The masses of the negative parity 70-plet baryons are analyzed in large N c QCD to order 1/N c and to first order in SU (3) symmetry breaking. The existing experimental data are well reproduced and twenty new observables are predicted. The leading order SU (6) spin-flavor symmetry breaking is small and, as it occurs in the quark model, the subleading in 1/N c hyperfine interaction is the dominant source of the breaking. It is found that the Λ(1405) and Λ(1520) are well described as three-quark states and spin-orbit partners. New relations between splittings in different SU (3) multiplets are found. † Fellow of CONICET, Argentina.

Starting from hyperbolic dispersion relations, we present a system of Roy-Steiner equations for pion Compton scattering that respects analyticity and unitarity requirements, gauge invariance, as well as crossing symmetry, and thus all symmetries of the underlying quantum field theory. To suppress the dependence on the high-energy region, we also consider onceand twice-subtracted versions of the equations, where the subtraction constants are identified with dipole and quadrupole pion polarizabilities. We consider the resolution of the γγ → ππ partial waves by a Muskhelishvili-Omnès representation with finite matching point, and discuss the consequences for the two-photon coupling of the σ resonance as well as its relation to pion polarizabilities.

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In the large N c limit, the mass spectrum of the L = 1 orbitally excited baryons N * has a very simple structure, with states degenerate in pairs of spins J = (2 ), corresponding to irreducible representations (towers) of the contracted SU (4) c symmetry group. The mixing angles are completely determined in this limit. Using a mass operator approach, we study 1/N c corrections to this picture, pointing out a four-fold ambiguity in the correspondence of the observed baryons with the large N c states. For each of the four possible assignments, we fit the coefficients of the quark operators contributing to the mass spectrum to O(N −1 c ). We comment on the implications of our results for the constituent quark model description of these states. *

The decays of non-strange negative parity baryons via the emission of single π and η mesons are c , is given and tested with the available data. Up to a few exceptions, a good description of the partial decays widths is already obtained at that order. Because of the rather large errors in the empirical input data the next to leading order fit fails to pin down with satisfactory accuracy the subleading effective coefficients.

We report on a dispersion relation for the γγ → (ππ) I S-wave in isospin I emphasizing the low energy region. The f 0 (980) signal that emerges in γγ → ππ is also discussed. Our results could be used to distinguish between different ππ isoscalar S-wave parameterizations. We also calculate the width of the σ resonance to γγ and obtain the value Γ(σ → γγ) = (1.68 ± 0.15) KeV. Finally, we elaborate on the size of the f 0 (980) coupling to ππ and show that its smallness compared to the KK one is not related to the OZI rule.

Several experimental investigations have observed parity violation in nuclear systems-a consequence of the weak force between quarks. We apply the 1/Nc expansion of QCD to the P-violating T-conserving component of the nucleon-nucleon (NN) potential. We show there are two leadingorder operators, both of which affect pp scattering at order Nc. We find an additional four operators at order N 0 c sin 2 θW and six at O(1/Nc). Pion exchange in the PV NN force is suppressed by 1/Nc and sin 2 θW , providing a quantitative explanation for its non-observation up to this time. The large-Nc hierarchy of other PV NN force mechanisms is consistent with estimates of the couplings in phenomenological models. The PV observed in pp scattering data is compatible with natural values for the strong and weak coupling constants: there is no evidence of fine tuning. The strong-nuclear and electromagnetic forces play the most prominent role in proton-proton (pp) scattering. There are also parity-violating (PV) pp interactions, which manifest the presence of weak interactions between the quarks inside each proton. Measurements of longitudinal beam asymmetries ∼ 10 and TRIUMF [3] demonstrate that PV nucleon-nucleon (NN) forces exist. PV in NN systems is also probed via an asymmetry in the reaction np → dγ [4,5]. And ab initio calculations of few-nucleon systems allow us to take models of the PV NN force and predict, e.g., the longitudinal asymmetry in 3 He( n, p) 3 H [6], which is soon to be measured [7]. Nuclear parity violation is also observed in, e.g., the radiative decay of the first excited state of 19 F, but there theoretical uncertainties in the relationship between the observable and the model of the PV NN force are harder to quantify. Much work has gone into constraining the PV NN force from a variety of nuclear experiments, see Refs. [8,9] for recent reviews.The prevailing paradigm in such analyses is based on single-meson exchange between nucleons, most commonly in the framework developed by Desplanques, Donoghue, and Holstein (DDH) [10]. The quantum numbers of the exchanged mesons determine the operator structures that contribute, while operator coefficients involve products of strong and weak meson-nucleonnucleon coupling constants. In this paper we show that Standard Model (SM) couplings and the 1/N c expansion of QCD predict the operators, and the sizes of the associated coefficients, which appear in the PV NN potential.An alternative framework-suitable for studying PV at very low energies-that systematizes pioneering studies [11,12] has recently emerged [13][14][15], but has, as yet, been applied to far fewer experiments. The extension of chiral perturbation theory to few-nucleon systems, χEFT [16] has also been invoked [17][18][19][20][21]. In χEFT the one-pion-exchange piece of the PV NN force dominates, with all other effects suppressed by two orders in the chiral expansion.One-pion exchange gives the long-distance parityconserving potential, and drives many of the properties of light nuclei. But, thus far, experiment...