FBS: From Experimental Data to Pole Parameters in a Direct Way (Angle Dependent Continuum Ambiguity and Laurent + Pietarinen Expansion)

Paper published in Few-Body Syst (2018) 59: 96. , First Online: 08 June 2018

From Experimental Data to Pole Parameters in a Direct Way (Angle Dependent Continuum Ambiguity and Laurent + Pietarinen Expansion)

A. Švarc, Y. Wunderlich, H. Osmanović, M. Hadžimehmedović, R. Omerović, J. Stahov, V. Kashevarov, K. Nikonov, M. Ostrick, L. Tiator, R. Workman

Few-Body Syst (2018) 59: 96.
DOI: https://doi.org/10.1007/s00601-018-1410-y

This article belongs to the Topical Collection “NSTAR 2017—The International Workshop on the Physics of Excited Nucleons”.

Unconstrained partial-wave amplitudes obtained at discrete energies from fits to complete sets of eight independent observables which are required to uniquely reconstruct reaction amplitudes do not vary smoothly with energy, and are in principle non-unique. We demonstrate how this behavior can be ascribed to the continuum ambiguity. Starting from the spinless scattering case, we demonstrate how an unknown overall phase depending on energy and angle mixes the structures seen in the associated partial-wave amplitudes making the partial wave decomposition non-unique, and illustrate it on a simple toy model. We then apply these principles to pseudo-scalar meson photoproduction and show that the non-uniqueness effect can be removed through a phase rotation generating “up-to-a-phase” unique set of SE partial wave amplitudes. Extracting pole positions from partial wave amplitudes is the next step. Up to now, there was no reliable way to extract pole parameters from SE partial waves, but a new and simple single-channel method (Laurent + Pietarinen expansion) applicable for continuous and discrete data has been recently developed. It is based on applying the Laurent decomposition of partial wave amplitude, and expanding the non-resonant background into a power series of a conformal-mapping, quickly converging power series obtaining the simplest analytic function with well-defined partial wave analytic properties which fits the input. The generalization of this method to multi- channel case is also developed and presented. Unifying both methods in succession, one constructs a model independent procedure to extract pole parameters directly from experimental data without referring to any theoretical model.

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PRC: Fixed-t analyticity as a constraint in single-energy partial-wave analyses of meson photoproduction reactions

Paper published in Phys. Rev. C 97.015207 - Published 31 January 2018

Fixed-t analyticity as a constraint in single-energy partial-wave analyses of meson photoproduction reactions

H. Osmanović, M. Hadžimehmedović, R. Omerović, J. Stahov, V. Kashevarov, K. Nikonov, M. Ostrick, L. Tiator, and A. Švarc

DOI:https://doi.org/10.1103/PhysRevC.97.015207

Partial-wave amplitudes of meson photoproduction reactions are an important source of information in baryon spectroscopy. We investigate a new approach in single-energy partial-wave analyses of these reactions. Instead of using a constraint to theoretical models in order to achieve solutions which are continuous in energy, we enforce the analyticity of the amplitudes at fixed values of the Mandelstam variable t. We present an iterative procedure with successive fixed-t amplitude analyses which constrain the single-energy partial-wave analyses and apply this method to the $\gamma p \rightarrow \eta p$. We use pseudodata, generated by the EtaMAID model, to test the method and to analyze ambiguities. Finally, we present an analytically constrained partial-wave analysis using experimental data for four polarization observables recently measured at MAMI and GRAAL in the energy range from threshold to $\sqrt{s}=1.85 GeV$.

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EPJa: N* resonances from KΛ amplitudes in sliced bins in energy

Paper published in The European Physical Journal A - Published 22 December 2017 

N* resonances from K ΛΛ amplitudes in sliced bins in energy 

A. V. Anisovich, V. Burkert, M. Hadžimehmedović, D. G. Ireland, E. Klempt, V. A. Nikonov, R. Omerović, A. V. Sarantsev, J. Stahov, A. Švarc, U. Thoma 

DOI: https://doi.org/10.1140/epja/i2017-12443-x

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 $J^P=1/2+,3/2+,1/2−,3/2−,5/2−$ .

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PRL: Strong evidence for nucleon resonances near 1900 MeV

Paper published in Phys. Rev. Lett. 119, 062004 – Published 11 August 2017
Accepted paper in Physical Review Letters - 10 July 2017.

https://journals.aps.org/prl/accepted/db072Yc4O111a856188a77c50a1ae4f7572e4182f

Strong evidence for nucleon resonances near 1900 MeV - A. V. Anisovich, V. Burkert, M. Hadžimehmedović, D. G. Ireland, E. Klempt, V. A. Nikonov, R. Omerović, H. Osmanović, A. V. Sarantsev, J. Stahov, A. Švarc, and U. Thoma 

Abstract:

Data on the reaction $\gamma p \to K^+\Lambda$ 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 which 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 photo-induced reactions within the Bonn-Gatchina (BnGa) partial wave analysis. The study confirms the existence of poles due to nucleon resonances with spin-parity J^P = 1/2^-; 1/2^+, and 3/2^+ in the region at about 1.9\,GeV.

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