CERN unveils its new European strategy - it does not rule out a new particle accelerator

An updated European strategy for particle physics research was long overdue. CERN, the European Organization for Nuclear Research, has finally unveiled its roadmap. This exercise in scientific multilateralism by CERN’s 23 member states will define the main directions of physics research not only at CERN, but for Europe, over much of the coming decade.

The strategy does not rule out the construction of a new, and even more powerful, particle accelerator to eventually replace the Large Hadron Collider (LHC), whose famous course snakes its way underground in a 27 kilometer loop of countryside along the Swiss-Franco border near its Geneva headquarters. But it doesn’t formally recommend one at this stage either.

Feasibility studies must continue, and it is not until the next update of the strategy, later in 5 - 7 years' time, that the issue will be put on the table again.

Why this is strategic. While research in particle physics takes time, the construction of large accelerators takes even longer, not to mention the costs involved, which means that several member states have to be convinced to invest. The construction of CERN’s Large Hadron Collider (LHC), the world’s largest and most powerful particle accelerator, was approved by the CERN Council in 1994, but the first particle beam was not emitted until 2008. All in all, CERN currently operates a complex of eight accelerators, of various sizes and functions, as well as two decelerators. From a scientific point of view, physicists must pool infrastructure as much as possible, and thus their main areas of research several years in advance.

Key points. The strategy does not only concern CERN, but the broader direction of European research in particle physics. The document released last week by the CERN Council, its member state decision-making body, which was drawn up following consultations with the the physics community in Europe, sets out the main research priorities as follows:

• Continuing research of the Higgs boson, discovered in 2012, leading to a Nobel Prize. Physicist Halina Abramowitz, Chair of the CERN commission that was responsible for updating the European strategy, explains:

"Since we discovered the Higgs boson, it's true that we don't know exactly where to go from here. But we believe that the study of the Higgs boson is one of the tools that will allow us to answer questions on the subject that remain unanswered".

• The exploration of high-energy collisions. CERN Director General Fabiola Gianotti expounds:

"We know that there is a whole field of physics that lies beyond the Standard Model. Exploring the high-energy field will allow for the development of new technologies, such as superconducting magnets, which could lead to new discoveries."

• Concerning the study of neutrinos, it was decided to continue collaborations with Japan and the United States, where large detectors such as the Super-Kamiokande or the Deep Underground Neutrino Experiment (DUNE) have already been built, or are under construction.

Projects in progress. This mainly revolves around the High-Luminosity LHC. This second-generation LHC, involving a series of staged improvements, is expected to begin operation sometime after 2025. It would generate more particle collisions, from a beam that is 5-7 times more concentrated than the one used today, and thus increase the chances of obtaining exotic, and perhaps unknown particles.

The LHC is expected to remain the major focus of European particle physics research until 2038, when the present-day collider infrastructure would be retired.

Projects under consideration. In the longer term, the strategy mentions the construction of a "Higgs boson factory" capable of mass-producing the elusive particle from collisions between electrons and positrons (or anti-electrons). Construction of such a collider could begin around 10 years after the High-Luminosity LHC goes out of service, i.e. in 2048.

A 100 kilometer “Future Circular Collider” (FCC) also traveling underground in a loop through the Franco-Swiss region around CERN would provide the infrastructure for the collider. The FCC, the subject of a preliminary study in 2019, is projected to cost between 9 and 20 billion Euros (21 billion CHF). Alternatively, the organization could opt for a linear and less pharaonic project, the Compact Linear Collider (CLIC), currently estimated at around 6 billion CHF.

At an even later date, i.e. after 2050, the CERN Council also recommends that feasibility of a very high-energy collider be assessed. This would aim to generate particle collisions at an energy level of as much as 100 TeV (teraelectronvolts), whereas the current LHC only reaches 13 TeV. To attain such energy levels, an accelerator as large as the FCC would anyway be needed.

Even so, it remains to be seen how CERN will choose to orchestrate these investments, if they were finally approved: 6 billion CHF by 2040 for the CLIC, or up to 21 billion CHF directly for the FCC?

The question of finance. CERN underlined that the strategy is not, at this stage, the basis for a funding decision; it is above all an expression of European scientific vision. At a press conference, Ursula Bassler, President of the CERN Council, explained:

"Feasibility and R&D work on accelerator projects such as the Compact Linear Collider (CLIC) or the Future Circular Collider (FCC) will have to continue before the CERN Council can issue specific recommendations on them. This will be one of the issues at stake in the next update of the European strategy on particle physics. This usually takes place every 5 to 7 years."

Abramowitz also reminds us:

"These revisions are also based on the scientific results obtained with the infrastructures of the moment. With the High Luminosity LHC that will gradually come into operation by 2027, we will have new results at that time to refine the strategy."

Physics, between international collaboration and competition. Other countries, such as Japan with its International Linear Collider project, or China with its Circular Electron Positron Collider (CEPC) project, are also planning to build new accelerators. But these giants have become so expensive that they are now a full-fledged issue in international science policy. Ursula Bassler recalls:

"In the 1990s, there were still three separate accelerators in Europe. Today, the trend is towards pooling infrastructure and costs."

Under these circumstances, CERN's attitude can also be interpreted as one of healthy caution. Before deciding to build the FCC or CLIC, it is necessary to ensure that another accelerator meeting similar research needs is not about to be built somewhere else in the world.

Uncertainties related to the Covid-19 crisis. More generally, is funding for basic research likely to be affected by the economic crisis following the Covid-19 pandemic? Fabiola Gianotti reminds us of the central role played by science in the political response to the health crisis:

"We are of course living in difficult times, and investments in research may be affected. But at this stage, almost all Member States have paid their contributions in full, on time. What is interesting in these times is that people are listening to the voice of science; governments have set up scientific committees to deal with the health crisis... I hope that this attention to science will continue after the pandemic, because it is research that allows us to be prepared for crises of all kinds: health, environmental...".