arXiv subjects
Publications and source records attributed to Alfred Svarc.
A unified Chew-Mandelstam description of single-pion photoproduction data, together with pion- and eta-hadroproduction data, has been extended to include measurements carried out over the last decade. We consider photo-decay amplitudes evaluated at the pole with particular emphasis on ng couplings and the influence of weighting on our fits. Both energy-dependent and single-energy analysis (energy-binned data) are considered.
Unfortunately, yes, it is. In ref.[1] it has been shown that without fixing the reaction-amplitude phase, many combinations of partial waves at neighboring energies in single-energy, single-channel partial wave analysis reproduce experimental data identically, but are discontinuous and disconnected. To obtain the continuous solution, the phase has to be fixed to some continuous value. In the same reference it has also been shown that the change of angular part of reaction-amplitude phase mixes partial waves, so the pole structure of any single-energy single-channel partial wave analysis depends on the chosen phase. As in any single-channel analysis the overall reaction-amplitude phase cannot be determined because of continuum ambiguity, it is in principle free and has to be taken from some coupled-channel model. Because of the difference in the angular part of the phase, choosing different phases results in the change of the pole content of the obtained solution. Therefore, single-energy single-channel partial wave analysis is inherently model dependent, and the number of poles it contains strongly depends on the choice of the phase. In that reference these facts have been illustrated on the pseudo-scalar meson production toy model. This truth is now for the measured observables demonstrated on the realistic model of ref. [2] which combines amplitude and partial wave analysis in one self-consistent scheme for the the world collection of {\eta}-photoproduction data, and fixes the phase to BG2014-2 solution [3]. We show that the pole structure of the solution whose phase is very close to the BG2014-2 theoretical ED phase is very similar to the pole structure of BG2014-2 solution, but the agreement with the data is not ideal. However, improving the agreement with the data requires the departure from BG2014-2 ED phase, and this spoils analytic structure of the obtained solution.
A method to extract resonance pole information from single-channel partial-wave amplitudes based on a Laurent (Mittag-Leffler) expansion and conformal mapping techniques has recently been developed. This method has been applied to a number of reactions and provides a model-independent extraction procedure which is particularly useful in cases where a set of amplitudes is available only at descrete energies. This method has been generalized and applied to the case of a multi-channel fit, where several sets of amplitudes are analysed simultaneously. The importance of unitarity constraints is discussed. The final result provides a powerful, model-independent tool for analyzing partial-wave amplitudes of coupled or connected channels based entirely on the concepts of analyticity and unitarity.
Resonances are extracted from a number of energy-dependent and single-energy fits to scattering data. The influence of recent, precise EPECUR data is investigated. Results for the single-energy fits are derived using the L+P method of analysis and are compared to those obtained using contour integration applied to the global energy-dependent fits.
We analyze the current status of three P-wave baryon states $N(1710){1/ 2}^+$, $N(1900){3/2}^+$, and $Δ(1600){3/2}^+$ as given in the Review of Particles Physics (RPP). Since the evidence for a particle's existence is linked to its RPP "star" rating, we discuss its subjective present definition. We also present the accumulating evidence supporting these states and give our new "star" rating recommendations.
Poles of partial wave scattering matrices have also in hadron spectroscopy recently been established as a sole link between experiment and QCD theories and models [1-4]. We have, however, noticed the fact that Karlsruhe-Helsinki (KH) partial wave analyses, one of the two partial wave analyses which have been "above the line" in PDG [1] for over three decades, does calculate and PDG compiles the abundance of Breit-Wigner (BW) parameters obtained by local BW fit, but has produced only a limited number of pole information obtained by using speed plot (SP) method [5]. We stress the fact that in KH method only Mandelstam analyticity is used as a theoretical constraint, so KH partial wave solutions are as model independent as possible. Therefore, they represent a very confident and valuable input, so it is of extreme importance to extract as many resonance pole parameters from them as possible. In addition, the troubling fact is that BW parameters given in PDG have been obtained from KH80 solution, while pole parameters have been obtained from KA84 version of KH solution. To remedy this, we have used newly developed Laurent+Pietarinen expansion method [6,7] for obtaining pole positions for all partial waves for KH80 and KA84 solutions, compared them, and shown that the differences in pole parameters are, with very few discussed exceptions, negligible for all partial waves. We finally give a full set of pole parameters for both solutions for all partial waves; results which are until now unpublished.
We present a new approach to quantifying pole parameters of single-channel processes based on a Laurent expansion of partial-wave T-matrices in the vicinity of the real axis. Instead of using the conventional power-series description of the non-singular part of the Laurent expansion, we represent this part by a convergent series of Pietarinen functions. As the analytic structure of the non-singular part is usually very well known (physical cuts with branch points at inelastic thresholds, and unphysical cuts in the negative energy plane), we find that one Pietarinen series per cut represents the analytic structure fairly reliably. The number of terms in each Pietarinen series is determined by the quality of the fit. The method is tested in two ways: on a toy model constructed from two known poles, various background terms, and two physical cuts, and on several sets of realistic piN elastic energy-dependent partial-wave amplitudes (GWU/SAID - [1, 2], and Dubna-Mainz-Taipei - [3, 4]). We show that the method is robust and confident using up to three Pietarinen series, and is particularly convenient in fits to amplitudes, such as single-energy solutions, coming more directly from experiment; cases where the analytic structure of the regular part is apriori unknown.
We present a new approach to quantifying pole parameters of single-channel processes based on Laurent expansion of partial wave T-matrices. Instead of guessing the analytical form of non-singular part of Laurent expansion as it is usually done, we represent it by the convergent series of Pietarinen functions. As the analytic structure of non-singular term is usually very well known (physical cuts with branhcpoints at inelastic thresholds, and unphysical cuts in the negative energy plane), we show that we need one Pietarinen series per cut, and the number of terms in each Pietarinen series is automatically determined by the quality of the fit. The method is tested on a toy model constructed from two known poles, various background terms, and two physical cuts, and shown to be robust and confident up to three Pietarinen series. We also apply this method to Zagreb CMB amplitudes for the N(1535) 1/2- resonance, and confirm the full success of the method on realistic data. This formalism can also be used for fitting experimental data, and the procedure is very similar as when Breit-Wigner functions are used, but with one modification: Laurent expansion with Pietarinen series is replacing the standard Breit-Wigner T-matrix form.
Luscher has shown that in single channel problem (elastic region below first inelastic threshold) there exists a direct link between the discrete value of the energy in a finite QCD volume and the scattering phase shift at the same energy. However, when the extension of the theorem is made to the baryon resonance sector (multi-channel situation in the inelastic region above first inelastic threshold), eigenphases (diagonal multi-channel quantities) replace phase shifts (single channel quantities). It is necessary to stress that the renowned pi/2 resonance criterion is formulated for eigenphases and not for phase shifts, so the resonance extracting procedure has to be applied with utmost care. The potential instability of extracting eigenphases from experimental data which occurs if insufficient number of channels is used can be reduced if a trace function which explicitly takes multi-channel aspect of the problem into account is used instead of single-channel phase shifts.
The old knowledge about interrelation among T-matrix, K-matrix and bare poles is summarized and put into modern perspective.
This is the summary of the 'International Workshop on eta--Nucleus Physics' held in Juelich, May 8--12, 2006. Each talk is represented by an extended abstract. At the same time the document provides a progress report of the corresponding working group.
Inspired by anomalies which the standard scattering matrix pole-extraction procedures have produced in a mathematically well defined coupled-channel model, we have developed a new method based solely on the assumption of partial-wave analyticity. The new method is simple and applicable not only to theoretical predictions but to the empirical partial-wave data as well. Since the standard pole-extraction procedures turn out to be the lowest-order term of the proposed method the anomalies are understood and resolved.
Total cross section for pp->pp eta process has been calculated in meson exchange model. Eta production T-matrices are from our PWA. Initial and final state interactions are disregarded for the moment, due to large uncertainties. Preliminary results do not show rho meson exchange dominance.
The check whether the presently available multiresonance coupled channel analyses can explain the more complex processes is made. The process chosen is a well measured proces of eta production in proton proton collisions.
A short, and definitely not a complete representation of ηproduction processes on hadrons is given. First of all, the different ways of obtaining the πN \to ηN and πN \to ηN amplitudes are presented. After that, an overview of results obtained using these amplitudes as input for calculating processes like: NN \to ηNN, pd \to η^{3}He, πd \to ηNN and ηd, η^{3}He and η^{4}He as well as η-light nuclei bond states, will be given. The experimental and theoretical results will be reviewed. The opened problems and the way how to solve them will be presented.
A three-channel, multi-resonance, unitary model developed in 1995 is used to determine the $πN \rightarrow ηN$ and $ηN \rightarrow ηN$ amplitudes using as input the latest data for the dominant $S_{11}$ $πN$ elastic scattering partial wave following suggestions of Prof. G. Höhler. The sign error in the numerical evaluation of the dispersion integral in the original publication is eliminated. The remaining weighted data set for the $πN \rightarrow ηN$ total and differential cross sections is used as in the original publication. The correction of the numerical error influences the $ηN$ cusp effect and improves the quality of the fit to the input data. However, our new result for the $ηN$ scattering length, $a_{ηN }= (0.717 \pm 0.030) + i (0.263 \pm 0.025)$ fm, is a sole consequence of the correction of the $S_{11}$ input and suggests that the $ηd$ system is unbound or loosely bound.
We show that the S-wave $ηN$ scattering length can be extracted in a model independent way within the scope of the multichannel model, but with the restricting assumption that only one resonance is included per partial wave. One has only to use the information on $πN$ elastic S-wave T-matrix at the $η$ production threshold, and near threshold $π^{-} p \rightarrow ηn$ total cross section. The results are independent of the particular parametrization of the elastic T-matrix and of the number of channels. For the described model the unitarity requires that the number of channels must be at least three. These assumptions are more general then the assumptions of the existing single resonance models up to now used for extracting the $ηN$ S-wave scattering length. The agreement of our approach with former estimates is anticipated and confirmed for all cases for which the input data agree with the commonly accepted values.
The S-wave scattering length for eta-N elastic scattering is extracted from the S-wave T-matrix in a three coupled channel, multiresonance unitary model. Results are compared with values already reported in literature which are obtained applying multichannel, but single resonance -- no background models. A dispersion among the previously published values of the real part of the S-wave scattering length is observed. We demonstrate that the reported spread originates from the strong sensitivity of the scattering length upon the small variation of the used input resonance parameters. In addition, we show that eta-N scattering length value obtained in single resonance -- no background models significantly increases if background term is added in a unitary way. We question the reliability of previously reported values based only on the single resonance -- no background models, and demonstrate that the value of the eta-N S-wave scattering length obtained in this publication is much more realistic because of the multiresonance and unitary approach.