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How do particle accelerators work?
Asked by: Angelo Garufi
School: St. John the Evanglist
Teacher: Anu Rai
Hobbies/Interests: Soccer, drawing, reading, video games
Career Interest: Artist, cartoonist
Answer from Stephen Levy
Assistant Professor of Physics, Binghamton University
Research area: Biophysics, microfluidics
Interests/hobbies: Tennis, chess, Wookiees
Particle accelerators work by speeding up beams of particles to high energies using electric fields and then colliding the beams with each other or another target. Let’s take the world’s largest particle accelerator, the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) near Geneva, Switzerland, as an example.
At the LHC, beams of protons collide with each other. The starting point for the LHC’s accelerator is a small canister of hydrogen gas. A hydrogen atom consists of a positively charged proton orbited by a negatively charged electron. The gas is pushed through a tube where an electric field strips away the electrons, leaving only the protons. The protons are then accelerated through narrow circular underground pipes, or rings, by applying a pulsed electric field — kind of like pumping your legs on a swing to get higher. The air in these pipes has been removed to avoid collisions with the speeding protons. Powerful magnets located around the tunnel force the protons to move in a circular path.
The protons are then transported into successively larger rings where this acceleration process continues. There are two final rings, each about 17 miles around, sitting on top of each other that the protons travel through in opposite directions at nearly the speed of light. Einstein’s special theory of relativity tells us that increasing the energy of a particle can be considered the same as increasing its mass. These protons have had their mass increased by about 7,000 times compared to a proton that isn’t moving.
Powerful magnets are again used to collide the two proton beams with each other at several locations around the ring. The protons collide about 600 million times per second. The energy of the collision can create new subatomic particles. Some of these only exist for a trillionth of a trillionth of a second before they decay into other particles. Experimenters have assembled massive detectors, similar to cameras, surrounding the collision points to measure the identity and velocity of the emerging particles. Due to the high collision rate, customized electronics are used to decide which events are interesting enough to record for further analysis.
Sifting through these collisions, the experimenters at CERN recently measured the mass of a long sought-after particle called the Higgs boson. The Nobel Prize in physics in 2013 was awarded to two scientists who predicted the existence of this particle 50 years ago.
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