LHC will run at nearly double its previous energy, smashing protons to open up the subatomic world and look for more varieties of Higgs boson

Researchers on the Large Hadron Collider (LHC) near Geneva are making final preparations to restart the giant underground machine and with it their quest to understand the fundamental laws of nature.

Engineers at Cern, the European particle physics lab, have spent the past two years refurbishing the world’s most powerful collider in its 17-mile tunnel under the French-Swiss countryside so it can run at nearly double its previous energy.

Rolf-Dieter Heuer, Cern’s director general, said the rebuilt machine could be loaded with proton beams in the next two weeks. The first particle collisions, the most energetic ever created by humans, are due to follow near the end of May.

In 2012, scientists working on the LHC’s multipurpose Atlas and CMS detectors discovered the long-sought Higgs boson, earning the Edinburgh-based physicist Peter Higgs and the Belgian, Francois Englert, the 2013 Nobel prize in physics for postulating the particle’s existence nearly half a century earlier.

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The Higgs boson was the missing piece of what particle physicists call the Standard Model: a mathematical formulation of the known particles that make up the universe, and the forces that act on them. But while the Standard Model ranks as one of the greatest successes of modern particle physics, researchers know it is incomplete. It says nothing about gravity. Nor the 95% of the observable universe that is not made up of normal matter.

Among the questions the LHC will look to answer is whether there is only one Higgs boson or a family of the particles. In a theory called supersymmetry, the Higgs boson comes in five different varieties. Finding those would be more dramatic than the discovery of the Higgs boson itself.

But there are far more mysteries that researchers want the machine to answer. What is the curious but invisible dark matter that seems to lurk around galaxies and make up most of the universe? Why is everything around us made of matter, instead of antimatter, which was created in equal supply in the early universe? And why is gravity so much weaker than all the other forces? It may be that it spreads through extra dimensions, leaving us to feel only a fraction of its strength. Those extra dimensions might reveal themselves in the patterns of subatomic debris that are scattered from collisions in the machine.

“It is high time to find a crack in the Standard Model, because there is 95% of the universe still unknown to us. We have to find out what it is,” said Heuer. “I have a dream. I want to see the first light in the dark universe. If I see that then nature is kind to me.”

Inside the collider, bunches of protons are whipped up to nearly the speed of light and then smashed together to create tiny fireballs that recreate conditions that existed in the primordial universe, a fraction of a second after the big bang.

“I have a dream. I want to see the first light in the dark universe.” Rolf-Dieter Heuer, Cern’s director general

The Large Hadron Collider closed down for engineering work in 2013 after a highly successful run at an energy of 8 TeV, meaning that the energy in the particles it smashed together reached 8 trillion electron volts. This year, the machine will ramp up that energy to an unprecedented 13TeV. The beams carry enough energy to melt a tonne of copper on impact.

To switch the $6bn (£4bn) machine on without damaging it takes several months. Cern’s management, and the thousands of scientists waiting to study the data it churns out, want to avoid a repeat of the incident that happened in September 2008, when an electrical connection shorted out and led to a massive helium explosion that knocked the machine out of service for a year. The past two years’ of engineering work have fixed the weak connections and made the machine more robust, but restarting is still a cautious time.

The first run of the LHC was focused on bagging the Higgs boson, a particle that almost had to exist. This time the situation is different. There is no easy target, or place to look for an exciting new discovery. “We are headed now, when it turns on, into unchartered territory. It’s going to be another era of science and we’ll see what we find,” said Dave Charlton, spokesperson for the Atlas collaboration. “It’s very open this time. We don’t know when a discovery is going to appear. It could be this year. It could be that it takes us 20 years. We don’t know the answers before we start.”

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At the backs of scientists’ minds is the knowledge that the machine may not find anything new beyond the Higgs boson and the Standard Model. That would leave particle physics in a bad place. There are proposals for new international colliders once the LHC closes down in 2035, but without serious hints of new discoveries on the horizon, they might never be built. Major questions about the nature of gravity, and the make up of most of the universe, would go on hold for generations.

The risk is real, but Heuer argues that the $6bn LHC is still worth the cost. “All education, all training, everything, has its cost. And when mankind stops paying at least a small amount of money with respect to the overall amount for educating people, especially in sciences, technology, engineering and maths, then I don’t think the world can evolve.”

Tara Shears, professor of experimental particle physics at Liverpool University on the LHCb team said: “Pure, blue skies research like this makes the unimagined possible. What we develop, what can spin out of our technologies, can find new homes in the most unexpected places. We can’t tell what or when, like we can’t tell what we’ll discover in LHC’s run 2, but what’s important is that this research gives us the potential to do it.”

John Ellis, professor of theoretical physics at King’s College London, added: “We’re trying to find out how the universe works and every time we increase the energy of the LHC we are probing what happened further back in time, closer to beginning of the universe, so that way we can hopefully find answers to more profound questions to what makes the whole thing tick.”