Hadron Structure Calculations Research Articles

Frontiers in lattice nucleon structure

In this paper, I review recent progress in lattice-QCD calculations of hadron structure with an emphasis on nucleon structure. A wide range of nucleon observables are being studied in modern lattice calculations, and important progress has been made at physical pion mass, including the spin decomposition of the nucleon and the Bjorken-[Formula: see text] dependence of hadron structure. Challenges and perspectives for future lattice hadron-structure calculations will be discussed.

Nucleon and Roper electromagnetic elastic and transition form factors

We compute nucleon and Roper e.m. elastic and transition form factors using a symmetry-preserving treatment of a contact-interaction. Obtained thereby, the e.m. interactions of baryons are typically described by hard form factors. In contrasting this behaviour with that produced by a momentum-dependent interaction, one achieves comparisons which highlight that elastic scattering and resonance electroproduction experiments probe the infrared evolution of QCD's running masses; e.g., the existence, and location if so, of a zero in the ratio of nucleon Sachs form factors are strongly influenced by the running of the dressed-quark mass. In our description of baryons, diquark correlations are important. These correlations are instrumental in producing a zero in the Dirac form factor of the proton's d-quark; and in determining d_v/u_v(x=1), as we show via a formula that expresses d_v/u_v(x=1) in terms of the nucleon's diquark content. The contact interaction produces a first excitation of the nucleon that is constituted predominantly from axial-vector diquark correlations. This impacts greatly on the gamma*p->P_{11}(1440) form factors. Notably, our quark core contribution to F_2*(Q^2) exhibits a zero at Q^2~0.5mN^2. Faddeev equation treatments of a hadron's quark core usually underestimate its magnetic properties, hence we consider the effect produced by a dressed-quark anomalous e.m. moment. Its inclusion much improves agreement with experiment. On the domain 0<Q^2<2GeV^2, meson-cloud effects are important in making a realistic comparison between experiment and hadron structure calculations. Our computed helicity amplitudes are similar to the bare amplitudes in coupled-channels analyses of the electroproduction process. Thus supports a view that extant structure calculations should directly be compared with the bare-couplings, etc., determined via coupled-channels analyses.

Hadron structure in lattice QCD

Recent progress in hadron structure calculations within lattice QCD is reviewed. Results on key observables such as the axial charge, the quark momentum fraction and the spin content of the nucleon are discussed with focus on open issues. Lattice QCD studies of the γ∗N→Δ transition as well as the Δ form factors are also presented.

Lattice QCD Calculations of Hadron Structure: Status and Perspectives

The last years have seen significant advances in hadron structure calculations in lattice QCD. While extrapolations to the physical point and direct comparisons with experiment are still very challenging, lattice calculations provide already very valuable insights into a number of fundamental physics questions related to, e.g., the distribution of charge and momentum inside hadrons and the nucleon spin structure. To illustrate the recent achievements, we present and discuss latest lattice results for a small number of observables related to nucleon form factors and generalized parton distributions. We also present a study of the decomposition of the nucleon spin 1/2 in terms of quark spin and orbital angular momentum contributions. We briefly discuss results and limitations of the relevant chiral fits and extrapolations.

Lattice charge overlap: towards the elastic limit

A numerical investigation of time-separated charge overlap measurements is carried out for the pion in the context of lattice QCD using smeared Wilson fermions. The evolution of the charge distribution function is examined and the expected asymptotic time behavior ∼exp[−( E q− m π t], where t represents the charge density relative time separation, is clearly visible in the Fo urier transform. Values of the pion form factor are extracted using point-to-smeared correlation functions and are seen to be consistent with the expected monopole form from vector dominance. The implications of these results for hadron structure calculations is briefly discussed.