The Electromagnetic Self-Energy Contribution to M_p - M_n and the Isovector Nucleon Magnetic Polarizability

We update the determination of the isovector nucleon electromagnetic self-energy, valid to leading order in QED. A technical oversight in the literature concerning the elastic contribution to Cottingham's formula is corrected and modern knowledge of the structure functions is used to precisely determine the inelastic contribution. We find \delta M_{p-n}^\gamma = 1.30(03)(47) MeV. The largest uncertainty arises from a subtraction term required in the dispersive analysis, which can be related to the isovector magnetic polarizability. With plausible model assumptions, we can combine our calculation with additional input from lattice QCD to constrain this polarizability as: \beta_{p-n} = -0.87(85) x 10^{-4} fm^3.Comment: 5 pages, version accepted for publication in PR

A Lattice Test of 1/N_c Baryon Mass Relations

1/N_c baryon mass relations are compared with lattice simulations of baryon masses using different values of the light-quark masses, and hence different values of SU(3) flavor-symmetry breaking. The lattice data clearly display both the 1/N_c and SU(3) flavor-symmetry breaking hierarchies. The validity of 1/N_c baryon mass relations derived without assuming approximate SU(3) flavor-symmetry also can be tested by lattice data at very large values of the strange quark mass. The 1/N_c expansion constrains the form of discretization effects; these are suppressed by powers of 1/N_c by taking suitable combinations of masses. This 1/N_c scaling is explicitly demonstrated in the present work.Comment: 13 pages, 20 figures; v2 version to be published in PR

Electromagnetic Polarizabilities: Lattice QCD in Background Fields

Chiral perturbation theory makes definitive predictions for the extrinsic behavior of hadrons in external electric and magnetic fields. Near the chiral limit, the electric and magnetic polarizabilities of pions, kaons, and nucleons are determined in terms of a few well-known parameters. In this limit, hadrons become quantum mechanically diffuse as polarizabilities scale with the inverse square-root of the quark mass. In some cases, however, such predictions from chiral perturbation theory have not compared well with experimental data. Ultimately we must turn to first principles numerical simulations of QCD to determine properties of hadrons, and confront the predictions of chiral perturbation theory. To address the electromagnetic polarizabilities, we utilize the background field technique. Restricting our attention to calculations in background electric fields, we demonstrate new techniques to determine electric polarizabilities and baryon magnetic moments for both charged and neutral states. As we can study the quark mass dependence of observables with lattice QCD, the lattice will provide a crucial test of our understanding of low-energy QCD, which will be timely in light of ongoing experiments, such as at COMPASS and HI\gamma S.Comment: 3 pages, talk given by B. C. Tiburzi at PANIC 201

Multichannel 1 -\u3e 2 transition amplitudes in a finite volume

We perform a model-independent, nonperturbative investigation of two-point and three-point finite-volume correlation functions in the energy regime where two-particle states can go on shell. We study three-point functions involving a single incoming particle and an outgoing two-particle state, relevant, for example, for studies of meson decays (e. g., B-0 -\u3e K*l(+)l(-) - \u3e pi Kl(+)l(-)) or meson photo production (e.g., pi gamma* - \u3e pi pi). We observe that, while the spectrum solely depends on the on-shell scattering amplitude, the correlation functions also depend on off-shell amplitudes. The main result of this work is a generalization of the Lellouch-Lscher formula relating matrix elements of currents in finite and infinite spatial volumes. We extend that work by considering a theory with multiple, strongly coupled channels and by accommodating external currents which inject arbitrary four-momentum as well as arbitrary angular momentum. The result is exact up to exponentially suppressed corrections governed by the pion mass times the box size. We also apply our master equation to various examples, including the two processes mentioned above as well as examples where the final state is an admixture of two open channels

Electromagnetic Self-Energy Contribution to M-p-M-n and the Isovector Nucleon Magnetic Polarizability

We update the determination of the isovector nucleon electromagnetic self-energy, valid to leading order in QED. A technical oversight in the literature concerning the elastic contribution to Cottingham\u27s formula is corrected, and modern knowledge of the structure functions is used to precisely determine the inelastic contribution. We find delta M-p-n(gamma) = 1.30(03)(47) MeV. The largest uncertainty arises from a subtraction term required in the dispersive analysis, which can be related to the isovector magnetic polarizability. With plausible model assumptions, we can combine our calculation with additional input from lattice QCD to constrain this polarizability as: beta(p-n) = -0.87(85) x 10(-4) fm(3)

Nuclear Forces and High-Performance Computing: The Perfect Match

High-performance computing is now enabling the calculation of certain hadronic interaction parameters directly from Quantum Chromodynamics, the quantum field theory that governs the behavior of quarks and gluons and is ultimately responsible for the nuclear strong force. In this paper we briefly describe the state of the field and show how other aspects of hadronic interactions will be ascertained in the near future. We give estimates of computational requirements needed to obtain these goals, and outline a procedure for incorporating these results into the broader nuclear physics community