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A. Wirzba

Publications and source records attributed to A. Wirzba.

At least 37 records · Page 2Linked to original sources
arXiv2012

Proceedings of the second International PrimeNet Workshop

These are the proceedings of the second PrimeNet Workshop, held in September 26-28, 2011, at the campus of Forschungszentrum Jülich, Germany. This workshop is part of the activities in the project "Study of Strongly Interacting Matter" (acronym HadronPhysics2), which is an integrating activity of the Seventh Framework Program of EU. This HP2 project contains several activities, one of them being the network PrimeNet having the focus on Meson Physics in Low-Energy QCD. This network is created to exchange information on experimental and theoretical ongoing activities on mainly eta and eta-prime physics at different European accelerator facilities and institutes.

nucl-ex↗
arXiv2011

Model-independent approach to eta -> pi+ pi- gamma and eta' -> pi+ pi- gamma

We present a new, model-independent method to analyze radiative decays of mesons to a vector, isovector pair of pions of invariant mass square below the first significant pion-pion threshold in the vector channel. It is based on a combination of chiral perturbation theory and dispersion theory. This allows for a controlled inclusion of resonance physics without the necessity to involve vector meson dominance explicitly. As an example, the method is applied to an analysis of the reactions eta -> pi+ pi- gamma and eta'->pi+ pi- gamma.

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arXiv2009

Neutron-Proton Mass Difference in Nuclear Matter and in Finite Nuclei and the Nolen-Schiffer Anomaly

The neutron-proton mass difference in (isospin asymmetric) nuclear matter and finite nuclei is studied in the framework of a medium-modified Skyrme model. The proposed effective Lagrangian incorporates both the medium influence of the surrounding nuclear environment on the single nucleon properties and an explicit isospin-breaking effect in the mesonic sector. Energy-dependent charged and neutral pion optical potentials in the s- and p-wave channels are included as well. The present approach predicts that the neutron-proton mass difference is mainly dictated by its strong part and that it markedly decreases in neutron matter. Furthermore, the possible interplay between the effective nucleon mass in finite nuclei and the Nolen-Schiffer anomaly is discussed. In particular, we find that a correct description of the properties of mirror nuclei leads to a stringent restriction of possible modifications of the nucleon's effective mass in nuclei.

nucl-th↗
arXiv2008

Neutron-proton mass difference in finite nuclei and the Nolen-Schiffer anomaly

The neutron-proton mass difference in finite nuclei is studied in the framework of a medium-modified Skyrme model. The possible interplay between the effective nucleon mass in finite nuclei and the Nolen-Schiffer anomaly is discussed. In particular, we find that a correct description of the properties of mirror nuclei leads to a stringent restriction of possible modifications of the nucleon's effective mass in nuclei.

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arXiv2007

Neutron-proton mass difference in isospin asymmetric nuclear matter

Isospin-breaking effects in the baryonic sector are studied in the framework of a medium-modified Skyrme model. The neutron-proton mass difference in infinite, asymmetric nuclear matter is discussed. In order to describe the influence of the nuclear environment on the skyrmions, we include energy-dependent charged and neutral pion optical potentials in the s- and p-wave channels. The present approach predicts that the neutron-proton mass difference is mainly dictated by its strong part and that it strongly decreases in neutron matter.

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arXiv2007

Remarks on N N -> N N pi beyond leading order

In recent years a two-scale expansion was established to study reactions of the type N N -> N N pi within chiral perturbation theory. Then the diagrams of some subclasses that are invariant under the choice of the pion field no longer appear at the same chiral order. In this letter we show that the proposed expansion still leads to well defined results. We also discuss the appropriate choice of the heavy baryon propagator.

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arXiv2007

Neutron-proton mass difference in nuclear matter

Isospin-breaking effects in nuclear matter are studied in the framework of a medium-modified Skyrme model. The proposed effective Lagrangian incorporates both the medium influence of the surrounding nuclear environment on the single nucleon properties and an explicit isospin-breaking effect in the mesonic sector. The approach predicts that the neutron-proton mass difference decreases in isospin-symmetric nuclear matter but by a very small amount only.

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arXiv2004

Nucleon-nucleon potential in finite nuclei

We consider the spin-isospin-independent central part of the residual nucleon-nucleon potential in finite spherical nuclei taking into account the deformation effects of the nucleons within the surrounding nuclear environment. It is shown that inside the nucleus the short-range repulsive contribution of the potential is increased and the intermediate attraction is decreased. We identify the growth of the radial component of the spin-isospin independent short-range part of the in-medium nucleon-nucleon interaction as the responsible agent that prevents the radial collapse of the nucleus.

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arXiv2002

Self energies of the pion and the delta isobar from the ^3He(e,e'pi^+)^3H reaction

In a kinematically complete experiment at the Mainz microtron MAMI, pion angular distributions of the $^3$He(e,e'$π^+)^3$H reaction have been measured in the excitation region of the $Δ$ resonance to determine the longitudinal ($L$), transverse ($T$), and the $LT$ interference part of the differential cross section. The data are described only after introducing self-energy modifications of the pion and $Δ$-isobar propagators. Using Chiral Perturbation Theory (ChPT) to extrapolate the pion self energy as inferred from the measurement on the mass shell, we deduce a reduction of the $π^+$ mass of $Δm_{π^+} = (-1.7^{+ 1.7}_{- 2.1})$ MeV/c$^2$ in the neutron-rich nuclear medium at a density of $ρ= (0.057^{+ 0.085}_{- 0.057})$ fm$^{-3}$. Our data are consistent with the $Δ$ self energy determined from measurements of $π^0$ photoproduction from $^4$He and heavier nuclei.

nucl-ex↗
arXiv2001

Nucleon deformation in finite nuclei

The deformation of a nucleon embedded in various finite nuclei is considered by taking into account the distortion of the chiral profile functions under the action of an external field representing the nuclear density. The baryon charge distribution of the nucleon inside light, medium-heavy and heavy nuclei is discussed. The mass of the nucleon decreases as it is placed deeper inside the nucleus and reaches its minimum at the center of the nucleus. We discuss the quantization of non-spherical solitons and its consequences for the mass splitting of the delta states. We show that bound nucleons acquire an intrinsic quadrupole moment due to the deformation effects. These effects are maximal for densities of nuclei about ρ(R)\sim 0.3...0.35 ρ(0). We also point out that scale changes of the electromagnetic radii can not simply be described by an overall swelling factor.

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arXiv1996

Small Disks and Semiclassical Resonances

We study the effect on quantum spectra of the existence of small circular disks in a billiard system. In the limit where the disk radii vanish there is no effect, however this limit is approached very slowly so that even very small radii have comparatively large effects. We include diffractive orbits which scatter off the small disks in the periodic orbit expansion. This situation is formally similar to edge diffraction except that the disk radii introduce a length scale in the problem such that for wave lengths smaller than the order of the disk radius we recover the usual semi-classical approximation; however, for wave lengths larger than the order of the disk radius there is a qualitatively different behaviour. We test the theory by successfully estimating the positions of scattering resonances in geometries consisting of three and four small disks.

chao-dyn↗
arXiv1996

In-Medium Pion Properties from Chiral Perturbation Theory

Two-point functions related to the pion weak decay constant $f_π$ are calculated from the generating functional of chiral perturbation theory in the mean field approximation and the heavy-baryon limit. The aim is to demonstrate that Lorentz invariance is violated in the presence of background matter. This fact manifests itself in the splitting of both $f_π$ and the pion mass into uncorrelated time- and space-like parts. We emphasize the different in-medium renormalizations of the correlation functions, show the inequivalence between the in-medium values of $f_π$ deduced from Walecka-type models, on the one hand, and QCD sum rules, on the other hand, and elaborate on the importance for some nuclear physics observables.

hep-ph↗
arXiv1996

Studies of chaotic Dynamics in a Three-Dimensional Superconducting Microwave Billiard

We present first measurements on a superconducting three-dimensional, partly chaotic microwave billiard shaped like a small deformed cup. We analyze the statistical properties of the measured spectrum in terms of several methods originally derived for quantum systems like eigenvalue statistics and periodic orbits and obtain according to a model of Berry and Robnik a mixing parameter of about 25%. In numerical simulations of the classical motion in the cup the degree of chaoticity has been estimated. This leads to an invariant chaotic Liouville measure of about 45%. The difference between this figure and the mixing parameter is due to the limited accuracy of the statistical analysis, caused by both, the fairly small number of 286 resonances and the rather poor desymmetrization of the microwave cavity. Concerning the periodic orbits of the classical system we present a comparison with the length spectrum of the resonator and introduce a new bouncing ball formula for electromagnetic billiards.

chao-dyn↗
arXiv1994

Higher Resonance Contributions to the Adler-Weisberger Sum Rule in the Large N_c Limit

We determine the $N_c$--dependence of the resonance contributions to the Adler--Weisberger sum rule for the inverse square $1/g_A^2$ of the axial charge coupling constant and show that in the large $N_c$ limit the contributions of the Roper-like excitations scale as $O(1/N_c)$. Consistency with the $1/N_c^2$ scaling of the $1/g_A^2$ term in the sum rule requires these contributions to cancel against each other.

hep-ph↗
arXiv1993

Dibaryons as axially symmetric skyrmions

Dibaryons configurations are studied in the framework of the bound state soliton model. A generalized axially symmetric ansatz is used to determine the soliton background. We show that once the constraints imposed by the symmetries of the lowest energy torus configuration are satisfied all spurious states are removed from the dibaryon spectrum. In particular, we show that the lowest allowed state in the $S=-2$ channel carries the quantum numbers of the H particle. We find that, within our approximations, this particle is slightly bound in the model. We discuss, however, that vacuum effects neglected in the present calculation are very likely to unbind the H.

hep-ph↗
arXiv1992

Nuclear Matter Aspects of Skyrmions

As an alternative approach to the infinite-array description of dense matter in the Skyrme model, we report about the properties of a single skyrmion on a compact 3-sphere of finite radius. The density of this matter can be increased by decreasing the hypersphere radius. As in the array calculations one encounters a transition to a distinct high density phase characterized by a delocalization in energy and baryon charge and by increased symmetries. We will argue that the high density phase has to be interpreted as chirally restored one. The arguments are based on the formation of complete chiral multiplets and the vanishing of the pionic massless Goldstone modes in the high-density fluctuation spectrum. We show that the restoration of chiral symmetry is common to any chirally invariant extension of the usual Skyrme model - whether via higher-order contact terms or via the introduction of stabilizing vector mesons which act over a finite range. (Talk at the workshop on ``Baryons as Skyrme Solitons'', Siegen, Germany, Sept. 27-30, 1992)

hep-ph↗