The computational challenge of lattice chiral symmetry - Is it worth the expense?

The origin of the low-lying position of the Roper resonance in the nucleon energy spectrum has been the subject of significant interest for many years, including several investigations using lattice QCD. It has been claimed that chiral symmetry plays an important role in our understanding of this resonance. We present results from our systematic examination of the potential role of chiral symmetry in the low-lying nucleon spectrum through the direct comparison of the clover and overlap fermion actions. After a brief summary of the background motivation, we specify the computational details of the study and outline our comparison methodologies. We do not find any evidence supporting the claim that chiral symmetry plays a significant role in understanding the Roper resonance on the lattice.

Structure of the Nucleon and its Excitations

The structure of the ground state nucleon and its finite-volume excitations are examined from three different perspectives. Using new techniques to extract the relativistic components of the nucleon wave function, the node structure of both the upper and lower components of the nucleon wave function are illustrated. A non-trivial role for gluonic components is manifest. In the second approach, the parity-expanded variational analysis (PEVA) technique is utilised to isolate states at finite momenta, enabling a novel examination of the electric and magnetic form factors of nucleon excitations. Here the magnetic form factors of low-lying odd-parity nucleons are particularly interesting. Finally, the structure of the nucleon spectrum is examined in a Hamiltonian effective field theory analysis incorporating recent lattice-QCD determinations of low-lying two-particle scattering-state energies in the finite volume. The Roper resonance of Nature is observed to originate from multi-particle coupled-channel interactions while the first radial excitation of the nucleon sits much higher at approximately 1.9 GeV.

NUCLEON RESONANCES AND GPDS IN AdS/QCD

We discuss an holographic soft wall model to describe nucleon properties. We pay special attention to nucleon spectrum, GPDs in the skewness case for nucleons and electroproduction of the N (1440) Roper resonance in soft-wall AdS/QCD.

EXCITED BARYONS IN A COVARIANT THREE-BODY BETHE-SALPETER APPROACH

We present first results for the excited nucleon spectrum in the framework of Poincaré-covariant three-body Bethe-Salpeter equations using the Rainbow-Ladder truncation of the interaction kernel. As expected, this truncation does not provide the mechanisms for a correct description of the spectrum. We also comment on possible steps beyond Rainbow-Ladder.