A composite of elementary building blocks called quarks, it contained not two quarks like mesons or three like the protons and neutrons that constitute all visible matter, but four — a number that theoretical physicists had come to think the laws of nature did not permit. This candidate "tetraquark" disintegrated so quickly that it seemed a stretch to call it a particle at all. But as similar formations appeared in experiments around the world, they incited a fierce debate among experts about the correct picture of matter at the quantum scale.

Most believed tetraquarks were a new kind of miniature molecule — essentially, two orbiting mesons, each made of one regular quark and one antimatter quark, or antiquark — while a smaller contingent saw them as stand-alone particles in which the two quarks and two antiquarks overlapped in the same small volume of space.