Excited Baryons and Chiral Symmetry Breaking of QCD

N* masses in the spin-1/2 and spin-3/2 sectors are computed using two non-perturbative methods: lattice QCD and QCD sum rules. States with both positive and negative parity are isolated via parity projection methods. The basic pattern of the mass splittings is consistent with experiments. The mass splitting within the same parity pair is directly linked to the chiral symmetry breaking QCD.Comment: contribution to Baryon 2002, 5 pages, 3 figure

Path Integrals in Lattice Quantum Chromodynamics

I discuss the use of path integrals to study strong-interaction physics from first principles. The underlying theory is cast into path integrals which are evaluated numerically using Monte Carlo methods on a space-time lattice. Examples are given on progress related to nuclear physics.Comment: 6 pages, 5 figures. Talk given at the International Conference on Path Integrals - New Trends and Perspectives, 23-28 September 2007, Dresden, German

Magnetic Moments of Δ++\Delta^{++} and Ω−\Omega^- from QCD Sum Rules

QCD sum rules for the magnetic moments of $\Delta^{++}$ and $\Omega^-$ are derived using the external field method. They are analyzed by a Monte-Carlo based procedure, using realistic estimates of the QCD input parameters. The results are consistent with the measured values, despite relatively large errors that can be attributed mostly to the poorly-known vacuum susceptibility $\chi$. It is shown that a 30% level accuracy can be achieved in the derived sum rules, provided the QCD input parameters are improved to the 10% level.Comment: 10 pages, RevTex, 1 PS figure embedded with psfig.st

Neutron electric polarizability

We use the background field method to extract the "connected" piece of the neutron electric polarizability. We present results for quenched simulations using both clover and Wilson fermions and discuss our experience in extracting the mass shifts and the challenges we encountered when we lowered the quark mass. For the neutron we find that as the pion mass is lowered below 500\MeV, the polarizability starts rising in agreement with predictions from chiral perturbation theory. For our lowest pion mass, m_\pi=320\MeV, we find that \alpha_n = 3.8(1.3)\times 10^{-4}\fm^3, which is still only one third of the experimental value. We also present results for the neutral pion; we find that its polarizability turns negative for pion masses smaller than 500\MeV which is puzzling.Comment: 11 pages, 8 figures, presented at the XXVII International Symposium on Lattice Field Theory, July 26-31, 2009, Peking University, Beijing, Chin

Neutron electric polarizability from unquenched lattice QCD using the background field approach

A calculational scheme for obtaining the electric polarizability of the neutron in lattice QCD with dynamical quarks is developed, using the background field approach. The scheme differs substantially from methods previously used in the quenched approximation, the physical reason being that the QCD ensemble is no longer independent of the external electromagnetic field in the dynamical quark case. One is led to compute (certain integrals over) four-point functions. Particular emphasis is also placed on the physical role of constant external gauge fields on a finite lattice; the presence of these fields complicates the extraction of polarizabilities, since it gives rise to an additional shift of the neutron mass unrelated to polarizability effects. The method is tested on a SU(3) flavor-symmetric ensemble furnished by the MILC Collaboration, corresponding to a pion mass of m_pi = 759 MeV. Disconnected diagrams are evaluated using stochastic estimation. A small negative electric polarizability of alpha =(-2.0 +/- 0.9) 10^(-4) fm^3 is found for the neutron at this rather large pion mass; this result does not seem implausible in view of the qualitative behavior of alpha as a function of m_pi suggested by Chiral Effective Theory.Comment: 36 pages, 11 figures. Note added concerning analytic continuation in the external electric field; some notation made more precis

Scattering phaseshift formulas for mesons and baryons in elongated boxes

We derive L\"{u}scher phaseshift formulas for two-particle states in boxes elongated in one of the dimensions. Such boxes offer a cost-effective way of varying the relative momentum of the particles. Boosted states in the elongated direction, which allow wider access to energies, are also considered. The formulas for the various scenarios (moving and zero-momentum states in cubic and elongated boxes) are compared and relations between them are clarified. The results are applicable to a wide set of meson-meson and meson-baryon elastic scattering processes, with the two-particle system having equal or unequal masses.Comment: 28 pages, 2 figures, 16 tables. Updated to match published versio