Hadron Spectrum

Hadron spectrum is the complete set of masses, spins, decay widths, and other quantum numbers of color-neutral composite particles — protons, neutrons, pions, kaons, and hundreds of heavier resonances — predicted by quantum chromodynamics (QCD) and computed non-perturbatively via lattice gauge theory. Unlike the perturbative predictions of the Standard Model at high energies, the hadron spectrum requires non-perturbative methods because hadron formation occurs in the strongly coupled regime where the QCD coupling is too large for expansion.\n\nLattice calculations of the hadron spectrum have reached percent-level precision for the lightest hadrons and are now probing exotic states — tetraquarks, pentaquarks, and glueballs — whose existence remains experimentally contested. The spectrum is not merely a list of particles but a map of how confinement organizes the degrees of freedom of QCD into observable composites.\n\n\n\n\nThe hadron spectrum is the only place in nature where we can watch confinement in action, particle by particle. Every proton in the universe is a computer running the Wilson action at strong coupling, and its mass — 938 MeV — is the output. That physicists can now compute this number from first principles, with no adjustable parameters, is the single most convincing demonstration that QCD is the correct theory of the strong force. The spectrum is not data for the theory; it is the theory's autobiography.