arXiv subjects
Publications and source records attributed to Derek B. Leinweber.
Strange quark contributions to the proton magnetic moment are estimated from a consideration of baryon magnetic moment sum rules. The environment sensitivity of quark contributions to baryon moments is emphasized. Pion cloud contributions to proton charge radii are examined in the framework of Chiral Perturbation Theory. The absence of scalar-diquark clustering in the nucleon is discussed.
The short-time regime of QCD two-point correlation functions is examined through a QCD-Sum-Rule-inspired continuum model. QCD Sum Rule techniques are tested and alternate nucleon interpolating fields are discussed. The techniques presented here may be of practical use in determining heavy-light meson decay constants.
Two-pion induced self-energy contributions to the $ρ$-meson mass are examined in relation to the quenched approximation of QCD, where the physics associated with two-pion intermediate states has been excluded from vector-isovector correlation functions. Corrections to quenched QCD calculations of the $ρ$-meson mass are estimated to be small at the order of a few percent of the $ρ$-meson mass. The two-pion contributions display nonanalytic behavior as a function of the pion mass as the two-pion cut is encountered. The implications of this nonanalytic behavior in extrapolations of full QCD calculations are also discussed. We note that for full QCD, the error made in making a linear extrapolation of the $ρ$ mass, neglecting nonanalytic behavior, increases as one approaches the two-pion cut.
A gauge invariant method for the investigation of scalar diquark clustering in the nucleon ground state is presented. The method focuses on a comparison of quark distributions in the nucleon with those in the $Δ$ baryon resonance. Recent lattice QCD calculations of these quark distribution radii are analyzed in a search for evidence of scalar diquark clustering. The analysis indicates the lattice results describe the negative squared charge radius of the neutron with little resort to hyperfine clustering between $u$-$d$-quark pairs. This result contrasts both quark-diquark and nonrelativistic quark models where hyperfine attraction between $u$ and $d$ quarks in the nucleon is argued to play a significant role. Comparison of light quark distributions in $Λ^0$ and $Σ^{*0}$ indicate only a small reduction of the scalar diquark distribution radius relative to the vector diquark distribution. Current lattice QCD determinations of baryon charge distributions do not support the concept of substantial $u$-$d$ scalar diquark clustering as an appropriate description of the internal structure of the nucleon.
The electromagnetic transition moments of the $SU(3)$-flavor baryon octet to decuplet are examined within a lattice simulation of quenched QCD. The magnetic transition moment for the $N \; γ\to Δ$ channel is found to be in agreement with recent experimental analyses. The lattice results indicate $μ_{p Δ} / μ_p = 0.88(15)$. In terms of the Particle Data Group convention, $f_{M1} = 0.231(41)$ GeV${}^{-1/2}$ for $p \; γ\to Δ^+$ transitions. Lattice predictions for the hyperon $M1$ transition moments agree with those of a simple quark model. However the manner in which the quarks contribute to the transition moments in the lattice simulation is different from that anticipated by quark model calculations. The scalar quadrupole form factor exhibits a behavior consistent with previous multipole analyses. The $E2/M1$ multipole transition moment ratios are also determined. The lattice results suggest $R_{EM} \equiv -{\cal G}_{E2}/{\cal G}_{M1} = +3\pm 8$ \% for $p \; γ\to Δ^+$ transitions. Of particular interest are significant nonvanishing signals for the $E2/M1$ ratio in $Ξ^-$ and $Σ^-$ electromagnetic transitions.
Diagrammatic methods and Large $N_c$ QCD are used to argue that the nucleon as calculated in quenched QCD contains physics which can be ascribed to the pion cloud of the nucleon. In particular, it is argued that the physics corresponding to one-pion loops in chiral perturbation theory are included in the quenched approximation.
Logarithmic divergences in pion and proton charge radii associated with chiral loops are investigated to assess systematic uncertainties in current lattice determinations of charge radii. The chiral corrections offer a possible solution to the long standing problem of why present lattice calculations yield proton and pion radii which are similar in size.
The electromagnetic properties of the SU(3)-flavor baryon decuplet are examined within a lattice simulation of quenched QCD. Electric charge radii, magnetic moments, and magnetic radii are extracted from the E0 and M1 form factors. Preliminary results for the E2 and M3 moments are presented giving the first model independent insight to the shape of the quark distribution in the baryon ground state. As in our octet baryon analysis, the lattice results give evidence of spin-dependent forces and mass effects in the electromagnetic properties. The quark charge distribution radii indicate these effects act in opposing directions. Some baryon dependence of the effective quark magnetic moments is seen. However, this dependence in decuplet baryons is more subtle than that for octet baryons. Of particular interest are the lattice predictions for the magnetic moments of $Ω^-$ and $Δ^{++}$ for which new recent experimental measurements are available. The lattice prediction of the $Δ^{++}/p$ ratio appears larger than the experimental ratio, while the lattice prediction for the $Ω^-/p$ magnetic moment ratio is in good agreement with the experimental ratio.