Colloquium: Roper resonance: Toward a solution to the fifty year puzzle

The Roper resonance in the strong interaction spectrum has defied understanding for over 50 years. This puzzling unstable particle appears to be an exact copy of the proton except that its mass is 50% greater. In modern terms, the Roper resonance is the proton's lightest radial excitation, consisting of a dressed-quark core augmented by a cloud of mesons. This Colloquium describes the experimental and theoretical developments that have enabled the picture of the Roper resonance to be drawn.