Next Steps Taken Toward the Future of Particle Physics in Europe

By: Hannah Pell

Since the discovery of the Higgs Boson (or, more infamously, the “God Particle”) in 2012 at the Large Hadron Collider (LHC) based at the European Center for Nuclear Research (CERN), particle physicists have sought to expand their ability to probe Nature’s deepest secrets. Although the Higgs was the last missing piece of the Standard Model — a well-tested physical theory describing fundamental particles and their interactions — some physical phenomena still remain unexplained, notably the existence of dark matter. In order to search for new physics beyond the Standard Model, physicists have argued that they will need tools to smash particles together at unprecedented energies and scales. (The alternative, that the LHC finds nothing new or unexpected regardless of upgrades, is commonly referred to as the “nightmare scenario”). 

 On June 19th, 2020, after more than two years of debate, the CERN Council unanimously approved and published the 2020 Update of the European Strategy for Particle Physics, which outlines priorities and aims for the European particle physics community over the next several decades. “This Strategy presents exciting and ambitious scientific goals that will drive technological and scientific exploration into new and uncharted territory for the benefit of the field and of society,” the document explains. An important assertion is a commitment to “an electron-positron Higgs factory” as the highest-priority initiative. A new collider with a center-of-mass energy of at least 100 TeV encompassed in a 100 km circumference network of tunnels and magnets. Such is the plan for the Future Circular Collider (FCC).
European Strategy for Particle Physics Update. Image Credit: CERN.
 The Strategy Update: What does it mean? 
The FCC conceptual design was initially published in January 2019 in a four-volume series report authored by the FCC Study, an international collaboration of scientists from more than 150 universities. Before the CERN Council’s vote, several other options for future colliders were on the table, but the strategy update signals that now CERN will shift its focus from debating potential alternatives to researching how they can make the FCC happen.
 “The strategy is above all driven by science and presents the scientific priorities for the field,” said Ursula Bassler, president of the CERN Council, in the CERN Courier. “We have started to concretely shape CERN’s future after the LHC, which is a difficult task because of the different paths available.”
 The strategy document outlines a two-fold plan. The first is that CERN will build a new electron-positron collider for the purpose of producing Higgs bosons. This first machine will then be dismantled to be replaced by a proton-proton smasher that can reach energies of up to 100 TeV (the LHC currently operates at 14 TeV). The FCC will eventually accelerate beams of electrons and positrons around a circular ring more than 3 times longer than the LHC at more than 7 times the current operating energies. 
 But is it always true that bigger is better? CERN’s emphasis on the importance of the FCC for the future of particle physics in Europe has naturally raised questions regarding its feasibility, affordability, and our scientific priorities. 
 A Bold Step? 
Before CERN can begin construction on the FCC, which is currently projected to start in 2038, they will need to secure the estimated €21 billion to fund the machine. The costly price tag will inevitably force CERN to seek additional funding beyond its revenue from the current group of 23 member states. Former CERN director-general Chris Llewellyn Smith told Nature that additional countries, including the United States, China, and Japan, might need to join CERN to expand the global organization. “Almost certainly it will need a new structure,” he said. 
 Although the update has been framed as yet another “bold step” in collider physics, not all physicists agree that the FCC is the best path forward. Sabine Hossenfelder, a theoretical physicist based at the Frankfurt Institute for Advanced Studies in Germany, has emerged as an outspoken critic of the plan, among other decisions made in the particle physics community. In an op-ed for Scientific American — which was published on the same day as the European Strategy for Particle Physics Update — she argues that the price tag is too extraneous for the scientific justification as it currently stands, and that the funds would be more useful if directed toward socially relevant scientific problems, such as climate change and pandemic modeling. 
 “But as the strategy update reveals, particle physicists have not woken up to their new reality. Building larger particle colliders has run its course. It has today little scientific return on investment, and at the same time almost no societal relevance. Large scientific projects tend to generally benefit education and infrastructure, but this is not specific to particle colliders. And if those side effects are what we are really interested in, then we should at least put our money into scientific research with societal relevance,” she writes. 
 Jared Kaplan, a theoretical physicist at Johns Hopkins University, has echoed this sentiment. “There are a lot of other experiments that are proposed and ongoing that are much cheaper. There are still huge mysteries in other domains of physics. A lot of the experiments that search for dark matter are $10 million, not $20 billion. It might make more sense to fund a hundred of those experiments than build one collider for 10 times as much money.”
 However, proponents of the project argue that these plans are more of an all-or-nothing scenario. Nima Arkani-Hamed, a theoretical physicist at the Institute for Advanced Study in Princeton, New Jersey, pointed out in a 2019 interview with the CERN Courier that science is not a “zero-sum game.” “It’s not a question of, ‘do we want to spend tens of billions on collider physics or something else instead,’ it is rather ‘do we want to spend tens of billions on fundamental physics experiments at all.’” The failure of the United States to construct the Superconducting Super Collider in the 1990s is another painful reminder of this fact. 
 Other Future Colliders 
Countries outside of Europe are still considering other plans for future particle colliders. Physicists in Japan are still considering the International Linear Collider (ILC). The ILC is also proposed to function as a “Higgs Factory,” colliding electrons and positrons with one another at energies up to 0.5 TeV. The ILC has yet to receive formal approval from the Japanese government, which will only be given if sufficient international funding is pledged and if consensus support within the Japanese physics community is established. CERN views the ILC and FCC as “complementary” machines, stating that the “timely realization of the electron-positron [ILC] in Japan would be compatible with this strategy and, in that case, the European particle physics community would wish to collaborate.” 
 A plan for a new collider called the Circular Electron Positron Collider/Super Proton-Proton Collider (CEPC-SPPC) is also underway in China. First proposed in 2012, CEPC-SPPC would unfold in two phases: the first, an electron-positron “Higgs Factory” collider housed in a 100-km tunnel (sound familiar?), and the second, a proton-proton collider with energies up to 100 TeV. However, the future of this plan is uncertain, and the CEPC-SPPC has also sparked similar criticisms
 Needless to say, there seems to be plenty of future opportunities for collaboration and competition in the global particle physics community. 
 The Future of Particle Physics 
Particle physics is complex, exciting, and dynamic. The fact that the global particle physics community thrives by collaborating across borders of differing kinds — geographic, linguistic, and cultural — by seeking answers to some of the most profound questions about the physical world is nothing short of inspiring. The technological, socio-economic, cultural benefits from massive particle collider projects should not be overlooked as we argue the importance of this basic research.
 However, we cannot deny that there could be a better way. Perhaps shifting our focus to prioritizing smaller-scale experiments could unlock the new physics we seek to grasp. Perhaps it’s time to realign our scientific priorities toward other equally profound questions within physics or by reconsidering the role by which particle physics contributes in the broader scientific landscape. 
 For now, it seems that only time will tell.

From the FCC Conceptual Design Report. Image credit: CERN.

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