Author: Emily Conover / Source: Science News
If particle physicists get their way, new accelerators could one day scrutinize the most tantalizing subatomic particle in physics — the Higgs boson. Six years after the particle’s discovery at the Large Hadron Collider, scientists are planning enormous new machines that would stretch for tens of kilometers across Europe, Japan or China.
The 2012 discovery of the subatomic particle, which reveals the origins of mass, put the finishing touch on the standard model, the overarching theory of particle physics (SN: 7/28/12, p. 5). And it was a landmark achievement for the LHC, currently the world’s biggest accelerator.
Now, physicists want to delve further into the mysteries of the Higgs boson in the hope that it could be key to solving lingering puzzles of particle physics. “The Higgs is a very special particle,” says physicist Yifang Wang, director of the Institute of High Energy Physics in Beijing. “We believe the Higgs is the window to the future.”
But the LHC — which consists of a ring 27 kilometers in circumference, inside which protons are accelerated to nearly the speed of light and smashed together a billion times a second — can take scientists only so far. That accelerator was great for discovering the Higgs, but not ideal for studying it in detail.
So particle physicists are clamoring for a new particle collider, specifically designed to crank out oodles of Higgs bosons. Several blueprints for powerful new machines have been put forth, and researchers are hopeful these “Higgs factories” could help reveal solutions to glaring weak spots in the standard model.
“The standard model is not a complete theory of the universe,” says experimental particle physicist Halina Abramowicz of Tel Aviv University. For example, the theory can’t explain dark matter, an unidentified substance whose mass is necessary to account for cosmic observations such as the motions of stars in galaxies. Nor can it explain why the universe is made up of matter, while antimatter is exceedingly rare.
Carefully scrutinizing the Higgs boson might point scientists in the direction of solutions to those puzzles, proponents of the new colliders claim. But, among scientists, the desire for new, costly accelerators is not universal, especially since it’s unclear what exactly the machines might find.
Scientists have drawn up a variety of plans for new accelerators that would follow the Large Hadron Collider, or LHC.
| Accelerator | Style | Location | Particles collided | Energy (eV) |
|---|---|---|---|---|
| LHC | 27 km circular | Europe | Protons | 13 trillion |
| ILC | 20 km linear | Japan | Electrons and positrons | 250 billion |
| CLIC | 11–50 km linear | Europe | Electrons and positrons | 380 billion–3 trillion |
| FCC | 100 km circular | Europe | Electrons and positrons | 90 billion–365 billion |
| Protons | 100 trillion | |||
| CEPC | 100 km circular | China | Electrons and positrons | 240 billion |
| Protons | TBD |
Sources: CERN, L. Evans, IHEP and Y. Wang
Closest to inception is the International Linear Collider in northern Japan. Unlike the LHC, in which particles zip around a ring, the ILC would accelerate two beams of particles along a straight line, directly at one another over its 20-kilometer length. And instead of crashing protons together, it would collide electrons and their antimatter partners, positrons.
But, in an ominous sign, a multidisciplinary committee of the Science Council of Japan came down against the project in a December 2018 report, urging the government to be cautious with its support and questioning whether the expected scientific achievements justified the accelerator’s cost, currently estimated at around $5 billion.
Supporters argue that the ILC’s plan to smash together electrons and positrons, rather than protons, has some big advantages. Electrons and positrons are elementary particles, meaning they have no smaller constituents, while protons are made up of smaller particles called quarks. That means that proton collisions are messier, with more useless particle debris to sift through.
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