever the best facilities were. From that moment on, CERN became an accelerator laboratory aspiring to be always in the vanguard of technology to enable the best science. It was a wise decision, and one that I was reminded of while listening to the presentations at the European Strategy for Particle Physics Open Symposium in Granada earlier this month. Because among the conclusions of this very lively meeting was the view that providing world-leading accelerator and experimental facilities is precisely the role the community needs CERN to play today.
There was huge interest in the Symposium, as witnessed by the 600-plus participants, including many from the nuclear and astroparticle physics communities, as well as, of course, particle physicists. The vibrancy of the field was fully on display, with future hadron colliders offering the biggest leap in energy reach for direct searches for new physics. Precision electroweak studies at the few per cent level, particularly for the Higgs particle, will obtain sensitivities for similar mass scales. The LHC, and soon the High-Luminosity LHC, will go a long way towards achieving that goal of precision. Indeed, it’s remarkable how far the LHC experiments have come in overturning the old adage that hadrons are for discovery and leptons for precision – the LHC has established itself as a precision tool, and this is shaping the debate as to what kind of future we can expect. Nevertheless, however precise proton-proton physics becomes, it will still fall short in some areas. To fully understand the absolute width of the Higgs, for example, a lepton machine will be needed, and no fewer than four implementations were discussed. So, one key conclusion is that if we are to cover all the bases, no single facility will suffice. One way forward was presented by the ACFA Chair, Geoff Taylor, representing the Asian view, who advocated a lepton machine for Asia, while Europe would focus on advancing the hadron frontier.
Interest in muon colliders was rekindled, not least because of some recent reconsiderations in muon cooling. The great and recent progress of plasma wakefield accelerators, including AWAKE at CERN, calls for further research in this field so as to render the technology usable for particle physics.
Methods of dark matter searches abound and are an important element of the discussion on physics beyond colliders, using single beams at CERN.
The Granada meeting was a town meeting on physics. Yet, it is clear to all that we can’t make plans solely on the basis of the available technology and a strong physics case, but must also consider factors such as cost and societal impact in any future strategy for European particle physics. With all the available technology options and open questions in physics, there’s no doubt that the future should be bright. The European Strategy Group, however, has a monumental challenge in plotting an affordable course to propose to the CERN Council in March next year.
There were calls for CERN to diversify and lend its expertise to other areas of research, such as gravitational waves: one speaker even likened interferometers to accelerators without beams. In terms of the technologies involved, that statement stands up well to scrutiny, and it is true that technology developed for particle physics at CERN can help the advancement of other fields. CERN already formally collaborates with organisations like ITER and the ESS, sharing our innovation and expertise. However, for me, the strongest message from Granada is that it is CERN’s focus on remaining at the forefront of particle physics that has enabled the Organization to contribute to a diverse range of fields. CERN needs to remain true to that founding vision of being a world-leading centre for accelerator technology. That is the starting point. From it, all else follows.