On-shell consistency of the Rarita-Schwinger field formulation

We prove that any bilinear coupling of a massive spin-3/2 field can be brought into a gauge invariant form suggested by Pascalutsa by means of a non-linear field redefinition. The corresponding field transformation is given explicitly in a closed form and the implications for chiral effective field theory with explicit Delta (1232) isobar degrees of freedom are discussed.Comment: 9 pages, 1 figur

Gauge invariance in two-particle scattering

It is shown how gauge invariance is obtained for the coupling of a photon to a two-body state described by the solution of the Bethe-Salpeter equation. This is illustrated both for a complex scalar field theory and for interaction kernels derived from chiral effective Lagrangians.Comment: 16 pages, 2 figures, references added and commented o

Towards an understanding of isospin violation in pion-nucleon scattering

We investigate isospin breaking in low-energy pion-nucleon scattering in the framework of chiral perturbation theory. This work extends the systematic analysis of [1] to the energy range above threshold. Various relations, which identically vanish in the limit of isospin symmetry, are used to quantify isospin breaking effects. We study the energy dependence of the S- and P-wave projections of these ratios and find dramatic effects in the S-waves of those two relations which are given in terms of isoscalar quantities only. This effect drops rather quickly with growing center-of-mass energy.Comment: 12 pp, REVTeX, 8 figs, FZJ-IKP(TH)-2000-2

Pion photoproduction off the proton in a gauge-invariant chiral unitary framework

We investigate pion photoproduction off the proton in a manifestly gauge-invariant chiral unitary extension of chiral perturbation theory. In a first step, we consider meson-baryon scattering taking into account all next-to-leading order contact interactions. The resulting low-energy constants are determined by a fit to s-wave pion-nucleon scattering and the low-energy data for the reaction pi- p --> eta n. To assess the theoretical uncertainty, we perform two different fit strategies. Having determined the low-energy constants, we then analyse the data on the s-wave multipole amplitudes E0+ of pion and eta photoproduction. These are parameter-free predictions, as the two new low-energy constants are determined by the neutron and proton magnetic moments.Comment: 23 pages, 17 figure

The S-wave \Lambda\pi phase shift is not large

We study the strong interaction S-wave \Lambda\pi phase shift in the region of the \Xi mass in the framework of a relativistic chiral unitary approach based on coupled channels. All parameters have been previously determined in a fit to strangeness S= -1 S-wave kaon-nucleon data. We find 0^\circ \le \delta_0 \le 1.1^\circ in agreement with previous chiral perturbation theory calculations (or extensions thereof). We also discuss why a recent coupled channel K-matrix calculation gives a result for \delta_0 that is negative and much bigger in magnitude. We argue why that value should not be trusted.Comment: 3 pages, REVTe

Scattering phases for meson and baryon resonances on general moving-frame lattices

A proposal by L\"uscher enables one to compute the scattering phases of elastic two-body systems from the energy levels of the lattice Hamiltonian in a finite volume. In this work we generalize the formalism to S--, P-- and D--wave meson and baryon resonances, and general total momenta. Employing nonvanishing momenta has several advantages, among them making a wider range of energy levels accessible on a single lattice volume and shifting the level crossing to smaller values of $m_\pi L$.Comment: 41 pages, 3 figures. References added, minor edits to text. Version to be published in Phys. Rev.

Searching for signatures around 1920 MeV of a N∗N^* state of three hadron nature

We provide a series of arguments which support the idea that the peak seen in the $\gamma p \to K^+ \Lambda$ reaction around 1920 MeV should correspond to the recently predicted state of $J^P=1/2^+$ as a bound state of $K \bar{K}N$ with a mixture of $a_0(980)N$ and $f_0(980)N$ components. At the same time we propose polarization experiments in that reaction as a further test of the prediction, as well as a study of the total cross section for $\gamma p \to K^+ K^- p$ at energies close to threshold and of $d \sigma/d M_{inv}$ for invariant masses close to the two kaon threshold.Comment: published versio

Strange chiral nucleon form factors

We investigate the strange electric and magnetic form factors of the nucleon in the framework of heavy baryon chiral perturbation theory to third order in the chiral expansion. All counterterms can be fixed from data. In particular, the two unknown singlet couplings can be deduced from the parity-violating electron scattering experiments performed by the SAMPLE and the HAPPEX collaborations. Within the given uncertainties, our analysis leads to a small and positive electric strangeness radius, $= (0.05 \pm 0.09) fm^2$. We also deduce the consequences for the upcoming MAMI A4 experiment.Comment: 7 pp, REVTeX, uses epsf, minor correction

Effective field theory for shallow P-wave states

We discuss the formulation of a non-relativistic effective field theory for two-body P-wave scattering in the presence of shallow states and critically address various approaches to renormalization proposed in the literature. It is demonstrated that the consistent renormalization involving only a finite number of parameters in the well-established formalism with auxiliary dimer fields corresponds to the inclusion of an infinite number of counterterms in the formulation with contact interactions only. We also discuss the implications from the Wilsonian renormalization group analysis of P-wave scattering.Comment: 23 pages, 5 figures. Contribution to Special Issue of Few-Body Systems: Celebrating 30 years of Steven Weinberg's papers on Nuclear Forces from Chiral Lagrangian