resonance

Most of the hadrons ever discovered are not particles that fly across a detector and leave a track. They are gone almost the instant they form — so short-lived that they decay before they could travel even the width of an atomic nucleus. These ultra-fleeting hadrons are called resonances, and they are detected not by seeing them, but by the trace they leave in how often collisions happen.

A resonance is a very short-lived state that shows up as a peak — a bump — in a graph of how likely a reaction is, plotted against energy. As you tune the collision energy upward, the reaction rate stays low, then suddenly spikes at a particular energy, then drops again. That spike means a real, if momentary, particle is being formed and immediately decaying. The energy of the peak is the resonance's mass; the width of the peak tells you its lifetime, through the uncertainty principle: a broader peak means a shorter-lived state. The classic shape of such a peak is the Breit-Wigner curve.

Resonances are the bulk of the particle zoo. The vast majority of the hundreds of cataloged hadrons are resonances — excited states of quark combinations, living for less than a trillionth of a trillionth of a second. Far from being a nuisance, they are how the spectrum of the strong force reveals itself: each bump is a rung on the energy ladder of some quark combination, and finding and measuring these bumps is the daily work of hadron spectroscopy.