The discovery of the Higgs boson by the ATLAS and CMS collaborations at the LHC in 2012 marked a turning point in particle physics. Not only was it the last of the Standard Model particles to be found, but it is completely different to any particle seen before: a fundamental scalar, with profound connections to the structure of the vacuum. Extensive measurements so far suggest that the particle is the simplest possible version that nature permits. But the study of the Higgs boson is still in its infancy and its properties present enigmas, including why it is so light, which the Standard Model cannot explain. Particle physics is entering a new era of exploration to address these and other outstanding questions, including unknowns in the universe at large, such as the nature of dark matter.
The 2020 update of the European strategy for particle physics (ESPPU), which was released today during the 199th session of the CERN Council, sets out an ambitious programme to carry the field deep into the 21st century. Following two years of discussion and consultation with particle physicists in Europe and beyond, the ESPPU has identified an electron–positron Higgs factory as the highest priority collider after the LHC. The ultraclean collision environment of such a machine (which could start operation at CERN within a timescale of less than 10 years after the full exploitation of the high-luminosity LHC in the late 2030s) will enable dramatic progress in mapping the diverse interactions of the Higgs boson with other particles, and form an essential part of a research programme that includes exploration of the flavour puzzle and the neutrino sector.
We have started to concretely shape CERN’s future after the LHCUrsula Bassler
To prepare for the longer term, the ESPPU prioritises that Europe, together with its international partners, explore the technical and financial feasibility of a future proton–proton collider at CERN with a centre-of-mass energy of at least 100 TeV. In addition to allowing searches for new phenomena at unprecedented scales, this machine would enable the detailed study of how the Higgs boson interacts with itself – offering a deeper understanding of the electroweak phase transition in the early universe after which the vacuum gained a non-zero expectation value and particles were enabled to acquire mass.
“The strategy is above all driven by science and presents the scientific priorities for the field,” said Ursula Bassler, president of the CERN Council. “We have started to concretely shape CERN’s future after the LHC, which is a difficult task because of the different paths available.”
Setting the stage
The strategy update is the second since the process was launched in 2005. It aims to ensure the optimal use of global resources, serving as a guideline to CERN and enabling a coherent science policy in Europe. Building on the previous strategy update, which concluded in 2013, the 2020 update states that the successful completion of the high-luminosity LHC should remain the focal point of European particle physics, together with continued innovation in experimental techniques. Europe, via the CERN neutrino platform, should also continue to support the Long Baseline Neutrino Facility in the US and neutrino projects in Japan. Diverse projects that are complementary to collider projects are an essential pillar of the ESPPU recommendations, which urge European laboratories to support experiments enabling, for example, precise investigations of flavour physics and electric or magnetic dipole moments, and searches for axions, dark-sector candidates and feebly interacting particles.
The continuing ability of CERN, European laboratories and the particle-physics community to realise compelling scientific projects is essential for scientific progress, states the report. Cooperative programmes between CERN and research centres and national institutes in Europe should be strengthened and expanded, in addition to building strong collaborations with the astroparticle and nuclear physics communities.
Exploring the next frontier
The 2013 ESPPU recommended that options for CERN’s next machine after the LHC be explored. Today, there are four possible options for a Higgs factory in different regions of the world: an International Linear Collider (ILC) in Japan, a Compact Linear Collider (CLIC) at CERN, a Future Circular Collider (FCC-ee) at CERN, and a Circular Electron Positron Collider (CEPC) in China. As Higgs factories, the ESPPU finds all four to have comparable reach, albeit with different time schedules and with differing potentials for the study of physics topics at other energies. While not specifying which facility should be built, the ESPPU states that the large circular tunnel necessary for a future hadron collider at CERN would also provide the infrastructure needed for FCC-ee as a possible first step. In addition to serving as a Higgs factory, FCC-ee is able to provide huge numbers of weak vector bosons and their decay products that would enable precision tests of electroweak physics and the investigation of the flavour puzzle.
Considering colliders at the energy frontier, a 3 TeV CLIC and a 100 TeV circular hadron collider (FCC-hh) were considered in depth. While the proposed 380 GeV CLIC also offers a Higgs factory as a first stage, the dramatic increase in energy possible with a future hadron collider compared to the 13 TeV of the LHC has led the ESPPU to consider this technology as the most promising for a future energy-frontier facility. Europe together with international partners will therefore begin a feasibility study into building such a machine at CERN with the FCC-ee Higgs and electroweak factory as a possible first stage, to be established as a global endeavour and completed on the timescale of the next strategy update later this decade. It is also expected that Europe invests further in R&D for the high-field superconducting magnets for FCC-hh while retaining a programme in the advanced accelerator technology developed for CLIC, which also has significant potential applications in accelerator-based science beyond high-energy physics.
Europe should keep the door open to participate in other headline projectsHalina Abramowicz
The report also notes that the timely realisation of the ILC in Japan would be compatible with this strategy and, in that case, the European particle physics community would wish to collaborate. “The natural next step is to explore the feasibility of the highest priority recommendations, while continuing to pursue a diverse programme of high-impact projects,” explains Halina Abramowicz, chair of the European Strategy Group, which was charged with organizing the 2020 update. “Europe should keep the door open to participate in other headline projects which will serve the field as a whole.”
Ramping up accelerator R&D
To achieve the ambitious ESPPU goals, particle physicists are urged to undertake vigorous R&D on advanced accelerator technologies, in particular concerning high-field superconducting magnets including those based on high-temperature superconductors. Europe should develop a technology roadmap, taking into account synergies with international partners and other communities such as photon and neutron science, fusion energy and industry, urges the ESPPU report, which also stresses the proven ability of innovative accelerator technology to drive many other fields of science, industry and society. In addition to high-field magnets, the roadmap should include R&D for plasma-acceleration schemes, an international design study for a muon collider, and R&D on high-intensity, multi-turn energy-recovery linacs.
It is an historic day for CERN and for particle physics in Europe and beyondFabiola Gianotti
The ESPPU recommendations strongly emphasise the need to continue with efforts to minimise the environmental impact of accelerator facilities and maximise the energy efficiency of future projects. Europe should also continue to vigorously support theoretical research covering the full spectrum of particle physics, pursuing new research directions and links with cosmology, astroparticle physics and nuclear physics. The development of software and computing infrastructures that exploit recent advances in information technology and data science are also to be pursued in collaboration with other fields of science and industry, while particle physicists should forge stronger relations with the European Commission and continue their leadership in promoting knowledge-sharing through open science.
“It is an historic day for CERN and for particle physics in Europe and beyond. We are all very excited and we are ready to work on the implementation of this very ambitious but cautious plan,” said CERN Director-General Fabiola Gianotti following the unanimous adoption of the resolution to update the strategy by the CERN Council’s national representatives. “We will continue to invest in strong cooperative programmes between CERN and other research institutes in CERN’s member states and beyond. These collaborations are key to sustained scientific and technological progress and bring many societal benefits.”