(Inside Science) -- In 2012, particle physicists detected the long-sought-after Higgs boson for the first time. This particle was the last missing puzzle piece of what physicists call the Standard Model -- the most thoroughly tested set of physical laws that govern our universe. The Higgs discovery was made possible by a giant machine in Europe, known as the Large Hadron Collider that uses a 27-kilometer ring of superconducting magnets to accelerate and then smash particles together at near the speed of light.
But the Standard Model is not the be-all and end-all of physics. It falls short in providing explanations for mysteries such as the existence of dark matter or dark energy, or why gravity is so different from other fundamental forces.
Like the unchartered territories that medieval mapmakers filled with fantastic beasts, the frontiers of physics have been filled with a wealth of hypotheses for what may lurk in the darkness. And in science, the only way to confirm or disprove these hypotheses is to gather more data -- data from better telescopes and microscopes and, perhaps, a brand-new, even bigger supercollider.
In 2012, the Institute of High Energy Physics of the Chinese Academy of Sciences announced a plan to build the next great supercollider. The planned Circular Electron Positron Collider will be 100 kilometers around, almost four times larger than the Large Hadron Collider, or LHC. Then in 2013, the LHC's operator, known as CERN, also announced their plan for a new collider, named simply the Future Circular Collider.
However, the price of exploring the unknown often doesn’t come cheap. With at least a 10-figure price tag, scientists and engineers are debating whether the endeavor will be worth the investment.
Although the detection of the Higgs boson marked the completion of the Standard Model in some ways, there is still plenty of work to be done.
“We still don't understand the mass of the Higgs boson. We don't understand the family problem, as in why there are three families of particles,” said CERN Director-General Fabiola Gianotti. “So, studying the Higgs boson with the highest possible precision is a must, and a future collider will do so.”
As particles crash into each other at near the speed of light inside a supercollider, some of their combined kinetic energy is converted into mass, creating new particles such as the Higgs. However, the LHC can produce only one Higgs boson about every billion collisions, so even with the ability to produce hundreds of millions of collisions every second, it still took LHC several years to produce enough data for the Higgs signal to rise above background noise. A more powerful collider can increase the rate of production and enable scientists to study the Higgs boson better.
To better understand why so much energy is needed to create more particles, imagine a game of bowling with millions of bowling pins in the lane, some light and some heavy. There are a lot more light pins than heavy ones -- for example one million 1-ounce pins to every 1-ton pin. Now to “create” a heavy particle like the Higgs, which is akin to knocking down a heavy pin, you need to throw the bowling ball hard enough not just to knock down the heavy pin, but also to plow through the millions of smaller pins in the way.
The energy required to create particles like the Higgs boson is measured in what are called gigaelectronvolts, or GeV. The LHC can generate collisions with an energy of 13,000 GeV -- more than a hundred times the 125 GeV mass-energy equivalence of the Higgs boson. It can produce only one Higgs boson for every 10 billion collisions, due to all the energy expended on all the lighter particles.
There might be even heavier particles that are beyond the technical capability of the LHC to produce, or the LHC could be generating them at such a low rate that it is not statistically possible to detect them. In other words, if we want to knock over more “heavy pins,” we will need more “muscle.”
“We are in a situation where the Standard Model cannot explain various phenomena,” said Gianotti. “There are many other theories, but we have no clue which one is the right one. And so, making a step forward in terms of energy scale … can help redirect our thoughts.”
One of the leading theories beyond the Standard Model is known as supersymmetry. Seemingly abstract at first glance, the basic concept of supersymmetry is actually rather straightforward. Supersymmetry predicts that for each of the 17 fundamental particles in the Standard Model, there exist a hypothetical partner particle -- thus the “symmetry” -- and each of these hypothetical particles would be heavier than their corresponding, already discovered partner -- thus the “super.”
First introduced during the late 1960s and early 1970s, supersymmetry looked promising due to its mathematical elegance and its ability to explain why gravity appears to be much weaker than the other fundamental forces and to resolve other mysteries such as dark matter.
However, as promising as supersymmetry looks as a theory that could help unify all fundamental forces in the universe, particle scientists have yet to see any direct evidence that supports it after decades of experiments. Generations of particle physicists have worked on the theory and many thought they would finally see traces of supersymmetric particles when the LHC first came online.
The view from inside the tunnel housing the LHC, underneath the France-Switzerland border near Geneva. The tunnel is 27 kilometers in circumference and 574 feet underground at its deepest point.”
Image credit: Yuen Yiu
“The vast majority of our field before the LHC turned on, maybe 90% of us, were sure that this new physics was going to show up,” said Nigel Lockyer, who directs the Fermi National Accelerator Laboratory near Chicago. “I had a friend who said you'll find it in three weeks.”
However, the initial optimistic expectations were dashed.
“This is a beautiful time, you know, because the best time to be an experimentalist is when the theorists have run out of ideas. Because then anything we discover is new,” said David Newbold, who directs the particle physics program at Rutherford Appleton Laboratory in the U.K. and is currently leading an effort to upgrade one of the main detectors at the LHC.
Right now, nobody can say for sure how much more power we will need to find the next new particles -- if there are any. It is entirely possible that the next collider may not see them at all.
Since their proposal, both the European and Chinese plans for a new supercollider have sparked criticism from those who doubt the projects will prove their worth. Their proponents, meanwhile, argue that the uncertainty of a payoff is inherent to the process of exploring the unknown. The endeavor is rewarding regardless of whether it yields exciting new particles, since we will still be able to refine our understanding of the universe by ruling out theories that don't fit the new data.
A Tale of Two Colliders
In their conceptual design report, CERN listed three possible avenues for their Future Circular Collider to take, each providing a different set of advantages and disadvantages in science, engineering and cost. The first is the construction of an electron-positron collider (FCC-ee) 100 km around that will provide high-precision studies of the Higgs boson and other known particles. The second would upgrade the FCC-ee into a new hadron collider (FCC-hh) with an energy seven times that of the LHC. This design could include a hadron-lepton interaction point (FCC-he). And finally, perhaps at the bottom of the wish list, is an upgrade to the LHC (HE-LHC) that will double its current power to 27,000 GeV.
The CEPC project in China is slightly different. Without existing accelerator infrastructure like the LHC, the CEPC will need to be constructed completely from scratch. Their conceptual design report separates the project into two phases. The first phase, an electron-positron collider to be housed in a 100-km circular underground tunnel, will begin construction as soon as 2022 and be finished by the end of the decade. It will serve as a “Higgs Factory” that can provide more precise measurement of the Higgs boson. The second phase, dubbed the Super Proton Proton Collider or SPPC, will share space in the same 100-km tunnel. Its construction is planned to begin around 2040 and be finished by the mid-2040s. It will be a proton-proton collider like the LHC but with a much higher energy of up to 100,000 GeV that will allow it to search for new physics such as supersymmetry. The proposed location for the facility is Qinhuangdao, a port city 300 km east of Beijing, which is reachable by high-speed train in one hour.
Image credit: Abigail Malate, Staff Illustrator
Image rights: Copyright American Institute of Physics
“To be able to rule out theoretical scenarios and redirect our thoughts is as important as making new discoveries,” said Gianotti. “For instance, look at the satellites WMAP and Planck -- they didn't discover anything, but they made very precise measurements of our universe that revolutionized our understanding of the cosmos. We should be careful not to think that success in science is just discoveries.”
While physicists know they cannot know the results without building the instruments and doing the experiment, the economics of such exploration is more open to debate. What kind of price are we willing to pay for a better understanding of our universe?
Chen-Ning Yang, a Nobel-winning particle physicist, brought the debate to public attention in China in 2016. In a widely shared blogpost, he criticized the quest for signs of supersymmetry by way of a new supercollider as “a guess on top of a guess.” He also expressed his worry that the project will have a negative effect on the funding for other research fields, especially those that “need pressing solutions, such as in environment, education and health.”
Yang pointed to the canceled Superconducting Super Collider of the U.S as a “painful experience” that became a “bottomless pit” of wasted funding. Originally proposed with a price tag of $4.4 billion in 1987, the estimated cost for that collider quickly ballooned up to $12 billion, before being canceled in 1993 after $2 billion had already been spent. In hindsight, this instrument could have helped the scientific community discover the Higgs boson years earlier.
Tian Yu Cao, a philosopher of science and politics from Boston University, is pessimistic about the future of China's Circular Electron Positron Collider, or CEPC. He pointed to China’s last Five-Year Plan published in 2016, which did not mention the CEPC among the 10 flagship projects announced in the report.
“They are definitely hesitant,” said Cao. “They are hesitant because there are objections from people from all branches of physics. How can they get so much money for this project when there are so many other projects that need funding?”
The CEPC's European counterpart, the FCC, has also garnered some opposing voices. Theoretical physicist Sabine Hossenfelder published an op-ed in the New York Times earlier this year, in which she wrote, “I still believe that slamming particles into one another is the most promising route to understanding what matter is made of and how it holds together. But $10 billion is a hefty price tag. And I’m not sure it’s worth it."
Both projects are now still in the research and development phase, but with a construction timeline planned to begin in the next decade, the projects will likely attract more scrutiny as their proponents attempt to secure funding.
“Right now, we've got five years of justification of the study to do, then probably another five years or so of detailed engineering design. Then we would proceed at whatever pace we could, which was limited by the money,” said Newbold. “It’ll probably be a minimum of 20 years from now and maybe longer.”
The teams at FCC and CEPC published their conceptual design reports last year, each hundreds of pages long and authored by more than a thousand scientists and engineers. At first glance, both projects are aiming to accomplish similar scientific goals, so the success of one may preempt the other. For the time being, the two projects are at the beginning of a long race into the unknown.