The world's most powerful particle accelerator, last seen discovering the Higgs boson, switches back on after a two-year hiatus nearly twice as powerful as before. Science reporter Ivan Semeniuk charts its new push into the unknown.
This story corrects an earlier version that stated the Large Hadron Collider was dormant for three years
Asimina Arvanitaki was just a small child growing up in Greece when plans were first being drawn up for the Large Hadron Collider. By the time its powerful proton beams were switched on for the first time in 2008, she had a newly minted PhD from Stanford University.
But only now, as a 35-year-old faculty member at the Perimeter Institute for Theoretical Physics in Waterloo, Ont., is Dr. Arvanitaki about to access a realm she has been waiting to explore her entire academic life.
This month, the Large Hadron Collider – the LHC – comes into its own.
By the numbers
Cost to build the Large Hadron Collider
50 to 175
Metres below ground
Number of times per second a proton will cross the French-Swiss border while moving around the ring
As the world’s most powerful particle accelerator, the LHC is already famous. The collider, located near the foothills of the Jura Mountains west of Geneva, Switzerland, is where scientists finally chased down the long-hypothesized particle known as the Higgs boson. That achievement, announced in 2012, effectively completed the Standard Model of particle physics, the most formidable description of the basic constituents of nature that humans have ever devised. It was the culmination of a quest that began more than a century earlier with J.J. Thomson’s discovery of the electron in 1897.
Yet even while physicists were hungering for the Higgs, it was only ever meant to be an appetizer for the LHC. “In a sense, the Higgs was a sure bet,” Dr. Arvanitaki said. “We knew it had to be there.”
Now comes round two and all bets are off. After a lengthy hiatus and a complete overhaul, the LHC is about to switch back on with its power nearly doubled. This time the goal is to push onward into the unknown. It means the curtain is about to rise on a period of raw discovery that is relatively rare in science. And after decades of work by thousands of researchers and many billions of dollars spent, it’s Dr. Arvanitaki’s generation that now find itself in the midst of the action.
A NEW MACHINE
The LHC is mind-bogglingly complex, but at its heart lies a simple idea: Smash particles together at high speeds to concentrate as much energy as possible, and then watch how that energy dissipates through processes that can involve the spontaneous creation of new particles.
Like a miniature Niagara, each collision resembles a waterfall – an energy-releasing cascade that pours over a precipice and tumbles onto the rocks below. In this metaphor, there are any number of possible pathways that water can take along the way down. And the higher you start, the more pathways there are to explore, which is why physicists are so keen to see the LHC running at higher energies.
“This is the machine that is at the frontier of what can be done,” said Tiziano Camporesi, spokesperson for one of the LHC’s giant particle detectors.
It was all supposed to have happened well before this. The LHC is made up of two opposing beams of protons that race around a 27-kilometre-long ring guided by superconducting magnets. Wherever the beams cross, protons can collide and the products of those collisions can be carefully measured.
Early on as the LHC was ramping up to full operations in 2008, a faulty electrical connection triggered a massive failure that partly tore some of the accelerator’s giant magnets from their concrete moorings. The hobbled machine was shut down for two years. When it was finally back online, it was only able to run at half its design energy.
That still proved enough to find the Higgs boson, vindicating a decision by the LHC’s managers that it was better to get some science done sooner rather than later. In 2013, with the Higgs in hand, they switched off the beams and set about rebuilding the collider.
“It’s practically a new machine,” said Rolf-Dieter Heuer, general director of the CERN, the sprawling pan-European research facility where the LHC is based.
A SHOT IN THE DARK
As early as March 23, protons will again be flying around the LHC ring as systems come back online, with the first collisions expected in May. With higher energy and more particles in the beams, it will certainly find the Higgs again. The questions is whether it will find anything else.
But while it’s possible nothing will turn up, there are good reasons to think there is something more for the LHC to discover.
Chief among them is the fact the universe is known to be full of a mysterious substance called dark matter that cannot be explained by the Standard Model. There are other possibilities too, including signs of extra-dimensions or the production of microscopic black holes that evaporate in a flash of particles. Alternatively, nothing so direct may emerge. Instead, the presence of new physics may be inferred, through unexpected quirks in the behaviour of the Higgs boson. There could be more than one kind of Higgs.
“The most important thing we can do as part two of this search is to ask is this really the Higgs we expected, or is this something else,” said Manuella Vincter, a physicist at Carleton University and a member of ATLAS, the LHC detector with which Canada has partnered.
The only way to be sure, of course, it to look and see.
“It will be great to be in the data-taking business again,” said Robert McPherson, an experimental physicist at the University of Victoria and co-spokesperson for ATLAS.
As a theorist, Dr. Arvanitaki agrees. “Basically,” she says, “experiments are the language by which nature speaks to us.”
Of quarks and leptons: LHC's search for dark matter
THE STANDARD MODEL
The most comprehensive theory of matter and energy to date, the Standard Model manages to boil down the dizzying complexity of nature into a set of basic interactions between 17 particles. In this picture, matter is made up of six quarks and six leptons. The most familiar lepton is the electron, a carrier of electric charge.
The lightest quarks are the up and the down, which combine to form protons and neutrons, the building blocks of atoms. The forces that operate between particles of matter are conveyed via four additional particles, including photons, which makes up light.
The final player in the company is the Higgs boson. First predicted in the 1960s but not seen until 2012 at the Large Hadron Collider, the Higgs is required by the Standard Model because it accounts for why some particles have mass while others do not.
Although the Standard Model is a powerful tool for making accurate predictions about how matter behaves, physicists know it is incomplete. It does not include gravity and gives no information about why there are so many fundamental particles and why they have the properties that they do. Now that the Higgs has been discovered, the mission of the LCH is to look for signs of new particles or phenomena that might lead to a deeper theory beyond the Standard Model.
A SEARCH FOR SUPERSYMMETRY
Astronomers have convincingly shown that the matter we see when we look up at the heavens is only a fraction of what is there. By a margin of about five to one, this so-called ordinary matter – that which makes up atoms, planets, stars and galaxies – is outweighed by something else called dark matter. This matter cannot be seen directly, but it makes its presence known through its gravitational pull.
Exactly what kind of particles make up dark matter remains a matter of debate, but they most certainly lie outside the Standard Model.
One of the most exciting possibilities in the coming year is that signs of dark matter might start showing up in the form of missing energy in the LHC’s detectors – energy moving through an unseen pathway. Such an outcome would allow researchers to study dark matter in detail and, depending on its properties, point the way to which new theory best accounts for its existence.
Among the most developed such theories is supersymmetry, which posits that all the known particles in nature must each have a mirror set of partners that can only be detected at extreme energies. In some versions of supersymmetry, the lightest and most stable of these particles would account for the dark matter astronomers see in the cosmos.
During the hunt for the Higgs boson, physicists were looking for early signs of supersymmetry in the LHC data and found none. It could mean the evidence is just around the corner and will turn up as early as this year. Or it could mean that supersymmetry is wrong and that dark matter will have to be explained through some other mechanism.
A DIFFERENT DIMENSION
In particle physics, higher energy enables the exploration of phenomena at smaller scales, far tinier than an atom. This, in turn, could allow the LHC to see hints of phenomena that only exist at such a granular scale, including the possibility that there are additional dimensions to the universe that we cannot sense at the scale of everyday life.
The existence of extra dimensions could explain why gravity is apparently so much weaker than the other forces of nature (allowing, for example, a tiny fridge magnet to easily pick up a paper clip, thereby defeating the gravitational pull of the entire planet). Instead of being inherently weaker, gravity may be losing much of its pull by spreading out into other dimensions. This would mean that gravity is stronger on a subatomic scale, and that collisions of sufficient energy at the LHC could produce microscopic black holes that quickly evaporate in a flash of particles. Instead of looking for signs of missing energy, physicists would see their detectors lighting up with every conceivable type of particle.
Such ideas have led to the popular meme that the LHC will create a black hole that devours Earth. It can’t happen, physicists says, and if it could, it would have happened long before the LHC was built. It may be humanity’s most powerful colldier, but cosmic rays coming in from deep space are bombarding our atmosphere at even higher energies on a daily basis.