A. Svarc

A. Svarc
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Nuclear Theory (33)
 
High Energy Physics - Phenomenology (22)
 
Nuclear Experiment (7)
 
High Energy Physics - Experiment (1)

Publications Authored By A. Svarc

2017May
Authors: D. Ho, P. Peng, C. Bass, P. Collins, A. D'Angelo, A. Deur, J. Fleming, C. Hanretty, T. Kageya, M. Khandaker, F. J. Klein, E. Klempt, V. Laine, M. M. Lowry, H. Lu, C. Nepali, V. A. Nikonov, T. O'Connell, A. M. Sandorfi, A. V. Sarantsev, R. A. Schumacher, I. I. Strakovsky, A. Švarc, N. K. Walford, X. Wei, C. S. Whisnant, R. L. Workman, I. Zonta, K. P. Adhikari, D. Adikaram, Z. Akbar, M. J. Amaryan, S. Anefalos Pereira, H. Avakian, J. Ball, M. Bashkanov, M. Battaglieri, V. Batourine, I. Bedlinskiy, W. J. Briscoe, V. D. Burkert, D. S. Carman, A. Celentano, G. Charles, T. Chetry, G. Ciullo, L. Clark, L. Colaneri, P. L. Cole, M. Contalbrigo, V. Crede, N. Dashyan, E. De Sanctis, R. De Vita, C. Djalali, R. Dupre, A. El Alaoui, L. El Fassi, L. Elouadrhiri, G. Fedotov, S. Fegan, R. Fersch, A. Filippi, A. Fradi, Y. Ghandilyan, G. P. Gilfoyle, F. X. Girod, D. I. Glazier, C. Gleason, W. Gohn, E. Golovatch, R. W. Gothe, K. A. Griffioen, M. Guidal, L. Guo, H. Hakobyan, N. Harrison, K. Hicks, M. Holtrop, S. M. Hughes, Y. Ilieva, D. G. Ireland, B. S. Ishkhanov, E. L. Isupov, D. Jenkins, H. Jiang, H. S. Jo, K. Joo, S. Joosten, D. Keller, G. Khachatryan, A. Kim, W. Kim, A. Klein, V. Kubarovsky, S. V. Kuleshov, L. Lanza, P. Lenisa, K. Livingston, I . J . D. MacGregor, N. Markov, B. McKinnon, T. Mineeva, V. Mokeev, R. A. Montgomery, A Movsisyan, C. Munoz Camacho, G. Murdoch, S. Niccolai, G. Niculescu, M. Osipenko, M. Paolone, R. Paremuzyan, K. Park, E. Pasyuk, W. Phelps, O. Pogorelko, J. W. Price, S. Procureur, D. Protopopescu, M. Ripani, D. Riser, B. G. Ritchie, A. Rizzo, G. Rosner, F. Sabatié, C. Salgado, Y. G. Sharabian, Iu. Skorodumina, G. D. Smith, D. I. Sober, D. Sokhan, N. Sparveris, S. Strauch, Ye Tian, B. Torayev, M. Ungaro, H. Voskanyan, D. P. Watts, M. H. Wood, N. Zachariou, J. Zhang, Z. W. Zhao

We report the first beam-target double-polarization asymmetries in the $\gamma + n(p) \rightarrow \pi^- + p(p)$ reaction spanning the nucleon resonance region from invariant mass $W$= $1500$ to $2300$ MeV. Circularly polarized photons and longitudinally polarized deuterons in $H\!D$ have been used with the CLAS detector at Jefferson Lab. The exclusive final state has been extracted using three very different analyses that show excellent agreement, and these have been used to deduce the {\it{E}} polarization observable for an effective neutron target. Read More

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. Read More

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. Read More

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. Read More

We analyze the current status of three P-wave baryon states $N(1710){1/ 2}^+$, $N(1900){3/2}^+$, and $\Delta(1600){3/2}^+$ 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. Read More

The pole structure of the current GW/SAID partial-wave analysis of elastic $\pi N$ scattering and $\eta N$ production data is studied. Pole positions and residues are extracted from both the energy-dependent and single-energy fits, using two different methods. For the energy-dependent fits, both contour integration and a Laurent+Pietarinen approach are used. Read More

We have applied a new approach to determine the pole positions and residues from pion photoproduction multipoles. The method is based on a Laurent expansion of the partial wave T-matrices, with a Pietarinen series representing the regular part of energy-dependent and single-energy photoproduction solutions. The method has been applied to multipole fits generated by the MAID and GWU/SAID groups. Read More

The mini-proceedings of the Light Meson Dynamics Workshop held in Mainz from February 10th to 12th, 2014, are presented. The web page of the conference, which contains all talks, can be found at https://indico.cern. Read More

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. Read More

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. Read More

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. Read More

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. Read More

The old knowledge about interrelation among T-matrix, K-matrix and bare poles is summarized and put into modern perspective. Read More

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. Read More

In ref. [1] we have used the Zagreb realization of Carnegie-Melon-Berkeley coupled-channel, unitary model as a tool for extracting pole positions from the world collection of partial wave data, with the aim of eliminating model dependence in pole-search procedures. In order that the method is sensible, we in this paper discuss the stability of the method with respect to the strong variation of different model ingredients. Read More

Each and every energy dependent partial-wave analysis is parameterizing the pole positions in a procedure defined by the way how the continuous energy dependence is implemented. These pole positions are, henceforth, inherently model dependent. To reduce this model dependence, we use only one, coupled-channel, unitary, fully analytic method based on the isobar approximation to extract the pole positions from the each available member of the worldwide collection of partial wave amplitudes which are understood as nothing more but a good energy dependent representation of genuine experimental numbers assembled in a form of partial-wave data. Read More

Within the previously developed Dubna-Mainz-Taipei meson-exchange model, the singularity structure of the pi N scattering amplitudes has been investigated. For all partial waves up to F waves and c.m. Read More

Eta meson production in the proton-proton scattering is dominated by the low-mass meson exchange. We present a brief study on how the type of the exchanged mesons within coupled-channel and multi-resonance model influences the scattering observables. We show under which circumstances the eta exchange may explain the shape of the observed cross sections, and present a few selected results: total cross section in the full energy range, and the proton-proton energy distribution at 15. Read More

It is argued in [1] that when the strong coupling to the K Lambda channel is considered, Breit-Wigner mass of the lightest orbital excitation of the nucleon N(1535) shifts to a lower value. The new value turned out to be smaller than the mass of the lightest radial excitation N(1440), which effectively solved the long-standing problem of conventional constituent quark models. In this Comment we show that it is not the Breit-Wigner mass of N(1535) that is decreased, but its bare mass. Read More

The physical meaning of bare and dressed scattering matrix singularities has been investigated. Special attention has been attributed to the role of well known invariance of scattering matrix with respect to the field transformation of the effective Lagrangian. Examples of evaluating bare and dressed quantities in various models are given. Read More

For last two decades different quark models have predicted diverse, sometimes contradictory collections of resonant states. To choose the best among them, the obtained sets had to be compared to available experimental values. In the absence of a more thorough understanding, quark-model resonant states have been directly identified with scattering-matrix singularities. Read More

A model-independent method for the determination of Breit-Wigner resonance parameters is presented. The method is based on eliminating the dependence on the choice of channel basis by analyzing the trace of the K and T matrices in the coupled-channel formalism, rather than individual matrix elements of the multichannel scattering matrix. Read More

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. Read More

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. Read More

In spite of prolonged polemics, the agreement on the existence of N(1710) P11 resonance has not until now been reached, and the Particle Data Group declares it as a 3-star resonance only. We show that the proper inclusion of inelastic channels in the coupled-channel formalism indisputably demands the existence of N(1710) P11 state, and that it presumably stays "hidden" within the continuum ambiguity of any typical single channel partial wave analyses. Consequently, its Particle Data Group confidence rating should be raised to a 4-star resonance. Read More

Single channel, single energy partial wave analyses (SE_PWA) are from the first principles non-unique in the inelastic region if only data from elastic channels are used, so we in details discuss mechanisms how the problem is eliminated in pion-nucleon scattering. The "continuum ambiguities" puzzle has been extensively discussed since early 1970es, and two major mechanisms for solving the problem have been suggested: either to ensure the continuity of Argand diagrams by imposing the T-matrix t-channel analyticity (Karlsruhe-Helsinki, VPI/GWU) or to restore the unitarity loss in the kinematical regime where the inelastic channels are successively opened by replacing the standard single channel PWA by the coupled channel formalism (CMB, Zagreb, Kent, Pittsburgh/ANL, Giessen). In both approaches the insufficiency of the single channel data is eliminated by introducing additional constraints using the data from other channels. Read More

The pole structure of the P11 pion-nucleon partial wave is examined with the emphasis on the 1700 MeV energy domain. The mechanism of eliminating continuum ambiguities in pion-nucleon partial wave analyses by using the coupled channel formalism, presented elsewhere for the piN -> etaN channel, is applied for the piN -> K Lambda channel, with the aim to clarify the issue whether physical reality requires none (VPI/GWU), one (KH80, CMB, Kent, Pittsburgh/ANL, Giessen), or possibly two (Zagreb) poles of the partial wave T-matrix in the 1700 MeV range. The role of second inelastic channel for resolving the dilemma is demonstrated. Read More

We have used the Breit-Wigner resonance model with S11, P11 and P13 resonances in the s-channel to re-analyze the old piN -> KLambda data with the aim to establish the origin of the prominent structure in the total cross section in the vicinity of 1700 MeV. In this paper we show that, at least in the Breit-Wigner resonance model, it is not possible to achieve the detailed reproduction of the narrow 1700 MeV total cross section peak using the standard partial widths. We have found the new set of resonance parameters enforcing the experimentally observed structure of the total cross section data simultaneously with the linear dependence of the differential cross sections with the cos(Theta) in the energy range 1650 MeV < W < 1800 MeV. Read More

An existing single resonance model with S11, P11 and P13 Breit-Wiegner resonances in the s-channel has been re-applied to the old pi N --> K Lambda data. It has been shown that the standard set of resonant parameters fails to reproduce the shape of the differential cross section. The resonance parameter determination has been repeated retaining the most recent knowledge about the nucleon resonances. Read More

The partial wave T-matrices for the pi N, eta N and pi2 N channels have been obtained within the framework of the coupled channel model using the pi N elastic and pi N --> eta N data base as input. It has been shown that for the P11 partial wave an equally good representation of the experimental data (namely the T(pi N,pi N) and T(pi N,eta N) T-matrices) can be obtained using either three, or four poles for the Green function propagator. However, the three Green function pole solution is not acceptable due to the structure of the extracted resonances. Read More

Re-analyzing the old pi N --> K Lambda data, the additional proof is given for the existence of the N(1710) P11 state, critically needed in light of reported observations of exotic Theta(1539) and Xi(1862) pentaquarks. An existing single-resonance model with S11, P11 and P13 Breit-Wiegner resonances in the s-channel has been applied. It has been shown that the standard set of resonant parameters fails to reproduce the shape of the differential cross section. Read More

Contrary to very early beliefs, the experimental cross section data for the eta production in proton-proton scattering are well described if pi and only eta meson exchange diagrams are used to calculate the Born term. The inclusion of initial and final state interactions is done in the factorization approximation by using the inverse square of the Jost function. The two body Jost functions are obtained from the S matrices in the low energy effective range approximation. Read More

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. Read More

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. Read More

Using the new set of dd --> eta alpha near threshold experimental data, the estimate of the importance of the nucleon-nucleon correlations for the eta alpha S-wave scattering length in the multiple scattering theory is obtained using the low-energy scattering length model. The contribution turns out to be much bigger then previously believed. The pi-eta mixing angle is extracted using the experimental data on the dd --> eta alpha and dd --> pi alpha processes. Read More

A short, and definitely not a complete representation of \eta production processes on hadrons is given. First of all, the different ways of obtaining the \pi N \to \eta N and \pi N \to \eta N amplitudes are presented. After that, an overview of results obtained using these amplitudes as input for calculating processes like: NN \to \eta NN, pd \to \eta ^{3}He, \pi d \to \eta NN and \eta d, \eta ^{3}He and \eta ^{4}He as well as \eta-light nuclei bond states, will be given. Read More

It is pointed out that the third of five low-lying P11 states predicted by a constituent quark model can be identified with the third of four states in a solution from a three-channel analysis by the Zagreb group. This is one of the so-called ``missing'' resonances, predicted at 1880 MeV. The fit of the Zagreb group to the pi N -> eta N data is the crucial element in finding this fourth resonance in the P11 partial wave. Read More

A three-channel, multi-resonance, unitary model developed in 1995 is used to determine the $\pi N \rightarrow \eta N$ and $\eta N \rightarrow \eta N$ amplitudes using as input the latest data for the dominant $S_{11}$ $\pi N$ elastic scattering partial wave following suggestions of Prof. G. H\"{o}hler. Read More

The total cross section for the near threshold eta production in proton-proton collisions has been investigated with the assumption that the production mechanism is due to the emission of a meson x (pi, eta, sigma) from one of the nucleons followed by a x N --> eta N transition on the second one. The x N --> eta N amplitudes are generated from the unitary multi-channel multi-resonance model which has recently been constructed in analyzing the pi N scattering and pi N --> eta N reaction. The initial and final pp distortions are calculated from a coupled-channel pi-NN model which describes the NN scattering data up to about 2 GeV. Read More

We show that the S-wave $\eta N$ 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 $\pi N$ elastic S-wave T-matrix at the $\eta$ production threshold, and near threshold $\pi^{-} p \rightarrow \eta n$ total cross section. The results are independent of the particular parametrization of the elastic T-matrix and of the number of channels. Read More

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. Read More

The pi N --> eta N and eta N --> eta N partial wave T-matrices for the eight lowest partial waves have been obtained in a three coupled channel model with unitarity manifestly imposed. The two physical channels are pi_N and eta_N, and the third channel, pi_pi_N is an effective, but unphysical two body channel which represents all remaining processes. The pi_N elastic phase shifts and the weighted data base of the pi- p --> eta n total and differential cross sections are chosen as the input for the fitting procedure. Read More

We study the posibility of large isospin fluctuations in high-energy heavy-ion collisions by assuming that pions are produced semiclassically both directly and in pairs through the isovector channel. The leading-particle effect and the factorization property of the scattering amplitude in the impact parameter space are used to define the classical pion field. In terms of the joint probability function $P_{II_{3}}(n_{0},n_{\_})$ for producing $n_{0}$ neutral and $n_{\_}$ negative pions from a definite isospin state $II_{3}$ of the incoming leading-particle system we calculate the two pion correlation parameters $f_{2,n_{\_}}^{0}$ and the average number of neutral pions $(< n_{0} >_{n_{\_}})$ as a function of negative pions $(n_{\_})$ produced. Read More

Assuming that leading particles in high-energy hadronic and nuclear collisions become sources of a classical pion field, we show that the direct production of pions favors Centauro (mainly charged) events and that the production of pions through the $ \rho$-type channel favors anti-Centauro (mainly neutral) events. We also observe a strong negative neutral-charged correlation in both cases. Read More