The recent proposal by CERN to construct a large 100 km circumference Future Circular Collider (FCC) capable of 100 TeV proton-proton collisions has generated both enthusiasm and scepticism in the particle physics community [1].
Proponents point to the FCC’s ability to push into uncharted territories of energy, luminosity, and precision, opening new windows into phenomena beyond the Standard Model (SM) like dark matter, quark-gluon plasma formation, and other fundamental questions inaccessible to past colliders like the Large Hadron Collider (LHC) [2]. However, the FCC’s estimated $21 billion price tag gives pause on whether such a massive investment is the optimal strategy for advancing particle physics compared to alternate approaches [3].
This ScienceShot examines the key motivations put forth for the FCC and contrasts them against the substantial economic, environmental, and opportunity costs incurred.
The FCC and The New Possible Physics
A central motivation underlying proposals for larger accelerators is expanding the energy frontier to probe new particles and interactions beyond what has been accessible historically. According to the Standard Model, many hypothesized particles like WIMPs, supersymmetric partners, or Z’ bosons could lie in the ~10-100 TeV range, necessitating colliders substantially more powerful than existing ones to produce them [4].
The FCC with 100 TeV proton beams would provide sufficient energy to survey a large swath of unexplored parameter space where new exotic particles may emerge. The 7-fold leap in energy over LHC’s 14 TeV and 30-fold increase in luminosity to 5 x 10^34 cm^-2 s^-1 also enables more copious production of rare processes, boosting experimental signatures and statistics [1,5].
addition to direct production of new Beyond Standard Model (BSM) particles, the FCC’s ultra-high luminosity enables high-precision measurements of SM particle properties. Deviations in precision tests of the SM could offer indirect evidence of new virtual particles and interactions [5]. The proposed FCC-ee (electron-positron) configuration with a tunable centre of mass energy from 90-350 GeV would enable 1-2 order of magnitude improvement in precision EW, QCD, and flavour measurements over LEP thanks to the production of 10^12-10^13 Z bosons, for example [6].
The 100 TeV proton beams also create extreme conditions of density and temperature rivalling the early universe, enabling new studies of the Quark Gluon Plasma (QGP) [7]. Measurements of collective phenomena in pp collisions could shed light on colour confinement and phases of nuclear matter. The higher energies lead to increased initial QGP temperatures and larger volumes, probing the structure of the QCD phase diagram under more extreme conditions.
While these capabilities present a compelling physics case, the question remains whether the scale and format of the FCC constitute the optimal investment.
Costs and Tradeoffs of Scaling to 100 km Colliders
The projected €21 billion ($24 billion) budget of the FCC gives considerable pause on whether alternative approaches in particle physics may reap higher returns on investment [3].
Constructing and operating 100 km rings requires vast expenditure in civil engineering, cryogenics, magnets, and detectors that limits funding available for other science areas. There is also a risk of unchecked cost overruns that plagued past colliders; the SSC atom smasher in Texas was cancelled in 1993 primarily due to its budget ballooning from $4.4 billion to $12 billion. Stringent contingency funds must thus be provisioned in any FCC proposal, inevitably increasing upfront costs [8].
The massive power consumption is another formidable cost. Though the FCC aims for 40% higher energy efficiency than LHC, it still requires a straggling 300-400 MW, enough to power 120,000 households [1, 9]. Operating costs over decades could amount to billions in electrical bills accrued.
There are also environmental impacts from the FCC’s footprint that must be accounted for. Excavation of 100 km of tunnels, installation of cryogenics and ventilation plants, and accumulation of activated waste materials disturb local habitats and generate substantial carbon emissions [10].
According to one study, the LHC’s estimated carbon footprint from materials and electricity usage is 120 kt/year CO2 equivalent, comparable to a small city like Liverpool or Bologna [11]. Scaled estimates suggest the FCC’s footprint would be 2-3 times higher even given efficiency gains. Regulatory costs associated with environmental assessments and emissions taxes could thus be considerable depending on hosting state policies.
The funds and resources consumed by the FCC also entail opportunity costs, limiting budgets available for alternative particle physics programs oriented around astroparticles, intense lasers/plasma accelerators, or exascale computing.
Investing billions in a single megaproject inhibits diversification into these complementary approaches compared to a balanced portfolio.
Alternate Strategies for Exploring the Energy Frontier
Innovative Particle Acceleration Techniques
Rather than simply scaling up existing collider architectures like FCC proposals, some alternate concepts employ innovative techniques to accelerate particles.
Laser-driven particle acceleration uses intense, ultrafast laser pulses focused onto a target to generate enormous electric fields strong enough to accelerate electrons to high energies in extremely compact distances [12]. As lasers with petawatt powers and femtosecond pulses become increasingly available through wider adoption in materials science and fusion research, they could serve as compact and inexpensive drivers of energetic particle beams. However, despite success in accelerating particles in the lab, laser-driven colliders currently lack the efficiency, beam quality, and repetition rates necessary for physics experiments.
Plasma-based wakefield acceleration is another highly promising technique that has gathered rapid interest recently [13, 14]. By propagating intense ultrarelativistic particle beams or lasers through an ionized medium, enormous electric fields on the order of 100 GV/m can be established. These large wakefields then give trailing particles an immense boost in energy compared to conventional cavities, with gradients orders of magnitude greater over far shorter distances measured in cm rather than km.
If such techniques can be further refined to control beam emittance and energy spread while scaling up beam powers and energies, plasma accelerators may eventually drive compact colliders compared to the civil engineering endeavours needed for FCC-scale machines. Ongoing experiments at FACET-II and other test facilities aim to provide wakefield gradients beyond 10 GV/m at high wall plug efficiency, stability, and beam brightness necessary to assess readiness for future colliders [15].
Studying Cosmic Messengers and Astroparticles
Rather than rely solely on human-made terrestrial accelerators, fundamental particle interactions can also be probed by studying beams naturally produced across the cosmos. Experiments like IceCube, ANITA, and Pierre Auger tackle mysteries like the origins of cosmic rays, neutrino masses and mixing parameters, dark matter, and other phenomena by examining particles that originate from energetic astrophysical events [16]. Space-based experiments can also open new windows, with recent successes like the Alpha Magnetic Spectrometer aboard the ISS collecting antimatter signatures and the upcoming CALET dark matter explorer [17, 18]. As detectors grow in scale harnessing techniques like radio or acoustic sensors spread over vast natural volumes, their sensitivities to rare processes continue improving.
Meanwhile, intensifying efforts to directly detect dark matter interactions underground like LUX, XENON, and SuperCDMS constrain WIMP properties complementing colliders [19]. Other fundamental physics areas from axions and axion-like particles to allowing violations can also be probed efficiently via table-top experiments on Earth [20]. Compared to costly beam acceleration, these astroparticles and small-scale experiments achieve substantial physics impact from natural processes at modest budgets.
Harnessing High-Performance Computing
The continual exponential advancement of computing hardware from teraflop to petaflop to upcoming exaflop capabilities also opens new avenues for particle collider research. HPC systems now allow sophisticated simulations of collisions using lattice Quantum Chromodynamics, modelling interactions down to first principles of quarks and gluons. Supercomputers have been pivotal in calculating crucial background processes needed to accurately interpret LHC data like multi-jet production [21].
Future exascale computing promises to greatly expand the role of simulation, enabling “virtual colliders” that accurately emulate experimental signatures of various phenomena to guide accelerator designs and theory [22]. With magnitudes lower power consumption than physical beamlines and no civil engineering needed, simulation-based research can stretch budgets considerably further than single experiments [23]. Machine learning techniques also boost analysis capabilities when applied to vast datasets from detectors [24].
Space-Based Particle Colliders
While technically formidable today, proposals have also been made for particle accelerators and colliders integrated into satellites and space stations that could provide advantages over Earth-bound facilities [25]. Orbiting colliders circumvent issues like footprint, electricity costs, tunnelling expenses, and activated background since acceleration and collisions occur away from inhabited areas [26]. The microgravity in space also enables the construction of colliders with very large circumferences not feasible on Earth, allowing longer straightaway sections for acceleration and cleaner collisions with fewer beamstrahlung effects that impose luminosity limits.
Colliders based on rings, recirculating linacs, wakefield acceleration, or laser-driven schemes with circumferences from 100 km up to 1000 km have been proposed, though enormous challenges exist in precision formation flying, attenuation, power, heat dissipation, and maintenance [26, 27]. Still, the promise of multi-TeV collisions in a flexible geometry has spurred proposals like the NASA Starlight program to continue advancing critical technologies needed to eventually realize space-based colliders [28]. Even proof-of-concept experiments like Small Accelerators in Space (SMILE) aim to launch MeV-level miniature accelerators and make steps towards high-energy frontier machines in orbit [29].
Our Opinion
The proposal for a 100 km, 100 TeV proton collider like CERN’s Future Circular Collider sets its sights on pushing the energy frontier substantially beyond the LHC’s capabilities.
However, while such a facility promises to expand our knowledge of fundamental particles and early universe conditions considerably through phenomena like dark matter, BSM physics, and Quark-Gluon Plasma, the enormous costs involved give pause on whether alternative approaches may yield higher returns on investment.
The $24 billion civil engineering megaproject also limits funding available for diversified particle physics programs oriented around developing novel acceleration methods, astroparticle physics, exascale computing, and other complementary techniques. We thus argue for a balanced portfolio approach that expends R&D effort advancing multiple collider concepts and detector technologies in parallel rather than narrow bets on a single facility. Incremental developments in accelerators based on intense lasers, plasma wakefields, and other innovative methodologies with further technological maturity could eventually realize more compact and economical colliders compared to simply replicating traditional ring architectures at a larger scale.
Meanwhile, astrophysics experiments harness naturally occurring cosmic beams, supercomputing brings powerful simulation capabilities rivalling physical setups, and possibilities even exist for space-based colliders if launch costs continue declining. While all alternatives carry respective challenges today, investing in such portfolio seedlings seems prudent to avoid missed opportunities and hedge risk given the sacrifices entailed in committing overwhelmingly to a single megaproject. Constructive debate on options must thus continue as we chart the future course of particle physics in the global community.
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