Source: The Guardian

As the largest scientific instrument on the planet enters its twilight years, Cern scientists have been facing the question of what next after the Large Hadron Collider (LHC). Following extensive debate, they appear to have landed on an answer: go bigger or go home.

Cern’s council is expected to announce its backing on Friday for a proposed new collider with a 100km circular tunnel, four times the size and six times as powerful as the LHC. A formal vote on the plan is due to go ahead on Friday.

The proposed particle-smashing machine, referred to as the Future Circular Collider (FCC), would allow scientists to hunt for particles and other new phenomena at higher energies, study the Higgs boson with greater precision, and may offer insights into dark matter.

Speaking before Cern’s council meeting, Prof Jon Butterworth, of University College London, said a scaled-up collider would allow scientists to make unprecedented measurements of nature at the subatomic scale. “It’s probing nature at the shortest distances and looking for the smallest things we can see … it’s a real exploratory mission,” he said. “Everyone agrees that’s what we need to do. The question has been: what’s the best machine to do it?”

But some question whether the scientific promise of the gigantic machine justifies the anticipated €20bn price tag for construction.

If approved in Friday’s vote, Cern is expected to assess the technical and financial feasibility of the FCC, with initial efforts going into a geological survey to check there are no underground lakes or other show-stopping features beneath Geneva that would require the plan to be reconsidered. Research and development efforts into high-field superconducting magnets and other technologies required for the proposed collider will also be ramped up.

There is no guarantee the machine will be built. Capital investment from EU member states and other Cern participants such as the UK will be required for construction to go ahead – and a commitment to continue funding operations into the 2050s. If the financial backing is secured, construction could begin within a decade and it would take 10 years to build, meaning it would not be operational until the 2040s.

The FCC proposal won out against alternative visions of what to do when the LHC winds down around a decade from now. Competing plans included a large linear collider, which some viewed as a safer option because it could have been extended in stages, whereas the size of a circular collider must be decided at the outset. A circular collider has the advantage of being able to house more experiments along the same tunnel.

The planned machine would be built up in stages. For the first phase, the FCC would collide electrons and their positive counterparts, positrons. Then it would collide electrons with much heavier nuclei of lead atoms. By the 2050s, it could be smashing protons together with an energy of 100 teraelectronvolts (TeV), around six times higher than the LHC’s maximum capability.

The new machine would act as a “Higgs factory”, allowing scientists to target the production of large numbers of Higgs bosons and study more precisely how they decay, which has not been possible with the LHC. Some theories suggest the Higgs particle may decay into dark matter particles, the nature of which remains one of the central mysteries in modern physics. “If there’s any missing, you know there’s something weird going on,” said Butterworth.

Moving to higher energy regimes could also uncover new phenomena yet to be predicted. However, there is not the same guaranteed win that the LHC offered before it was built, when scientists knew the collider would sweep through the predicted energy range for the Higgs and either confirm its existence or prove that theoretical physics needed a complete overhaul.

“With the LHC, we knew we’d either find the Higgs or break the Standard Model, and that’s a very luxurious position to be in,” said Butterworth. “There is no equivalent scenario now, it’s much more exploratory. It’s definitely higher risk.”

The LHC itself has highlighted the uncertain nature of progress. Many hoped that after finding the Higgs, further questions in fundamental physics would be resolved, such as a hypothesis called supersymmetry, which predicts that subatomic particles that make up normal matter possess mirror, or supersymmetrical, versions of themselves. However, the LHC is yet to observe any supersymmetric entities, or any dark matter particles, which had also been in scientists’ sights.

Sabine Hossenfelder, a theoretical physicist at the Frankfurt Institute for Advanced Studies in Germany, said open-ended exploration and the refining of existing knowledge was not enough to justify the enormous cost of the proposed machine. “On some level I find it irresponsible,” she said. “Why don’t we put the money into an international centre for climate models or pandemic models?”

The LHC is currently being upgraded and is due to restart in May 2021 and run until the end of 2024, with a final run expected to begin at the end of 2027.

Source: The Guardian