A particle resonance is an unstable excitation inferred from an enhanced scattering or decay response near a characteristic energy. It is associated with a pole of the analytically continued scattering amplitude, rather than a permanently normalizable state that survives indefinitely.

For an isolated narrow resonance, a Breit-Wigner form relates the peak scale to a mass and a width, with lifetime approximately $\hbar/\Gamma$. Different final states probe different partial widths and branching fractions. Interference with nonresonant amplitudes, nearby thresholds and detector resolution can shift or distort the observed peak, so a bump's width need not equal the intrinsic decay width without a model. Conservation laws and angular distributions help identify which quantum numbers are compatible with the signal.


 Particle resonance (source code)