resonance

Push a child on a swing at just the right rhythm and the swing soars; push at the wrong rhythm and almost nothing happens. That special, matched response is resonance, and it is everywhere in nature — from tuning a radio to shattering a glass with a singer's note. In particle physics, the same idea appears in a striking way: when you collide particles at just the right energy, the chance of a reaction can suddenly spike, because that energy happens to match a fleeting particle that wants to be created.

Here is what is really going on. If the collision energy equals the mass-energy of some unstable particle, the incoming particles can briefly merge into that particle before it decays. At that magic energy the reaction rate shoots up to a peak; tune away from it and the rate falls off again. A resonance, then, is both a phenomenon (a bump in the rate versus energy) and, more profoundly, the signature of a short-lived particle. Its peak position tells you the particle's mass, and the width of the peak tells you how fast it decays — a wider bump means a shorter-lived particle, through the uncertainty principle.

Resonances are how a huge number of particles were discovered. Many of the short-lived hadrons and the famous Z boson first showed up not as tracks in a detector but as peaks in graphs of reaction rate against collision energy. Honestly, calling a resonance a particle is a matter of degree: the shortest-lived resonances exist for so little time that they are really better thought of as enhancements in a scattering process than as objects with a definite, lasting identity. But the line between a long-lived particle and a fleeting resonance is one of lifetime, not of kind.