Author: Rüdiger Voss

The CERN Council has started to chart a roadmap for European Particle Physics in the post-LHC era

At an extraordinary meeting in Lisbon in 2006, the CERN Council approved for the first time the “European Strategy for Particle Physics”. This strategy is not only a roadmap for the CERN Laboratory in Geneva, but for the future of particle physics in Europe at large; its formal basis is Article II of the CERN Convention whereby the Organization provides, inter alia, for the organization and sponsoring of international co-operation in nuclear research, including co-operation outside the Laboratories. Since 2006, the strategy has been updated in seven-year intervals, first in 2013 and recently in June 2020. Regardless of its European perspective and scope, the strategy and its updates have always kept an eye on developments and roadmaps in other regions, in the interest of a global sharing of efforts at complementary research frontiers and to minimize an unwanted duplication of major research infrastructures.

The initial 2006 strategy focused on the completion and initial exploitation of the Large Hadron Collider (LHC) at CERN, and the 2013 update on the upgrade to the High-Luminosity LHC (HL-LHC) which is now under construction. Whereas HL-LHC remains the obvious near-term priority, the 2020 update also faced the challenge of developing a strategy for the post-LHC era. The Higgs boson that was discovered with the LHC in 2012 is a cornerstone of the successful Standard Model of particle physics, and there are compelling arguments for a new large electron-positron collider that would operate as a “Higgs factory” to study the unique properties of this fundamental particle with the highest accuracy in clean experimental conditions.

Different avenues can be charted to reach this goal. The shortest is a European participation in the “International Linear Collider” (ILC) which has been proposed for construction in Japan for many years; a candidate site for this machine has been identified where it could be built with a short lead time, and with mature and established technologies. It would reach a centre-of-mass energy of 500 GeV in a 30 km long tunnel, and could possibly be upgraded to 1 TeV by extending the length. However, the future of the ILC seems uncertain since thus far – even after reducing the initial energy to 250 GeV – it has failed to gain unequivocal support from the Japanese government and from the wider Japanese scientific community. Moreover, a straight tunnel does not open a direct path to a new discovery machine at the high-energy frontier of particle physics, where the LHC has firmly positioned Europe as the global leader.

A complementary, and much more ambitious, scenario is under discussion for CERN: the “Future Circular Collider” (FCC) in a new tunnel of 100 km circumference, which could initially accommodate a circular electron-positron collider (FCC-ee), that would be replaced at a later stage by a new hadron-hadron collider (FCC-hh) with a target energy of 100 GeV in the centre-of mass. This would replicate the successful LEP-LHC scenario on a larger scale. Whereas the energy of FCC-ee will be intrinsically limited to 365 GeV by synchrotron radiation, such a machine would be straightforward to build in principle. In contrast, for FCC-hh to attain seven to eight times higher proton and ion energies than the LHC in a 100 km long tunnel calls for novel technologies for superconducting bending magnets, which in turn require a massive development effort. R&D into a new generation of dipole magnets based on niobium-tin (Nb₃Sn) superconductors has started at CERN and has produced encouraging results, but there is still a long way ahead to reach the required field strengths, and to bring this technology to maturity for the mass production of magnets on an industrial scale. Last not least, HL-LHC will need to deliver results that help to sharpen the physics case for a new discovery machine.

Finally, a new facility of the dimensions of the FCC cannot be financed within CERN’s present institutional and budgetary framework. When the LHC was built, 90% of the total cost of the accelerator infrastructure were funded by the Member States through their regular contributions to the CERN budget; only 10% were contributed – mostly in-kind – by non-Member States (the non-Member State contributions to the LHC detectors are significantly higher). This model is not scalable to the much larger dimensions of the FCC, which can only be built through a truly global effort that will require new and innovative governance and funding mechanisms.

Against this background of complex scientific, technological and political imponderables, the European Strategy Group (ESG) which was charged to prepare the update for the CERN Council has been prudent not to voice explicit support for either of the two scenarios. The core recommendation is that Europe, together with its international partners, should investigate the technical and financial feasibility of a future hadron collider at CERN with a centre-of-mass energy of at least 100 TeV and with an electron-positron Higgs and electroweak factory as a possible first stage. In parallel, the strategy update keeps the door open for a European participation in the ILC.

Regardless of this restraint, the 2020 strategy update is a significant milestone on the long and arduous way to unravelling fundamental physics beyond the Standard Model, and to consolidating Europe’s leadership at the high energy frontier of particle physics.

For full details about the European Strategy for Particle Physics, the update process and the 2020 recommendations, see https://europeanstrategyupdate.web.cern.ch.