The most powerful particle collider currently lies about 100 meters underground, circling several French and Swiss towns northwest of Geneva. The Large Hadron Collider (LHC) revs up protons to relativistic speeds, smashing them together so scientists can sift through the debris for evidence of new physics.
But the LHC’s grand achievement — the discovery of the Higgs boson — has come and gone, and the collider is reaching the limits of its power output. More discoveries, it seems, will require a new collider. The question is, what type?
Physicists from around the world gathered February 27 through March 10 at UC Santa Barbara’s Kavli Institute for Theoretical Physics (KITP) to discuss the potential for a new kind of experiment: a muon collider. The rapid response program comes on the heels of a formal community discussion effort and aims to provide concrete recommendations to the two influential panels currently plotting the future of high energy physics in the United States.
“KITP was very happy to host this event,” said director Lars Bildsten. “Not only did it bring the particle physics community together to better understand the challenges, it also triggered debates and discussions that will inform those making recommendations to funding agencies about priorities for large investments.”
Proton-proton colliders, like the LHC, can reach high energies due to the proton’s relatively large mass. But protons are composite particles — made of three quarks bound together by the strong nuclear force — so their collisions exchange only a fraction of the energy carried in each proton. And they’re messy.
On the other hand, electron-positron colliders are precision machines. Electrons and positrons elementary particles, and because they are each other’s antiparticles, they completely annihilate and convert all of their energy into just a few products.
Unfortunately, the small mass of electrons and positrons means they lose a lot of energy during acceleration. Scientists can harness this synchrotron radiation for other purposes, but it prevents electron-positron accelerators from achieving high collision energies.
“The dichotomy is electron-positron machines are for precision and the proton-proton machines are for energy reach,” said event co-organizer Nathaniel Craig, a professor of physics at UC Santa Barbara.
Enter the muon, the electron’s big brother. “What’s exciting about the muon collider is that it can do both of these things at once,” explained Nima Arkani-Hamed, a physicist at the Institute for Advanced Study in Princeton, New Jersey.
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