Physical interpretation of the 2s excitation of the nucleon

Abstract

Lattice QCD calculations of the $2s$ radial excitation of the nucleon place the state at an energy of approximately 1.9 GeV, raising the possibility that it is associated with the $N1/2^+(1880)$ and $N1/2^+(1710)$ resonances through mixing with two-particle meson-baryon states. The discovery of the $N1/2^+(1880)$ resonance in pion photoproduction but not in $πN$ scattering and the small width of the $N1/2^+(1710)$ resonance suggest that a state associated with these resonances would be insensitive to the manner in which pions are permitted to dress it. To explore this possibility, we examine the spectrum of nucleon radial excitations in both 2+1 flavour QCD and in simulations where the coupling to meson-baryon states is significantly modified through quenching. We find the energy of the $2s$ radial excitation to be insensitive to this modification for quark masses close to the physical point. This invariance provides further evidence that the $2s$ radial excitation of the nucleon is associated with the $N1/2^+(1880)$ and $N1/2^+(1710)$ resonances.