“Congratulations,” Rolf-Dieter Heuer, the straight-talking director general at Cern, a particle physics lab near Geneva, told thousands of staff from the control room of the Large Hadron Collider. “Now the hard work starts.”



The pat on the back and call to arms marked the restart on Sunday morning of the world’s largest and most powerful particle accelerator. More than two years after it handed researchers the Higgs boson, and was closed down for crucial upgrade work, the machine is ready to make scientific history for a second time.

How that history will be written is unknown. High on the wishlist for discoveries are dark matter, the invisible material that appears to hang around galaxies and makes up more than 25% of the universe; hidden extra dimensions that would explain why gravity is so puny compared to other forces of nature; and an explanation for why the world around us is not made from antimatter.

But there is another history that keeps scientists awake at night: the possibility that the LHC’s discoveries begin and end with the Higgs boson, that it finds nothing else over the next 20 years it is due to run. As Steven Weinberg, a Nobel laureate and professor at the University of Texas in Austin, told the Guardian: “My thoughts on the possibility of the LHC telling us nothing new don’t go beyond hopeless fear.”

Sunday was not a time for despondency though. There were cheers in the control centre as the Large Hadron Collider stirred back to life. Thousands of scientists and PhD students around the world will build their careers on the data the machine generates over the coming years.

An instrument as complex as the LHC does not wake up and start working at the throw of a switch. For weeks it has been cooled and prepared to receive beams of protons that will hurtle in opposite directions around the collider’s 17 mile (27km) tunnel at nearly the speed of light.

Scientists confirmed at 10.41am local time on Sunday that the first beam of protons had made its way around the £3.74bn atom smasher. The second beam soon followed and, without a hitch, completed a lap in the other direction by 12.27pm. In anticipation of a long day at the lab, researchers had stocked up on croissants and the occasional chocolate Easter rabbit.

“The beam went smoothly through the whole machine. It’s fantastic to see it going so well after two years and such a major overhaul of the LHC,” said Heuer.

Sunday’s restart saw the beams circulating at low energy, but over the coming days the accelerator team will steadily turn them up, until the protons are whizzing around the machine at 13TeV or teraelectron volts, or nearly twice as much energy as before.

Facebook Twitter Pinterest Physicist Peter Higgs shared a Nobel prize with François Englert in 2013 for their work on the Higgs boson, which was identified by the LHC before it was shut down. Photograph: Murdo MacLeod

When the machine is operating at high energy, the Large Hadron Collider will start to live up to its name. At four points around the machine, scientists will cross the beams of protons, which belong to a class of particles called hadrons. Amid the head-on collisions that ensue, they hope to find hints of new laws of physics, or to create exotic new particles that have never been captured before.

Until now, the Large Hadron Collider has run at only half its design energy. The machine was restricted to 7TeV collisions after a weak connection led to a short circuit that caused an explosion less than two weeks after it was first switched on in September 2008. The blast covered half a kilometre of the machine with a thin layer of soot and closed the collider for more than a year. The repairs cost the lab £24m.

The machine was switched back on in 2009, but Cern took the precaution of running at half energy to slash the risk of another accident. The gamble paid off. On 4 July 2012, the lab’s Atlas and CMS detector teams declared they had discovered the Higgs boson months before the machine was shut down. A year later, Peter Higgs, the Edinburgh-based physicist, and François Englert from Brussels, won the Nobel prize for their work on the particle, which is thought to give mass to others.

Engineers have spent the past two years reinforcing more than 10,000 connections between the LHC’s components, and building in safety devices to prevent another catastrophic short circuit. “The emphasis throughout the shutdown from the accelerator teams has been on safety, to avoid another incident, and to make sure that things continue to run smoothly,” Prof David Charlton, head of the Atlas collaboration, told the Guardian.

Mike Lamont, head of accelerator operations at Cern, said teams would make sure that safety devices were in place over the next few days to make sure the high-energy beams could not damage the LHC if they ran out of control. “This beam has got a lot of destructive power,” he said. “We’ll spend a lot of time setting up our protective devices to make sure we can handle these beams safely.”

In ramping up to higher energy, the Large Hadron Collider will smash about five times as many protons in the next three years as it has done to date. The first high-energy collisions are expected in two months’ time. Energy can be converted into mass according to Einstein’s famous equation, E=mc2. The more energy that goes into the collisions, the more massive particles can be created.

The Higgs boson was the last piece of what physicists call the Standard Model, a series of equations that describe how all the known particles interact with one another. Though successful, the model is woefully incomplete, accounting for only 4% of the known universe. With the LHC, scientists hope to find physics beyond the Standard Model, a first step to explaining the majority of the cosmos that lies beyond our comprehension.

“The LHC will be running day and night. When we will get results we don’t know. What is important is that we will have collisions at energies we’ve never had before,” said Arnaud Marsollier, a Cern spokesman.



What is the Large Hadron Collider looking for?

Facebook Twitter Pinterest Scientists say the mystery dark matter will be a prime research target for the coming year

The Higgs boson Scientists on the Large Hadron Collider discovered the Higgs boson in 2012 but the machine was shut down for an upgrade only months later. They now want to make more Higgs particles and measure their properties accurately. If the particle behaves strangely, it could hold the secrets to entirely new theories of physics.



Supersymmetry Many scientists thought supersymmetry would have shown up by now in the Large Hadron Collider. The theory describes a universe in which all the particle types we know about have more massive, invisible twins, with names like squarks and winos. One version calls for five different types of Higgs boson.

Dark matter Galaxies do not move the way they should if visible matter is all that is out there. Dark matter is a mysterious substance that appears to cluster around them, exerting a huge gravitational pull, and giving a skeleton to the cosmos itself. Physicists believe that dark matter makes up 27% of the universe. Particles of dark matter could be made in the LHC and spotted as missing mass and energy.

Antimatter The universe was created, it is thought, with equal amounts of matter and antimatter. But all we see around us is made of matter. But there is no reason why antimatter couldn’t form anti-objects, including antimatter planets and antimatter life. What happened to all the antimatter? Exquisite measurements of particles called beauty quarks in the LHC could reveal the answer.

Extra dimensions The three familiar dimensions of space, along with time, make up the four dimensions of our reality, but there could be many more dimensions that we are unaware of. The existence of small extra dimensions could explain one of the greatest mysteries in physics: why gravity is so much weaker than the other forces of nature. According to the theory, gravity spreads through the extra dimensions, so we experience only a fraction of its force.