Ultraperipheral nuclear collisions

Bigger microscopes reveal finer detail. For physicists studying subatomic structure, the microscopes are accelerators, whose increasing size enables them to launch ever more energetic particles at each other. Accelerators generally fall into one of two classes. The first class accelerates hadrons (mesons or baryons, particles made from quarks). Usually protons or heavy nuclei are directed at each other, but occasionally experiments are run with secondary beams of pions or other particles. Those projectiles probe their targets by means of the strong force, and they are excellent for some purposes—for example, studying perturbative quantum chromodynamics (QCD) in reactions with high momentum transfers. But the coupling that controls the strong force is large, and gluons, the carriers of the strong force, couple to each other. So hadron reactions at colliders are almost invariably messy. For that reason, physicists also build electron accelerators or use muon beams to interrogate their targets electromagnetically via...