If particle physicists get their way, new accelerators may one day scrutinize the most curious subatomic particle in physics, the Higgs boson. Six years after the discovery of this particle at the Large Hadron Collider, physicists are planning huge new machines that will stretch tens of kilometers in Europe, Japan or China.
New colliders: what they will be
The discovery of this subatomic particle, which reveals the origin of mass, led to the completion of the Standard Model, the overarching theory of particle physics. And it also became a landmark achievement for the LHC, currently the world's largest accelerator - after all, it was built to search for the Higgs boson, though not only.
Now physicists want to delve deeper into the mysteries of the Higgs boson in the hope that it will be the key to solving the long-running problems of particle physics. “The Higgs is a special particle,” says physicist Yifang Wang, director of the Institute for High Energy Physics in Beijing. "We believe the Higgs is a window to the future."
The Large Hadron Collider, also known as the LHC, consisting of a 27-kilometer-long ring, inside which protons accelerate to almost the speed of light and collide billions of times per second, has almost reached its limit. He did an excellent job of finding the Higgs, but he is not suitable for detailed research.
Therefore, particle physicists are demanding a new particle collider specially designed to launch piles of Higgs bosons. Several designs have been proposed for these powerful new machines, and scientists hope these Higgs factories could help find solutions to the Standard Model's glaring weaknesses.
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"The Standard Model is not a complete theory of the universe," says experimental particle physicist Galina Abramovich of Tel Aviv University. For example, this theory does not explain dark matter, an unidentified substance whose mass is needed to account for cosmic observations such as the motion of stars in galaxies. It also fails to explain why the universe is made of matter, while antimatter is extremely rare.
Proponents of the new colliders argue that careful study of the Higgs boson could point scientists on the way to solving these mysteries. But among scientists, the desire for new expensive accelerators is not supported by everyone. Moreover, it is not clear what exactly such machines could be found.
Next in line
The first in line is the International Linear Collider in northern Japan. Unlike the LHC, in which particles move in a ring, the MLC accelerates two particle beams in a straight line, directly on top of each other, along its entire 20-kilometer length. And instead of pushing protons together, it pushes electrons and their antimatter partners, positrons.
However, in December 2018, an interdisciplinary committee of the Japan Scientific Council opposed the project, urging the government to be careful with its support and wondering if the expected scientific advances justified the cost of the collider, which is currently estimated at $ 5 billion.
Proponents argue that MLK's plan to collide electrons and positrons, rather than protons, has several major advantages. Electrons and positrons are elementary particles, that is, they do not have smaller components, and protons are composed of smaller particles - quarks. This means proton collisions will be more chaotic and create more useless particle debris that will have to be sifted through.
In addition, in collisions of protons, only part of the energy of each proton actually gets into the collision, whereas in electron-positron colliders, particles transfer the total energy into the collision. This means scientists can tune the collision energy to maximize the number of Higgs bosons produced. At the same time, the MLK would only require 250 billion electron volts to produce Higgs bosons, compared to 13 trillion electron volts at the LHC.
At MLK, "the quality of the data will be much better," says particle physicist Lyn Evans of CERN in Geneva. One out of every 100 collisions at the MLK will produce the Higgs boson, while at the LHC this happens once every 10 billion collisions.
The Japanese government is expected to make a decision on the collider in March. Evans says that if the MLK is approved, it will take about 12 years to build. Later, the accelerator can also be upgraded to increase the energy it can reach.
CERN has plans to build a similar machine, the Compact Linear Collider (CLIC). It will also collide electrons and positrons, but at higher energies than the MLK. Its energy will start at 380 billion electron volts and will rise to 3 trillion electron volts in a series of updates. To reach these higher energies, new particle acceleration technology needs to be developed, which means that CLIC will not appear before the MLK, says Evans, who is leading the research collaboration on both projects.
Running in a circle
The other two planned colliders, in China and Europe, will be as round as the LHC, but much larger: each with a circumference of 100 kilometers. This is a large enough circle to encircle the country of Liechtenstein twice. This is practically the length of the Moscow Ring Road.
The circular electron-positron collider, whose construction site in China has not yet been determined, will collide 240 billion electron-volt electrons and positrons, according to a conceptual plan officially unveiled in November and sponsored by Wang and the Institute for High Energy Physics. This accelerator could later be upgraded to collide high-energy protons. Scientists say they could start building this $ 5-6 billion machine by 2022 and complete it by 2030.
And at CERN the proposed Future Circular Collider, BKK, will also come into operation in stages, colliding electrons with positrons, and later protons. The ultimate goal will be to achieve proton collisions at 100 trillion electron volts, more than seven times the energy of the LHC.
Meanwhile, the scientists shut down the LHC for two years, upgrading the machine to run on higher energy. In 2026, the LHC with a high luminosity will start working, which will increase the frequency of proton collisions by at least five times.
Higgs portrait
When the LHC was built, scientists were confident enough to find the Higgs boson with it. But with new machines, it's not clear what new particles to look for. They will simply catalog how strongly the Higgs interacts with other known particles.
Measurements of Higgs interactions can confirm the expectations of the Standard Model. But if the observations differ from expectations, the discrepancy may indirectly indicate the presence of something new, such as the particles that make up dark matter.
Some scientists are hoping that something unexpected will happen. Because the Higgs boson itself is a mystery: these particles condense into a molasses-like liquid. Why? We have no idea, says particle theorist Michael Peskin of Stanford University. This fluid permeates the universe, slowing particles down and giving them weight.
Another mystery is that the Higgs mass is a million billion less than expected. This oddity may indicate that there are other particles. Scientists previously thought they could answer the Higgs problem with the help of supersymmetry theory - a consonant of which each particle has a heavier partner. But this did not happen, because the LHC did not find any traces of supersymmetric particles.
Future colliders may still find evidence of supersymmetry or otherwise hint at new particles, but this time scientists will not make promises. They are now more busy developing priorities and making arguments in favor of new colliders and other experiments in particle physics. One thing is for sure: the proposed accelerators will explore unknown territory with unpredictable results.
Ilya Khel