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The isobar model EtaMAID has been updated with new and high precision data for η and η′ photoproduction on protons and neutrons from MAMI, ELSA, GRAAL and CLAS. The background is described in a recently developed Regge-cut model, and for the resonance part the whole list of nucleon resonances has been investigated with 21 N* states contributing to η photoproduction and 12 N* states contributing to η′ photoproduction. Finally, the possibility of a narrow resonance near W = 1900 MeV is discussed, that would be able to explain unexpected energy and angular dependence of observables in p(γ, η′)p near η threshold.

The two reactions $\gamma p\rightarrow K^{+}\Lambda$ and $\pi^{-} p\rightarrow K^{0}\Lambda$ are analyzed to determine the leading photoproduction multipoles and the pion-induced partial wave amplitudes in slices of the invariant mass. The multipoles and the partial-wave amplitudes are simultaneously fitted in a multichannel Laurent+Pietarinen model (L+P model), which determines the poles in the complex energy plane on the second Riemann sheet close to the physical axes. The results from the L+P fit are compared with the results of an energy-dependent fit based on the Bonn-Gatchina (BnGa) approach. The study confirms the existence of several poles due to nucleon resonances in the region at about 1.9 GeV with quantum numbers $J^{P} = 1/2^{+}, 3/2^{+}, 1/2^{-}, 3/2^{-}, 5/2^{-}$ . [ABSTRACT FROM AUTHOR]

Abstract: We have reanalyzed the scattering data at low energy in the Coulomb-nuclear interference region as measured by the CHAOS group at TRIUMF with the aim to determine the pion–nucleon σ term. The resulting value , while in agreement with lattice QCD calculations and compatible with other recent analyses, is significantly lower than that from the GWU–TRIUMF analysis of 2002. [Copyright &y& Elsevier]

Dispersion relations along interior hyperbolas and a. set of hyperbolas passing through the Cheng-Dashen point are used to calculate the pion-nucleon sigma term. The t-channel input is updated using the recent GWU partial wave solution and n phase shifts from calculations based on Roy equations. Obtained values for the sigma term are still within the error bars of the previous Karlsruhe result. [ABSTRACT FROM AUTHOR]

It was recently proved that the invariance of observables with respect to angle-dependent phase rotations of reaction amplitudes mixes multipoles, changing also their relative strength [A. Švarc et al., Phys. Rev. C 97, 054611 (2018)]. Modern partial wave analyses (PWAs) in η photoproduction on protons in the center-of-mass energy range 1.5≤W≤2.0 GeV, either energy-dependent (ED) or single-energy (SE) ones, do not take this effect into consideration. It is commonly accepted that all PWAs give very similar results for the E0+ multipole, but notable differences in this and all other partial waves still remain. In this paper we demonstrate that once these phase rotations are properly taken into account, all ED and SE partial wave analysis in η photoproduction become almost identical for the dominant E0+ multipole, and the agreement among all other multipoles becomes much better. We also show that measured observables are almost equally well reproduced in all PWAs, and the remaining differences among multipoles can be attributed solely to the difference in predictions for unmeasured observables. So, new measurements are needed. [ABSTRACT FROM AUTHOR]

Unconstrained partial-wave amplitudes, obtained at discrete energies from fits to complete sets of eight independent observables, may be used to reconstruct reaction amplitudes. These partial-wave amplitudes do not vary smoothly with energy and are in principle nonunique. We demonstrate how this behavior can be ascribed to the continuum ambiguity. Starting from the spinless scattering case, we show how an unknown overall phase, depending on energy and angle, mixes the structures seen in the associated partial-wave amplitudes. This process is illustrated using a simple toy model. We then apply these principles to pseudoscalar meson photoproduction, showing how the above effect can be removed through a phase rotation, allowing a consistent comparison with model amplitudes. The effect of this phase ambiguity is also considered for Legendre expansions of experimental observables. [ABSTRACT FROM AUTHOR]

Data on the reaction γp→K+Λ from the CLAS experiments are used to derive the leading multipoles, E0+, M1-, E1+, and M1+, from the production threshold to 2180 MeV in 24 slices of the invariant mass. The four multipoles are determined without any constraints. The multipoles are fitted using a multichannel L+P model that allows us to search for singularities and to extract the positions of poles on the complex energy plane in an almost model-independent method. The multipoles are also used as additional constraints in an energy-dependent analysis of a large body of pion and photoinduced reactions within the Bonn-Gatchina partial wave analysis. The study confirms the existence of poles due to nucleon resonances with spin parity JP=1/2-, 1/2+, and 3/2+ in the region at about 1.9 GeV. [ABSTRACT FROM AUTHOR]

Electromagnetic resonance properties are uniquely defined at the pole and do not depend on the separation of the resonance from background or the decay channel. Photon-nucleon branching ratios are nowadays often quoted at the pole, and we generalize the considerations to the case of virtual photons. We derive and compare relations for nucleon to baryon transition form factors both for the Breit-Wigner and the pole positions. Using the MAID2007 and SAID SM08 partial wave analyses of pion electroproduction data, we compare the GM, GE, and GC form factors for the Δ(1232) resonance excitation at the Breit-Wigner resonance and pole positions up to Q²=5GeV². We also explore the E/M and S/M ratios as functions of Q². For pole and residue extraction, we apply the Laurent + Pietarinen method. [ABSTRACT FROM AUTHOR]