CMS detector

CERN's Large Hadron Collider: restarting the Big Bang Machine

Scientists are preparing to switch on CERN's upgraded Large Hadron Collider before the end of March. The reboot will let them smash protons with much more punch.

Panic. Stress. Fear. Worry. But, most of all, tangible excitement. Jean-Philippe Tock, an engineer at CERN, one of the world’s biggest particle physics labs in Geneva, Switzerland, crosses the echoing main magnet workshop. Its massive doors are wide open to a stunning view of the Jura Mountains. A dozen bright blue cylindrical magnets - 15 metres long and weighing a massive 28 tonnes each - line the floor.

In a tunnel a hundred metres below, identical magnets - set head to tail in a 27km long circle deep beneath scenic Franco-Swiss farmlands - form the world’s mightiest particle accelerator - the Large Hadron Collider. In a few days it will come to life after a two-year revamp, ready to fling protons through a narrow pipe inside these superconducting magnets. The particles are set to reach 6.5 trillion electron volts (TeV) - energies never before attempted by science and nearly double what the accelerator has achieved so far.

“We are all very impatient to get our beams flying and colliding again,” says Tock. But as he knows very well, a lot could go wrong.

The machine, with a US$5 billion price tag, earned its place in the history books in 2012 for discovering the much-hailed Higgs boson. It was the missing particle in the Standard Model that explains our physical world and crucially is believed to be responsible for giving mass to all other particles.

If the practice runs go well, the LHC will start in May to smash protons together, sending them in beams moving at nearly the speed of light towards head-on collisions. Four main detectors spread around the ring will then sift through the debris and look for short-lived, exotic particles thatmay help scientists understand, quite bluntly, why we are here.

Once the beams are switched on, the tunnels will be largely off-limits, except for maintenance. But during the two-year shutdown, Tock and his 300 colleagues in the Superconducting Magnets And Circuits Consolidation (SMACC) team made the tunnel their main workplace, spending about one million man-hours to reinforce the electrical connections inside the magnets and ensure that the machine survives the extra punch.

“We simply can’t afford a mistake. Not again,” says Tock.

He remembers all too well the morning of 19 September 2008, nine days after the LHC had been switched on for the first time, to great fanfare, following nearly a decade in the making. Two missed phone calls from his boss. A panicky rush to the main control room. And eerily black screens that were supposed to show key data from the tunnel such as temperature and pressure. “Nobody knew what had actually happened, but we knew it was serious,” remembers the 45-year-old engineer.

The problem was a faulty connection, or splice, between two magnets. There are more than 1,200 main magnets arranged end to end to make up the ring guiding the proton beams, reinforced by hundreds of other magnets. They steer and focus the protons around the loop; chilled to a frigid 1.9 kelvin (-271°C) the magnets are colder than deep space, because only then the cables carrying the electric current become superconductive and generate the required magnetic fields.

As the proton beams reach higher and higher energies, the magnets have to sustain higher and higher currents. But nearly seven years ago, one single faulty connection - or splice - failed and melted. Quickly, tonnes of supercooled helium leaked into the tunnel. This led to a rapid dumping of energy, which overheated several magnets and made them resistant to current - an event known as a quench. This in turn destabilised the vacuum conditions. “Pressure was so huge that some doors in the tunnel got blasted away, and a number of those enormous magnets were displaced by up to half a metre,” recalls Tock.
It took months to replace the damaged magnets, and more than a year to switch the machine back on.

“It was a blow,” says Roberto Saban, head of CERN’s engineering department.

To play safe, the team never cranked up the LHC to its full designed potential of 7TeV per beam. A single proton carrying 7TeV is about the energy of a flying mosquito - but since there are about 300 trillion protons in a beam, the energy of each beam is comparable to the energy of a freight train travelling at about 100 miles per hour, or a car at about 2000 miles per hour.

The energy maximum put into the beams so far has been 4TeV, producing 8TeV in head-on collisions. It was sufficient to end a 50-year hunt and pin down the Higgs, which earned theoreticians François Englert and Peter Higgs a Nobel Prize in physics in October 2013.

But physicists believe that there’s much else to discover and are eager to get their beams flying at full pelt.

Searching for supersymmetry

The Higgs may have been the missing piece to satisfy the Standard Model puzzle, which explains all known fundamental particles and forces except gravity; however, the model itself no longer satisfies the particle physics community. There’s simply too much that it can’t explain. For example, it describes ordinary matter well enough, but fails to explain the dark matter and dark energy that make up most of our universe. And the model depends on too many mathematical assumptions.

A popular extension to the Standard Model called supersymmetry, or tenderly SUSY by its fans, would allow physicists to get rid of many of these assumptions - by introducing heavier supersymmetric counterparts, or ‘sparticles,’ to all particles. The problem is that since the theory was dreamed up in the 1970s, all attempts to find any evidence for the sparticles have drawn a blank.

Physicists hope that the LHC’s higher energies could help them enter the realm of such exotic particles. But “if in a decade or so we find nothing, we’ll have to scratch our heads both on the machine side and on the theory side,” says Luca Malgeri, physics coordinator at Compact Muon Solenoid (CMS), one of the LHC experiments that hunts for any evidence of SUSY. Squinting at the sun as he looks out of his office at the snow-?capped Alps, he adds: “It’ll be time to go back to pen and paper.”

But the CERN team is willing to try, and the refurbished collider is their best shot - provided it can get as close as possible to its design energy.

Countdown to LHC’s reboot

To outsiders, the unprecedented overhaul may have “looked like we were all taking a break of two years,” says Mirko Pojer, a physicist and the engineer in charge of LHC operations. He stands in CERN’s main control room, the LHC’s nerve centre that was nearly empty up to a few weeks ago but has now become crowded and noisy. “It was not the case.”

At first glance, CERN indeed looked fairly deserted during the shutdown. But it was because physicists were all tucked away in their offices, analysing the data from the LHC’s first run of operations, while most engineers and technicians were working deep underground. There, in the tunnels, they were busy fitting the accelerator with new sensors and re-cabling everything to ensure that they immediately catch even the slightest surge in voltage.

Two years for the reboot of a machine may sound a long time, but then the LHC is no ordinary machine; it’s one of its kind.

Tock’s team had to reinforce more than 10,000 superconducting splices linking the magnets; they did this by fitting each with electrical shunts - low-resistance connections that would guide the current to an alternative path if a splice were to lose its superconducting state. This alone took more than a year. And it took a few more months to cool the magnets down to 1.9K, to get them ready to receive the beams. The LHC ring is divided into eight independent sections to speed up the chilling process.

Finally, in August 2014 the team started first electrical tests to make sure the ring would survive the energy boost without a quench.

Besides reinforcing the connections, the LS1 - CERN-speak for Long Shutdown 1 - was an opportunity to improve the protection of sensitive electronic equipment from radiation. When high-energy protons zoom through the tunnel, every now and then stray particles smash into the ultra-precise electronics crammed inside, resulting in microdamage that might send erroneous data to the control room.

And since it’s not only the particles’ energies that will be cranked up but there will also be more protons in the beam, the added shielding will “make sure that the electronics cope with the extra dosage of radiation at Run Two,” says Tara Shears, a physicist at LHCb, one of the four experiments at the collider.

The experiments themselves also got an extra kick. On the southern side of the LHC’s ring, and straddling the Swiss border, sits ATLAS, one of the two machines that independently identified the Higgs. ATLAS is as big as half of Notre Dame cathedral in Paris and weighs 7,000 tonnes - the same as the Eiffel Tower or 1,000 elephants.

During the upgrade, ATLAS received a new sub-detector, the Insertable B-Layer or IBL. A layer of silicon pixel, it is technically similar to the sensor of a digital camera. Placed very close to the centre of ATLAS, right where protons smash into each other to create a cascade of exotic subatomic particles, the new detector will be an additional and more accurate measurement point for new particles. “We can now look at particles closer up as they come from the collision,” says physicist Dave Charlton, the spokesman for the ATLAS experiment.

Some 8.5km away, on the other side of the ring, sits the other Higgs-catcher, called CMS. To drive there from ATLAS takes about 20 minutes. For Tock to go there along the collider’s body by bike - the most “practical, efficient and healthy way to move around in the tunnel” takes two hours. And a fully accelerated proton makes the entire 27km lap in less than one ten-thousandth of a second.

During the upgrade, four disc-shaped chambers were added to each end of the CMS to increase its sensitivity to muons. These charged particles are similar to electrons, but 200 times heavier. The four chambers are meant to improve greatly the detector’s ‘trigger’ - the mechanism that monitors the collision debris and guides researchers whether the data from the collision are interesting enough to keep.

Recreating the early universe

Yet another experiment had its computer system completely refurbished so that it can cope with more data; called ALICE, it is tasked with re-creating the conditions of the universe in the first fraction of a second after the Big Bang some 13.8 billion years ago.

ALICE was in the spotlight in 2010, when it produced a ‘mini-Big Bang’. By colliding lead ions instead of protons, physicists created temperatures a million times hotter than at the centre of the sun and generated a so-called quark-gluon plasma - primordial matter that is thought to have existed when not much else did.

This was a feat, but more questions linger. Why is matter much heavier than the weight of its constituents - for instance, why is a proton much heavier than the three quarks that make it up? “And what is this magic that means a quark cannot exist in isolation - why when we recreate them, they immediately condense into a more complex object?” says physicist Yves Schutz, ALICE’s deputy spokesman.

Higher energies may help solve these mysteries. They may allow the researchers to create the quark-gluon plasma in a much hotter state, which means that this primordial matter will take much longer to cool down and return to ordinary matter. This will give the team more time for observations and analysis. “It will make our life easier,” says Schutz.

So is the CERN team dead certain there won’t be a repeat of the 2008 meltdown?

“Of course we are not fully reassured,” says Tock. “But we did all we could to test, so far only measuring things partially. The final ‘well-done’ stamp will come when all the sectors are at the nominal energy and we have protons flying in the machine.”

Looking ahead

If all goes well, the LHC will operate at 6.5TeV for about a year, before the team cranks the machine up towards its design energy of 7TeV. But LS1 was not the final refurbishment of the collider. In total, Run Two is scheduled to last three and a half years - and later, in the mid-2020s, the LHC will undergo a major upgrade to give it an enormous boost in luminosity, physics speak for the intensity of collisions. “It will give us a lot more sensitivity to rare processes,” says Charlton. For the accelerator it means going back to sleep for another two to three years while the work is done. After that, CERN’s engineers and physicists hope to keep it running until about 2035.

And what if nothing is found then? “From a physics point of view, in some strange contorted way it’ll be brilliant,” says Shears. It will mean, however, that physicists may have to scrap their ideas and come up with something else - both in terms of theory and technology.

Already teams are working on ways to scale down future accelerators, instead of making them bigger and more expensive. The most promising concept pursued at CERN is called AWAKE, based on the so-called wakefield effect. Instead of shooting particles through 27km of vacuum, a much shorter pipe would be filled with super-heated plasma. Still, making the concept work at scale is at the moment beyond CERN’s scientists and engineers. That’s why, all eyes are on Tock, Sagan, and the rest of the team who have worked for two years to fortify the Large Hadron Collider.

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