P11 Partial Wave Research Articles

Genuine quark state versus dynamically generated structure for the Roper resonance

In view of the recent results of lattice QCD simulation in the P11 partial wave that has found no clear signal for the three-quark Roper state we investigate a different mechanism for the formation of the Roper resonance in a coupled channel approach including the $\pi N$, $\pi\Delta$ and $\sigma N$ channels. We fix the pion-baryon vertices in the underlying quark model while the $s$-wave sigma-baryon interaction is introduced phenomenologically with the coupling strength, the mass and the width of the $\sigma$ meson as free parameters. The Laurent-Pietarinen expansion is used to extract the information about the $S$-matrix pole. The Lippmann-Schwinger equation for the $K$ matrix with a separable kernel is solved to all orders. For sufficiently strong $\sigma NN$ coupling the kernel becomes singular and a quasi-bound state emerges at around 1.4~GeV, dominated by the $\sigma N$ component and reflecting itself in a pole of the $S$-matrix. The alternative mechanism involving a $(1s)^22s$ quark resonant state is added to the model and the interplay of the dynamically generated state and the three-quark resonant state is studied. It turns out that for the mass of the three-quark resonant state above 1.6~GeV the mass of the resonance is determined solely by the dynamically generated state, nonetheless, the inclusion of the three-quark resonant state is imperative to reproduce the experimental width and the modulus of the resonance pole.

Partial wave analysis of (γ/π)N→ηNreactions within a coupled channel unitary Lagrangian approach*

The recent Crystal Ball measurements of the eta-meson photoproduction on the proton are analyzed within an unitary coupled-channel Lagrangian approach. The dip observed in the reaction cross section at 1.68 GeV is explained in terms of the interference between the S 11(1535) and S 11(1650) states. Additional contributions from the P11 partial wave are also important. No strong indication for the narrow P11 state with a width of 15 MeV is found. Predictions for the double-spin asymmetry Cz are presented and discussed.

Relevance of complex branch points for partial wave analysis

A central issue in hadron spectroscopy is to deduce --- and interpret --- resonance parameters, namely pole positions and residues, from experimental data, for those are the quantities to be compared to lattice QCD or model calculations. However, not every structure in the observables derives from a resonance pole: the origin might as well be branch points, either located on the real axis (when a new channel comprised of stable particles opens) or in the complex plane (when at least one of the intermediate particles is unstable). In this paper we demonstrate first the existence of such branch points in the complex plane and then show on the example of the piN P11 partial wave that it is not possible to distinguish the structures induced by the latter from a true pole signal based on elastic data alone.

Pion electro-production in the Roper region in chiral quark models

We present a method to calculate pion electro-production amplitudes in a coupled-channel framework incorporating quasi-bound quark-model states. The method offers a clear prescription how to extract the resonant part of the amplitudes, even in the presence of different decay channels and a strong mixing of neighbouring resonances. The method is applied to the calculation of the M1- and the S1- amplitudes in the P11 partial wave in a simple chiral quark model. A good agreement with the observed M1- amplitude is found with a significant contribution from the pion cloud. The same effect is also prominent in the S1- amplitude but a rather uncertain data prevent us to draw a definitive conclusion.

New results on the Roper resonance and the [formula omitted] partial wave

Properties of the Roper resonance, the first scalar excitation of the nucleon, are determined. Pole positions and residues of the P11 partial wave are studied in a combined analysis of pion- and photo-induced reactions. We find the Roper pole at {(1371±7)−i(92±10)} MeV and an elasticity of 0.61±0.03. The largest decay coupling is found for the Nσ (σ=(ππ)-S-wave). The analysis is based on new data on γp→pπ0π0 for photons in the energy range from the two-pion threshold to 820 MeV from TAPS at Mainz and from 0.4 to 1.3 GeV from Crystal Barrel at Bonn and includes further data from other experiments. The partial wave analysis excludes the possibility that the Roper resonance is split into two states with different partial decay widths.

Two channelN/D calculation of the p11 partial wave of ?N scattering at low energies

Two channelN/D calculation of the p11 partial wave of ?N scattering at low energies