The Large Hadron Collider’s hunt for the ‘fifth force’ of physics has begun

In 2012, the Large Hadron Collider found something that everyone already knew would be there.

The LHC’s discovery of the Higgs boson rightly got attention around the world. But its existence had been predicted 40 years earlier by the British physicist Peter Higgs. For the maths of particle physics to work, there had to be a particle, a boson, of a certain mass.

And the LHC was comfortably powerful enough to find particles of that mass. “The Higgs boson was a dead cert,” says Andrew Pontzen, a professor of cosmology at UCL. “If we hadn’t found it, that would have been such a shock that it would have been as exciting as finding it.”

Ten years later, the LHC has been switched on again, for the third time. It’s been upgraded, twice, since it was first turned on at the start of the last decade. Its beams are more tightly focused, so the particles it launches around its 20-mile track a shade under the speed of light crash into each other more reliably, so more data is gathered.

This time, though, scientists are not so confident that it will find anything groundbreaking. The jigsaw is not missing a single, obvious piece, as it was with Higgs. In a way, the jigsaw is complete. But that doesn’t mean that there’s nothing to be excited about.

There has, says Pontzen, to be a whole new jigsaw.

Dude, where’s my universe

It’s been obvious for a very long time that there are bits missing from physics. Our two main theories – relativity theory and quantum theory – are both extraordinarily successful. Relativity can describe the behavior of the very large and very fast with amazing precision, how light bends around black holes, how clocks on board a fast-moving satellite tick slightly slower than clocks on Earth. And quantum mechanics can predict the behavior of the very small, such as how many atoms of some radioactive material will decay, with equal accuracy.

But they can’t talk to each other. Quantum mechanics deals with subatomic particles, and the forces that govern them – electromagnetism, and the strong and weak nuclear forces that hold atoms together. Relativity deals with gravity, the force that holds planets together. And there is no room in quantum mechanics for gravity.

That’s far from the only thing that’s not right. You’ve probably heard of “dark matter” and “dark energy”. They’re somewhat annoying terms for stuff that’s missing.

When we look at galaxies, we can see they’re rotating at a certain speed. But we also know that the stars they’re made of weigh a certain amount, and that gravity is a certain strength. The three numbers don’t add up. The gravity of all those stars isn’t strong enough to hold the galaxies together if they’re spinning that fast. They should fly apart. There must be something, some matter, that is holding them together. Physicists call that unknown stuff dark matter.

Also, the universe is expanding. It was thrown apart by the Big Bang. But it should be slowing down, as gravity works against it, just like a ball thrown up will eventually start to slow and fall back to Earth. But it’s not. For some reason, all the galaxies are flying apart from each other at accelerating speeds. There must be something pushing it. Again, physicists call that unknown motivate force dark energy. Between them, dark energy and dark matter seem to make up about 95 per cent of everything in the universe.

And there’s more. Why is there something rather than nothing at all? When the universe was created, there was (it seems) an almost equal quantity of matter and antimatter. Antimatter is a mirror of matter: there are electrons (matter) and positrons (antimatter); protons (matter) and antiprotons (antimatter). When matter and antimatter come into contact, they annihilate each other. At the very start of the universe, that’s exactly what happened – the vast quantities of the two that were created destroyed each other. But, luckily for us, there was a tiny imbalance: there was slightly more matter than antimatter. Why? Why wasn’t the early universe symmetrical?

In a somewhat similar vein, in the very first moments after the big bang, the cosmos was a hot soup of energy, almost completely smooth. But there were tiny variations. “The universe appears to have been very nearly uniform,” says Pontzen. “But not quite.” The very slightly denser bits had slightly higher gravity, so they pulled other matter towards them, and – hundreds of millions of years later – formed galaxies. Why were there these imperfections? And what drove the strange expansion of space itself, known as “cosmic inflation”, in the earliest years of the universe?

There are 17 particles in the “standard model” of particle physics – the particles that quantum mechanics predicts and that experimental evidence has found. They make up the matter and the energy that we are aware of. But perhaps if one or more new ones were found, it would provide the key to unlock some of the remaining mysteries of physics.

Smashing particles, Gromit

The dream of high-energy physics is that we can do that by smashing particles together at extraordinary speed. Which is what the LHC does.

It uses magnets to accelerate two beams of subatomic particles to 99.9999991 per cent of the speed of light, about 7mph below the fundamental speed limit of the universe. Then it smashes them together.

Particle collisions at that speed release incredible amounts of energy, annihilating the particles themselves. But the interesting bit is that, sometimes, they also create new ones. In Albert Einstein’s famous equation, E=mc2 – that is: energy equals mass times the speed of light squared – if you get enough energy in one place, it creates new matter.

The more massive a particle is, the more energy is needed to create it, and the less likely it is to appear. Relatively small particles can be made with relatively small colliders, but heavy particles need hugely more.

The trouble is, it’s very unlikely that any one collision will have the exact amount of energy to create a given particle. That’s why the LHC hurls particles together in a billion collisions a second. Then it uses huge detectors to look for what flies out of them. The storm of different subatomic particles flying into those detectors makes a noisy signal, like listening for a distant radio station through static: but given enough collisions, if there are unexpected particles being made, it can be spotted.

In the search for the Higgs, physicists confidently expected the new particle to be around a certain mass. That made the search easier: they knew that given a certain number of collisions at a certain velocity, it would almost certainly show up.

Tom Whyntie, a medical physicist at Oxford University who previously worked on the LHC, says that the LHC’s predecessor, the Large Electron-Positron Collider (LEP), was almost powerful enough to look for it. “When the LEP was being shut down in the early 1990s,” he says, “they knew they had to build the LHC, because they knew what the mass of the Higgs was, and the LHC would definitely find it.” As it turned out, he says, the Higgs’s mass was at the lower end of what was expected, and they would probably have been able to find it with the LEP if they’d boosted it a bit. “They’d have saved themselves a lot of work,” he says.

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The LHC’s magnets have been improved since 2012. Its beams are a little faster and significantly more tightly focused, so more of the particles in those beams collide, and at higher energy. It means that it can look for possible particles that are somewhat bigger than the Higgs.

Unlike in 2012, there’s no obvious target, no obvious missing piece of the jigsaw. But, says Jon Butterworth, a professor of physics at UCL who works on the LHC’s Atlas detector, that doesn’t mean it’s not worth doing.

“It’s an exploration, now,” he says. “It’s not like with the Higgs, where we had a very firm prediction. There are a whole bunch of open questions which the standard model doesn’t address.”

His metaphor is one of a landscape. As high-energy physics has progressed, it’s been able to explore ever larger areas – new colliders can look for ever heavier particles. “What we’ll do with the LHC is explore a whole bunch of new territory,” he says. Extend the frontier.

He thinks that since Higgs, physicists have been too concerned with finding specific targets, “a bit locked into what we’re trying to prove and disprove”. “To me the key is the exploration,” he says. “We want to push the boundaries, explore what nature is doing at the highest energies and smallest scales. What we’ll find we don’t know.”

And it may be that they find nothing. “There could be a particle [with mass the LHC can find],” says Whyntie. “Or it could be thousands of times larger and we’d never find it.”

But there are reasons to think there might be. The LHC has detected that some particles, B-mesons, seem to form smaller particles at an unexpected rate: “Some decay rates don’t line up the way they should,” says Butterworth. Maybe they’re breaking up strangely because there’s some force which hasn’t been previously detected, beyond the four we know.

And any force in physics has an associated particle, just as electromagnetic radiation like light is carried by the photon. A “fifth force” would have its own, too. “We can explain those experimental events if you stick in a new force particle,” says Whyntie.

There are other tantalising hints. Fermilab in the US seems to have found that one particle, the W-boson, is slightly heavier than the standard model predicts and that another, the muon, behaves in an unexpected way. “These little quantum anomalies that show up give us hope that there may be some answers,” says Butterworth. “You can have some indirect sensitivity of things that are too high for you to detect.” And they just give a shade of hope that if there is something to be found, it’ll be light enough for the new, more powerful LHC to find.

Throw physics to the dogs; I’ll none of it

Not finding anything doesn’t mean the LHC has failed. It has its big, flagship discovery, the Higgs, and it’s been quietly doing less high-profile but nonetheless exciting physics on and off for a decade now. In general, physics has been making progress – “Look at the last few years!” says Pontzen. “The Higgs, gravitational waves; really fundamental aspects of our universe that 10 years ago we didn’t have.” The Higgs might have been confidently predicted, but it still had to be found.

And the LHC has driven engineering as well as science. Developing the ultra-powerful, precisely controlled superconductor magnets that guide its beams has moved the technology on, and led to improvements in magnets used to control fusion reactors. “It doesn’t operate in isolation,” says Butterworth. “It’s part of a science and tech ecosystem that feeds off itself.”

But while the latest upgrades are not the last – the team are already preparing the ground for the next stage – the LHC probably isn’t going to be able to reach all that much higher into the mass landscape. “I think it’s the last chance for the LHC,” says Whyntie, “before you have to fundamentally look for a paradigm shift in collider tech to make the next step up in energy.”

There are new ideas coming – he mentions plasma generators which can generate incredible temperatures. “It’s going to be interesting to see how it’s kind of played, in terms of where we go next,” says Whyntie. “It’s the last roll of the dice for the LHC in its current form, but it’s a good one, and a fun game to play.”

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