Andreas Wirzba | Forschungszentrum Juelich (original) (raw)

Papers by Andreas Wirzba

We argue that the effective pion mass in nuclear matter obtained from chiral effective lagrangian... more We argue that the effective pion mass in nuclear matter obtained from chiral effective lagrangians is unique and does not depend on off-mass-shell extensions of the pion fields as e.g. the PCAC choice. The effective pion mass in isospin symmetric nuclear matter is predicted to increase slightly with increasing nuclear density, whereas the effective time-like pion decay constant and the

QCD superconductors in the color-flavor-locked (CFL) phase sustain light Goldstone modes (that wi... more QCD superconductors in the color-flavor-locked (CFL) phase sustain light Goldstone modes (that will be referred to as generalized pions) that can be described as pairs of particle and/or hole excitations around a gapped Fermi surface. In weak coupling and to leading logarithm accuracy, their form factor, mass and decay constant can be evaluated exactly. These modes are found to satisfy an axial-Ward-identity, constraining the mass of the Goldstone modes in the CFL phase. #1 q = (q 0 , q) andψ T is the transposed and conjugated field with C ≡ iγ 2 γ 0 . #2 We are adopting the standard phase convention between ψψ and S(q). #3 This approximation assumes M † M ≈ 1 cf in the mass-shell condition. #4 Note that γ 0 Λ ± (q) = Λ ∓ (q)γ 0 , γ 5 Λ ± (q) = Λ ± (q)γ 5 and α ·q Λ ± (q) = ±|q| Λ ± (q).

Page 1. Classical and Quantum Chaos (Lecture Notes for 2002 Los Alamos Summer School) Daniel Adam... more Page 1. Classical and Quantum Chaos (Lecture Notes for 2002 Los Alamos Summer School) Daniel Adam Steck Theoretical Division (T-8), MS B285 Los Alamos National Laboratory, Los Alamos, NM 87545 16 July 2002 1 Classical Chaos ...

Physics Letters B, 2012

An exclusive measurement of the decay η → π + π − γ has been performed at the WASA facility at CO... more An exclusive measurement of the decay η → π + π − γ has been performed at the WASA facility at COSY. The η mesons were produced in the fusion reaction pd→ 3 He X at a proton beam momentum of 1.7 GeV/c. Efficiency corrected differential distributions have been extracted based on 13340±140 events after background subtraction. The measured pion angular distribution is consistent with a relative p-wave of the two-pion system, whereas the measured photon energy spectrum was found at variance with the simplest gauge invariant matrix element of η → π + π − γ. A parameterization of the data can be achieved by the additional inclusion of the empirical pion vector form factor multiplied by a first-order polynomial in the squared invariant mass of the π + π − system.

Physics Letters B, 1984

We show that under chiral SU(2) × SU(2) boundary conditions of the hedgehog type, the Dirac vacuu... more We show that under chiral SU(2) × SU(2) boundary conditions of the hedgehog type, the Dirac vacuum in chiral bag models picks up non-trivial chiral terms. The chiral Casimir energy and the net flux of axial current flowing through the surface of the bag are worked out in terms of the confined Dirac propagator and their subtle relationship is discussed. It is shown that both observables are plagued with ultra-violet divergences. The chiral Casimir energy diverges logarithmically, and the flux of axial current has both a logarithmic and a linear divergence. Using asymptotic expansions closed form expressions are derived and their relevance to chiral bag phenomenology is discussed.

Physics Letters B, 2007

In recent years a two-scale expansion was established to study reactions of the type N N → N N π ... more In recent years a two-scale expansion was established to study reactions of the type N N → N N π 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.

Physical Review Letters, 2011

We report on a high-statistics measurement of the basic double pionic fusion reaction pn → dπ 0 π... more We report on a high-statistics measurement of the basic double pionic fusion reaction pn → dπ 0 π 0 over the energy region of the so-called ABC effect, a pronounced low-mass enhancement in the ππ-invariant mass spectrum. The measurements were performed with the WASA detector setup at COSY. The data reveal the ABC effect to be associated with a Lorentzian shaped energy dependence in the integral cross section. The observables are consistent with a resonance with I(J P ) = 0(3 + ) in both pn and ∆∆ systems. Necessary further tests of the resonance interpretation are discussed.

Physical Review D, 2000

We discuss the Overhauser effect (particle-hole pairing) versus the BCS effect (particleparticle ... more We discuss the Overhauser effect (particle-hole pairing) versus the BCS effect (particleparticle or hole-hole pairing) in QCD at large quark density. In weak coupling and to leading logarithm accuracy, the pairing energies can be estimated exactly. For a small number of colors, the BCS effect overtakes the Overhauser effect, while for a large number of colors the opposite takes place, in agreement with a recent renormalization group argument. In strong coupling with large pairing energies, the Overhauser effect may be dominant for any number of colors, suggesting that QCD may crystallize into an insulator at a few times nuclear matter density, a situation reminiscent of dense Skyrmions. The Overhauser effect is dominant in QCD in 1+1 dimensions, although susceptible to quantum effects. It is sensitive to temperature in all dimensions.

Nuclear Physics A, 1995

Chiral effective lagrangians may differ in their prediction of meson-nucleon scattering amplitude... more Chiral effective lagrangians may differ in their prediction of meson-nucleon scattering amplitudes off-meson-mass-shell, but must yield identical S-matrix elements.

Europhysics Letters (EPL), 2005

PACS. 05.45.Mt -Quantum chaos; semiclassical methods. PACS. 46.40.Cd -Mechanical wave propagation... more PACS. 05.45.Mt -Quantum chaos; semiclassical methods. PACS. 46.40.Cd -Mechanical wave propagation (including diffraction, scattering, ...). PACS. 62.30.+d -Mechanical and elastic waves; vibrations.

The European Physical Journal A - Hadrons and Nuclei, 2003

We calculate electromagnetic form factors of the proton bound in specified orbits for several clo... more We calculate electromagnetic form factors of the proton bound in specified orbits for several closed shell nuclei. The quark structure of the nucleon and the shell structure of the finite nuclei are given by the QMC model. We find that orbital electromagnetic form factors of the bound nucleon deviate significantly from those of the free nucleon.

Annals of Physics, 1997

Most discussions of chaotic scattering systems are devoted to two-dimensional systems. It is of c... more Most discussions of chaotic scattering systems are devoted to two-dimensional systems. It is of considerable interest to extend these studies to the, in general, more realistic case of three dimensions. In this context, it is conceptually important to investigate the quality of semiclassical methods as a function of the dimensionality. As a model system, we choose various three dimensional generalizations of the famous three disk problem which played a central role in the study of chaotic scattering in two dimensions. We present a quantum-mechanical treatment of the hyperbolic scattering of a point particle off a finite number of non-overlapping and non-touching hard spheres in three dimensions. We derive expressions for the scattering matrix S and its determinant. The determinant of S decomposes into two parts, the first one contains the product of the determinants of the individual one-sphere S-matrices and the second one is given by a ratio involving the determinants of a characteristic KKR-type matrix and its conjugate. We justify our approach by showing that all formal manipulations in these derivations are correct and that all the determinants involved which are of infinite dimension exist. Moreover, for all complex wave numbers, we conjecture a direct link between the quantum-mechanical and semiclassical descriptions: The semiclassical limit of the cumulant expansion of the KKR-type matrix is given by the Gutzwiller-Voros zeta function plus diffractional corrections in the curvature expansion. This connection is direct since it is not based on any kind of subtraction scheme involving bounded reference systems. We present numerically computed resonances and compare them with the corresponding data for the similar twodimensional N -disk systems and with semiclassical calculations.