After the LHC, which will be crowned King Collider?
Particle physics is petrolhead science – a particle-revving, high-octane demolition derby near the speed of light. Cathal O’Connell looks ahead to new 'Higgs factories' on teraelectronvolt, megawatt and gigadollar scales.
The Large Hadron Collider is a wonder of the modern world, our version of the great pyramid at Giza, but built by 10,000 scientists and engineers.
It’s a giant ring, 27 kilometres around, running 100 metres beneath the surface of France and Switzerland. Physicists use it to rev protons up to 99.999999% the speed of light before smashing them together to reveal the building blocks of the universe.
Simply building the LHC was a monumental achievement – it has the largest refrigeration system on the planet and magnets coiled with enough superconducting wire to stretch to the sun and back five times.
And it works. In 2012, physicists used it to discover the Higgs boson, a result hailed as one of the great scientific breakthroughs in human history.
But the biggest machine ever built is not big enough.
The LHC is still young, with another 15 years of groundbreaking research to come. Yet physicists already know that some of the biggest challenges in particle physics lie just outside its reach. And they’re already planning bigger things.
There are two “big questions” the LHC’s successor will try to answer, says Geoff Taylor, a particle physicist at the University of Melbourne in Australia and the European Organisation for Nuclear Research (CERN), which operates the LHC.
“What are the details of the Higgs? And what is the dark matter particle – or particles?”
Finding the Higgs was “the physics version of the discovery of DNA”, according to Sir Peter Knight, president of the Institute of Physics in the UK.
And, like DNA or any other great discovery in science, the Higgs boson provoked more questions than it provided answers.
At just 130 times the mass of the proton, the Higgs was a lot lighter than many physicists expected. Why? We’ve so far only found one Higgs particle, yet some theories predict at least four other – do they exist? Does the Higgs mechanism, which gives mass to most subatomic particles, work just as the standard model says it should? What exactly can the Higgs decay into?
The LHC can’t answer these questions because it can’t produce Higgs bosons in great enough numbers or in conditions clean enough to observe clearly. To do that, we need a new collider, a so-called “Higgs factory”, tuned to produce the Higgs en masse.
This new field of Higgs physics could tie up many of the loose ends in the standard model of particle physics, but the ultimate prize would be to go beyond the standard model itself, and thereby solve one of the great mysteries in all physics.
From the seemingly off-kilter dance of galaxies we know the universe is made of more than meets the eye – “dark matter” makes up about 80% of the material in the universe.
Many physicists think dark matter must be made of a new kind of particle, or perhaps whole families of new particles. They hoped the LHC might produce them in collisions – perhaps in the form of so-called “superpartners” of the common electrons and quarks we’re made of.
As British theoretical physicist Peter Higgs said, the boson that bears his name was “not the most interesting thing that the LHC is looking for”.
Alas, despite some titillating signals, no more new particles have yet been found. To create these particles, if they exist, we’ll need much more energetic collisions than the LHC can ever muster.
Already, the wheels are in motion to set up the LHCs successors. Three gargantuan projects are on the cards: one in Japan, one in China and one in Europe. Each would, in turn, become the largest experiment ever devised.
The International Linear Collider, Japan
The first, or at least the easiest, cab off the rank is the International Linear Collider (ILC) destined for Japan, says Taylor.
Imagine a gun barrel 15.5 kilometres long. Imagine a second barrel, pointing down its throat. Now imagine firing two bullets towards each other at almost the speed of light and photographing the collision. That’s the ILC in a nutshell.
The ILC is an electron-positron collider, so each “bullet” is actually a bunch of 20 billion electrons or positrons.
On the face of it, the ILC’s collision energy of 500 gigaelectronvolts seems puny compared to the LHCs 13 teraelectronvolts. But it’s a totally different animal.
Proton colliders like the LHC are the sledgehammers of particle physics – used to break new ground. Electron colliders are akin to a magnifying glass to closely dissect a specific particle.
The reason for this is proton smashers can reach much higher energies, but they also produce an unholy mess. Protons are little bags of other particles: subatomic quarks and the gluons that stick them together. Each collision at the LHC produces millions of new particles and a huge part of the job is teasing out the telltale signature of a particular one, such as the Higgs.
Electrons and positrons are fundamental. They don’t have internal parts. Plus, as antimatter partners of one another, they annihilate on contact to form a clean burst of pure energy. The result: no mess.
THIS WOULD BE A CHANCE TO SEE THE HIGGS GETTING PHYSICAL WITH THE HEAVIEST KNOWN PARTICLE IN NATURE.
Instead, physicists can tune the collision energy to produce just one kind of particle, a bit like tuning a radio to a particular station. Set the dial at 250 gigaelectronvolts, and the ILC produces Higgs bosons in abundance, and not much else.
Tuned a bit higher, to 375 gigaelectronvolts, and the collision could produce Higgs particles along with a top quark. This would be a chance to see the Higgs getting physical with the heaviest known particle in nature.
The main advantage a linear collider has over a circular one is that it’s much easier to accelerate electrons (or positrons) in a straight line than in a circle. That’s because particles running in a circle shed lots of extra energy as gamma rays, which puts the brakes on.
After two decades of design work, the ILC and is ready to be built, Taylor says. All that’s needed is for the relevant parties to cough up the dough – around A$10 billion.
The current agreement says the host country should pay half, but Japan is trying to renegotiate this with its international partners in the US and Europe and that’s holding things up.
The Japanese government is also being cautious in other respects, commissioning studies on how the massive project will affect life and industry in the region. All this means we don’t have an ETA on the project yet.
Besides the ILC, another linear collider on the horizon is the Compact Linear Collider (CLIC). It would use a more advanced design to hit energies of three teraelectronvolts or even higher along a similar length to the ILC, but it’s only in a very early design stage.
The Circular Electron Positron Collider, China
Another option is the Circular Electron Positron Collider (CEPC), set for Qinhuangdao, a port city in the northern province of Hebei in China.
It would be a giant ring at least twice the size of the LHC. A few possible sizes are on the table: 54 kilometres, 70 kilometres and 88 kilometres – the latter chosen in part because eight is an auspicious number in Chinese numerology.
CEPC would start out as a Higgs factory, just like the ILC, smashing electrons and positrons at up to 250 gigaelectronvolts. The long-term goal is to upgrade the same tunnel to host a new facility called the Super Proton-Proton Collider (SPPC). It’s as a proton collider that the Chinese proposal would really shine, potentially reaching energies of 70 to 100 teraelectronvolts, whizzing physics into unexplored territory.
Particle colliders are tools for turning energy into mass. It’s that piece of magic contained in Albert Einstein’s famous equation: E = mc2.
Slam two particles together at breakneck speed, and you can convert that kinetic energy into a spray of new particles. The higher the collision energy, the heavier the particles you can create.
Taylor says, with proton-proton colliders, “you just bang them as hard as you can and see what comes out”.
The Chinese plans are not as far along as the ILC, though the team is working to finish its design by 2020. China has a reputation for striving ahead with massive infrastructure projects, which may give them the chance to jump to the front of the pack by assembling the world’s top atom smasher. Construction could start as early as 2021, with the first collisions set for 2028 – if the project can overcome early controversy.
The first electron-positron stage carries a US$6 billion price tag and China would be expected to fork out at least half this. Meanwhile, the SPPC upgrade, scheduled for 2042, would bring costs up to US$20 billion.
In September, the most famous particle physicist in China and Nobel laureate Chen Ning Yang said it was too expensive for a country with such pressing societal needs.
In rebuttal, Yifang Wang, director of China’s Institute of High Energy Physics, argued that building the collider would boost China’s development: to become world leaders in high energy physics and attract important intellectual capital.
The Future Circular Collider, France/Switzerland
CERN is not resting on its laurels. It’s working out plans to supersede the LHC. The planned Future Circular Collider (FCC) is the biggest on offer, with a planned circumference of 90 to 100 kilometres and a whopping 100-teraelectronvolt collision energy.
Ultimately, the FCC project would result in a proton-proton monster collider. But before reaching that end, it could also spend some time as an electron-positron collider (another possible Higgs factory) and an electron-proton collider (which are great for studying the quark-gluon interactions that hold a proton together).
CERN scientists are working out the details of each option and plan to deliver a design report by 2018, with construction earmarked some time in the mid-2030s.
There is some overlap in the capabilities of these three projects, and Taylor doubts they’ll all be built. But as to which, he can only speculate. “Perhaps one of the circular colliders, plus the ILC, because then we’d get two different sorts of machines.”
In making these decisions, governments and physics communities are wary of overshooting, and repeating the mistakes of the past.
In the 1980s, the US started to build the Superconducting Super Collider in Texas. Its ring circumference of 87.1 kilometres would have dwarfed the LHC, and may perhaps have been the site of the Higgs discovery, had it ever been finished.
The US spent US$2 billion before the project was canned in 1993 due to rising costs. That failure cemented a shift in the centre of particle physics from the US to Europe. Could that centre shift further east? Only time will tell.
For now, the LHC will remain King Collider at least another decade. Whatever knocks it off its perch will have to be spectacular – the next wonder of the modern world.