LHC accelerates its first “atoms”

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August 2, 2018

On 27 Jul 2018, Protons might be the Large Hadron Collider’s bread and butter, but that doesn’t mean it can’t crave more exotic tastes from time to time. On Wednesday, 25 July, for the very first time, operators injected not just atomic nuclei but lead “atoms” containing a single electron into the LHC. This was one of the first proof-of-principle tests for a new idea called the Gamma Factory, part of CERN’s Physics Beyond Colliders project.

Image above: During a special one-day run, LHC operators injected lead “atoms” containing a single electron into the machine (Image: Maximilien Brice/Julien Ordan/CERN).

“We’re investigating new ideas of how we could broaden the present CERN research programme and infrastructure,” says Michaela Schaumann, an LHC Engineer in Charge. “Finding out what’s possible is the first step.”

During normal operation, the LHC produces a steady stream of proton–proton collisions, then smashes together atomic nuclei for about four weeks just before the annual winter shutdown. But for a handful of days a year, accelerator physicists get to try something completely new during periods of machine development. Previously, they accelerated xenon nuclei in the LHC and tested other kinds of partially stripped lead ions in the SPS accelerator.

This special LHC run was really the last step in a series of tests,” says physicist Witold Krasny, who is coordinating a study group of about 50 scientists to develop new ways to produce high-energy gamma rays.

Accelerating lead nuclei with one remaining electron can be challenging because of how delicate these atoms are. “It’s really easy to accidentally strip off the electron,” explains Schaumann. “When that happens, the nucleus crashes into the wall of the beam pipe because its charge is no longer synchronised with the LHC’s magnetic field.”

During the first run, operators injected 24 bunches of “atoms” and achieved a low-energy stable beam inside the LHC for about an hour. They then ramped the LHC up to its full power and maintained the beam for about two minutes before it was ejected into the beam dump. “If too many particles go off course, the LHC automatically dumps the beam,” states Schaumann. “Our main priority is to protect the LHC and its magnets.”

After running the magnets through the restart cycle, Schaumann and her colleagues tried again, this time with only six bunches. They kept the beam circulating for two hours before intentionally dumping it.

Large Hadron Collider (LHC). Animation Credit: CERN

Physicists are doing these tests to see if the LHC could one day operate as a gamma-ray factory. In this scenario, scientists would shoot the circulating “atoms” with a laser, causing the electron to jump into a higher energy level. As the electron falls back down, it spits out a particle of light. In normal circumstances, this particle of light would not be very energetic, but because the “atom” is already moving at close to the speed of light, the energy of the emitted photon is boosted and its wavelength is squeezed (due to the Doppler effect).

These gamma rays would have sufficient energy to produce normal “matter” particles, such as quarks, electrons and even muons. Because matter and energy are two sides of the same coin, these high-energy gamma rays would transform into massive particles and could even morph into new kinds of matter, such as dark matter. They could also be the source for new types of particle beams, such as a muon beam.

Even though this is still a long way off, the tests this week were an important first step in seeing what is possible.


CERN, the European Organization for Nuclear Research, is one of the world’s largest and most respected centres for scientific research. Its business is fundamental physics, finding out what the Universe is made of and how it works. At CERN, the world’s largest and most complex scientific instruments are used to study the basic constituents of matter — the fundamental particles. By studying what happens when these particles collide, physicists learn about the laws of Nature.

The instruments used at CERN are particle accelerators and detectors. Accelerators boost beams of particles to high energies before they are made to collide with each other or with stationary targets. Detectors observe and record the results of these collisions.

Founded in 1954, the CERN Laboratory sits astride the Franco–Swiss border near Geneva. It was one of Europe’s first joint ventures and now has 22 Member States.

Related links:

Dark matter: https://home.cern/about/physics/dark-matter

Large Hadron Collider (LHC): https://home.cern/topics/large-hadron-collider

For more information about European Organization for Nuclear Research (CERN), Visit: https://home.cern/

Image (mentioned), Animation (mentioned), Text, Credits: CERN/Sarah Charley.

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