The Large Hadron Collider Is Back and Ready to Hunt for Dark Matter

Date:23 April 2022 Author: Juandre

In the control room at CERN (The European Center for Nuclear Research) is a row of empty champagne bottles. Scientists popped open each one to celebrate a successful landmark, like the discovery of the Higgs boson particle, the long-elusive particle that gives all other subatomic particles their mass.

Rende Steerenberg, CERN’s head of operations, hopes to toast coworkers with another bottle in July, when the Large Hadron Collider (LHC) is expected to achieve its next milestone: operating with a record-setting high-energy beam. Combined with a higher rate of proton collisions, the new setup will allow scientists to further investigate the Higgs boson after its discovery in 2012. Physicists especially look forward to the possibility of ushering in a new era of physics with the discovery of particles that could explain dark matter.

“When it’s a nice starry evening, and you look up to the sky, everything you see there is only five percent of what’s out there. The other 95 percent is still not understood,” Steerenberg tells Popular Mechanics. “It would be very good if we could find hints of what this dark matter is, and perhaps find a dark matter particle.”

Located over 300 feet beneath the Swiss-French border, the nearly 17-mile-long circular particle collider is at the heart of the discoveries made at CERN. The COVID-19 pandemic delayed a long shutdown for maintenance and upgrades, but the process to restart the collider for new experiments is already underway.

During a typical experiment in the particle accelerator, twin beams of protons travel at close to the speed of light in opposite directions through the collider. They meet in the middle, where “invisible fireworks” happen—protons smash into each other, creating new subatomic particles. While they aren’t visible to the human eye, the traces of these collisions are revealed in diagrams that look like fireworks, Steerenberg explains.

On Friday, operators took an important step toward restarting LHC, threading the first beam of protons, section by section, through the collider. During beam-conditioning, only two groups, or “packages,” of about 100 million proton particles make the circuit in opposite directions (see sidebar). Once the LHC is fully functional, the team will ratchet the beams up to at least billions of protons per package, with multiple packages loaded at one time.

Magnet Training

Getting ready to start up the collider involves literally tens of thousands of tests, Steerenberg says. The most time-consuming part is superconductor magnet-training.

Thousands of magnets, in coils of special electric cable, wind through the collider. The magnets are responsible for bending, focusing, and squeezing the particles into the best position for a maximum number of collisions. In order to work, the magnets need to be cooled to 1.9 kelvins, (-520.3 degrees Fahrenheit), a temperature colder than outer space. (Nothing is colder than zero on the kelvin scale.) To chill them, the collider uses 150 tons of liquid helium. This temperature allows the magnets to work with 13,000 amps of electric current. For comparison, a standard five-millimeter LED usually requires a maximum current of 20 milliamps.

large hadron collider at cern infographic of maintenance work on superconducting magnets

However, after its long period of non-use, the sudden drop in temperature causes great mechanical stress on the magnets. When the machine warms up, the magnetic windings lengthen by 262.5 feet over the nearly 17 miles of collider. When cooled, they contract by the same amount, Steerenberg says. During these shifts, the windings of the layers of electrical coils in the magnets move from their normal positions. That can impede healthy operation.

When operators start the current, a large magnetic field is generated. This creates a bit of friction between the magnetic windings, creating heat. That’s not good, since the magnet only works well at an extremely cold temperature. “If you heat up a superconductor it’s no longer superconducting. That’s what we call a quench,” Steerenberg says.

These complications mean you can’t just start up the LHC like a car engine. “I always compare it a little bit with a big orchestra where you have all the different instruments playing, and in order for the music to sound nice, it all has to be well synchronized,” Steerenberg says.

One of the first major steps requires “training” all the magnet machinery to stay in place. In order to get up to the energy level needed for operation, operators ramp up the current so that a few magnets quench. They pull down the current again, and repeat the cycle until all the magnets are quench-free. Usually, magnets only quench once, and then they tend to remain stable, Steerenberg says. When the team is confident that the magnets are trained, they are ready to send through a proton beam. (The magnet training process takes a few months.)

The CERN team has trained the magnets to operate at up to 6.8 tera electron volts of energy. That’s 6.8 million million electron volts, with one electron volt comparable to the energy a mosquito produces in flight. Imagine squeezing this much energy into a space about a million million times smaller than a mosquito. (You can learn more about it with the CERN glossary.)

The Proton Dance

Protons collide at nearly the speed of light in the LHC. This is how the collider uses proton beams.

“Imagine that you have two of these bunches [of protons] coming together in the experiments. Then these bunches will cross each other in the experiment and they will collide,” Steerenberg explains. “You will have 100 billion protons in one bunch and 100 billion in the other bunch. … Only 50 of the protons out of the 100 billion will actually collide. And the other 100 billion minus the 50 will continue [around the circuit], and at some point they will come back … and only 50 will collide again.”

But the 50 protons have collided and broken apart into smaller, subatomic particles. So, they leave behind a hole where they used to exist. After many laps around the collider, the bunch eventually depletes enough that the total number of protons drops, and the density of protons drops. Therefore, the rate of collisions drops too, to less than 50 per meeting, then less than 40, and so on. After about 12 or 13 hours in the collider, the density of the beam is so sparse that the remaining protons must be dumped.

A set of magnets directs the remains of the beam to a graphite block, which slows them down—just like running straight into waves on the beach slows a person down. The graphite eventually absorbs the protons. Then a fresh beam is loaded into the machine (which takes two to three hours) and the whole process restarts.

Ramping Up To Unprecedented Energy Levels

Now that the LHC has passed its first beam test, scientists will gradually increase the beam intensity and brightness. The freshly upgraded injector chain, a part of the complex LHC machinery that produces particle beams, can now produce brighter beams, with more particles per square millimeter. Therefore, the number of proton collisions in the LHC will rise.

If the testing process continues to go well, then the first experiments—sending a few packages at a time at 6.8 tera electron volts—could start as early as July 5, Steerenberg says. It would set a new world record for establishing particle collisions at this energy level. By the beginning of August, physicists should be able to start recording meaningful data.

Ultimately, the LHC team wants to reach 2,800 packages of protons in the clockwise beam, and the same number in the counter-clockwise beam. Each package would have 140 billion protons. The collider will shut down once again at the end of 2025 to make the necessary upgrades to allow the collider to “swallow” this intensity of protons, Steerenberg says.

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