After an intricate game of particle jockeying, physicists have now measured the radius of the helium nucleus five times more precisely than before. After banging into enough helium there, a muon will slow down and lose energy—and eventually get trapped by a final helium atom. The muon kicks out both the atom’s electrons, and what’s left is the helium nucleus—which is two protons and two neutrons—and a muon. Next they bathe those ions in laser light, hoping to bump the muons up to a higher-energy but unstable state. “If I were to clean it, I would be putting it into an unstable state because I can’t keep it clean very long.