What’s the carbon footprint of a Higgs boson? It varies — a lot

Artist's impression of a future detector for a collider: a metal tunnel with a polygonal cross section in a white hall.
Artist's impression of a future detector for a collider: a metal tunnel with a polygonal cross section in a white hall.

Future colliders will produce many Higgs bosons so researchers can probe the particle’s physics.Credit: Polar Media/CERN

Physicists around the world are vying to build the planet’s next super collider. Any such machine would be kilometres long and would consume as much energy as a medium-sized city — but the carbon footprints of the various designs could be vastly different, says an analysis led by a physicist at CERN, Europe’s particle-physics laboratory.

A machine proposed by CERN, the Future Circular Collider (FCC) near Geneva, Switzerland, would use just one-sixth of the energy of its most power-intensive rivals to achieve the same physics goals, finds the study, which was published last month in The European Physical Journal Plus1.

The FCC’s energy credentials get even better when the various sources of energy that power the electricity grid at each site are factored in, say the authors, who are long-time proponents of building the FCC. Accounting for the fact that more than 90% of electricity at CERN comes from carbon-free sources, such as nuclear power, the FCC’s carbon footprint would be just 1% of the most polluting alternative.

POWER-HUNGRY PROPOSALS. Graphic comparing carbon footprints of five ‘Higgs factory’ collider designs.

Source: Ref. 1

CERN already hosts the world’s most powerful particle accelerator, the Large Hadron Collider (LHC). In 2012, LHC physicists discovered the Higgs boson, the particle associated with the field that gives matter its mass. At the LHC, Higgs bosons are produced only rarely. Physicists now want a multibillion-dollar ‘Higgs factory’ dedicated to churning out the particles, in the hope that studying them in precise detail will lead to exciting discoveries.

Patrick Janot at CERN and Alain Blondel, a particle physicist at the University of Geneva, used published details of five leading ‘Higgs factory’ designs to calculate the energy consumption per Higgs boson for each. As well as the FCC and China’s proposed Circular Electron Positron Collider (CEPC), they looked at three proposals for linear colliders: a long-planned International Linear Collider (ILC) in Japan, CERN’s own Compact Linear Collider (CLIC) and the Cool Copper Collider (C3), a compact US-based accelerator.

It makes sense to look at energy consumption per Higgs boson, because “the ability to do science is directly related to the number of Higgs bosons produced”, says Janot. Circular colliders tend to have a similar annual energy consumption to linear ones, but they generate Higgs bosons faster, so they can meet the same science goals in a shorter time.

The FCC would use 3 megawatt hours of electricity per Higgs boson produced, the researchers found. The CEPC was close behind, with an energy consumption of 4.1 megawatt hours per boson. Linear machines performed worse on the energy-efficiency front, with C3 coming bottom of the pile at 18 megawatt hours per boson (see ‘Power-hungry proposals’).

When it comes to selecting the next accelerator, carbon emissions will be as important as cost, says Veronique Boisvert, a particle physicist at Royal Holloway University in London. The authors’ approach is a good first step towards estimating these emissions, she adds. But emissions produced by each facility’s construction, decommissioning and the greenhouse gases produced by detectors — which were not factored into this study — could end up being just as important as their operational footprints.

Changing assumptions

The carbon footprint of future colliders is a hugely important issue, agrees Wang Yifang, director of Beijing’s Institute of High Energy Physics (IHEP) and a pioneer of the CEPC. The study’s methods are sound, he adds, but many of the figures used are based on assumptions that could change. Ongoing efforts to hone technology for the CEPC could cut energy consumption by 10% compared with current estimates, he says.

Physicists behind the C3 collider are also working to improve energy-efficiency and exploring ways to power operations with renewable energy, says Caterina Vernieri, a particle physicist at Stanford University in California who is part of the design team.

The ramp up of renewable power in host countries might also cause the paper’s conclusions to “change dramatically”, Wang adds, noting that China plans to cut the proportion of its energy that comes from fossil fuels from 83% today to 64% in 2035, when it is hoped that the CEPC will become operational. Indeed, many potential host nations are aiming for zero-carbon grids by 2050 or earlier, says Boisvert.

However, emissions from civil-engineering processes — mostly from producing cement used for the tunnels — are likely to remain difficult to eliminate, she says. There, C3 might have an advantage: at just 8 kilometres long, compared with the FCC’s 90-kilometre circumference, its construction should have a lower carbon footprint.

For now, the carbon footprint associated with running a collider is generally larger than that of its construction, says Janot, who notes that by the time electricity is carbon-free, the emissions associated with civil engineering might also have fallen. Factoring in emissions from construction would roughly double the FCC’s carbon footprint per Higgs boson, but the total would still be much lower than the footprints of linear accelerators based on operations alone, he says.

The authors did the analysis partly because fighting global warming is hugely important to both of them personally, says Janot. But factoring in environmental concerns when choosing the world’s next collider will also help researchers to gain the backing of governments and the public, he says.

All collider designs should maximize energy efficiency, but the ultimate decision about which machine to build will be based on many factors, including the overall cost, input from the host country and international support, says Wang.

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