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Search Results: 1 - 10 of 223920 matches for " R Holzwarth "
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Skyrme-Model $πNN$ Form Factor and Nucleon-Nucleon Interaction
G. Holzwarth,R. Machleidt
Physics , 1996, DOI: 10.1103/PhysRevC.55.1088
Abstract: We apply the strong $\pi NN$ form factor, which emerges from the Skyrme model, in the two-nucleon system using a one-boson-exchange (OBE) model for the nucleon-nucleon (NN) interaction. Deuteron properties and phase parameters of NN scattering are reproduced well. In contrast to the form factor of monopole shape that is traditionally used in OBE models, the Skyrme form factor leaves low momentum transfers essentially unaffected while it suppresses the high-momentum region strongly. It turns out that this behavior is very appropriate for models of the NN interaction and makes possible to use a soft pion form factor in the NN system. As a consequence, the $\pi N$ and the $NN$ systems can be described using the same soft $\pi NN$ form factor, which is impossible with the monopole.
Electro-Magnetic Nucleon Form Factors and their Spectral Functions in Soliton Models
Gottfried Holzwarth
Physics , 1996, DOI: 10.1007/s002180050187
Abstract: It is demonstrated that in simple soliton models essential features of the electro-magnetic nucleon form factors observed over three orders of magnitude in momentum transfer $t$ are naturally reproduced. The analysis shows that three basic ingredients are required: an extended object, partial coupling to vector mesons, and relativistic recoil corrections. We use for the extended object the standard skyrmion, one vector meson propagator for both isospin channels, and the relativistic boost to the Breit frame. Continuation to timelike $t$ leads to quite stable results for the spectral functions in the regime from the 2- or 3-pion threshold to about two rho masses. Especially the onset of the continuous part of the spectral functions at threshold can be reliably determined and there are strong analogies to the results imposed on dispersion theoretic approaches by the unitarity constraint.
Formation of extended topological defects during symmetry breaking phase transitions in O(2) and O(3) models
G. Holzwarth
Physics , 1999, DOI: 10.1103/PhysRevD.59.105022
Abstract: The density of extended topological defects created during symmetry-breaking phase transitions depends on the ratio between the correlation length in the symmetric phase near $T_c$ and the winding length of the defects as determined by the momentaneous effective action after a typical relaxation time. Conservation of winding number in numerical simulations requires a suitable embedding of the field variables and the appropriate geometrical implementation of the winding density on the discrete lattice. We define a modified Kibble limit for the square lattice and obtain defect densities as functions of winding lengths in O(2) and O(3) models. The latter allows to observe formation of disoriented aligned domains within the easy plane. Their extent is severely limited by the momentaneous defect density during the course of the quench.
Baryon Number Conservation and the Formation of Disoriented Chiral Condensates
G. Holzwarth
Physics , 1996,
Abstract: Conservation of baryon number as topological charge in effective chiral field theories imposes a local constraint on the time evolution of field configurations in the commonly used O(4) model. Possible consequences for the formation of chiral condensates after a quench from random initial configurations are discussed. It is argued that efficient dissipative terms are necessary but not sufficient to unwind the randomly curled-up initial configuration, before collective motion towards the condensate can grow. The existence of soliton stabilizing mechanisms will further prolong or prevent this process.
Baryons as Solitons in Effective Chiral Field Theories
Gottfried Holzwarth
Physics , 1995, DOI: 10.1016/0146-6410(96)00021-X
Abstract: This lecture comprises some recent developments concerning the description of baryons as topological solitons in effective chiral meson theories. In the first part one-loop corrections to the classical tree approximation are discussed. This involves renormalization of low-energy coupling constants and evaluation of the finite next-to-leading-order terms in the $1/N_c$ expansion. Recent results for various nucleon observables are presented. In the second part electromagnetic nucleon form factors (FFs) with relativistic corrections are evaluated in a chiral soliton model including vector mesons. The magnetic FF $G_M^p$ is shown to agree well with new SLAC data for spacelike $Q^2$ up to 30(GeV/c)$^2$ if superconvergence is enforced. The electric FF $G_E^p$ is dominated by a zero in the few (GeV/c)$^2$ region. The third part describes how to extract the strong $\pi NN$ form factor from chiral soliton models, taking due care of the local metric created by the presence of the soliton. When used in a one-boson-exchange model for the nucleon-nucleon (NN) interaction, deuteron properties and phase parameters of NN scattering are reproduced as well as in conventional NN models that apply a hard monopole form factor at the $\pi NN$ vertex.
The electro-magnetic Form Factors of the Proton in Chiral Soliton Models
Gottfried Holzwarth
Physics , 1995,
Abstract: The electro-magnetic form factors of the proton are calculated in a chiral soliton model with relativistic corrections. The magnetic form factor $G_M$ is shown to agree well with the new SLAC data for spacelike $Q^2$ up to 30 (GeV/c)$^2$ if superconvergence is imposed. The direct continuation through a Laurent series to the timelike region above the physical threshold is in fair agreement with the presently available set of data. The electric form factor $G_E$ is dominated by a zero in the few (GeV/c)$^2$ region which appears to be in conflict with the SLAC data.
Electromagnetic Form Factors of the Nucleon in the Chiral Soliton Model
G. Holzwarth
Physics , 2002,
Abstract: Several years ago it was pointed out that the chiral soliton model allows naturally for satisfactory agreement with the experimentally well-determined proton magnetic form factor $G_M^p$. The corresponding result for the proton electric form factor at that time was in serious disagreement with the data because the calculated $G_E^p$ showed as a rather stable feature a zero for $q^2$ near 10 (GeV/c)$^2$ which was hard to avoid for reasonable choices of parameters, while the data at that time showed no indication for such a behaviour. Meanwhile, new data have confirmed those $G_E^p$ predictions in a remarkable way, so it appears worthwhile to have another look at that model, especially concerning its flexibility with repect to the electric neutron formfactor $G_E^n$ while trying to maintain the satisfactory results for the proton form factors.
Chiral Phase Transition and Baryon Number Conservation
G. Holzwarth
Physics , 1997, DOI: 10.1103/PhysRevD.57.3749
Abstract: In the standard $R^4$ embedding of the chiral O(4) model in 3+1 dimensions the winding number is not conserved near the chiral phase transition and thus no longer can be identified with baryon number. In order to reestablish conserved baryon number in effective low-energy models near and above the critical temperature $T_c$ it is argued that insisting in O(N) models on the angular nature of the chiral fields with fixed boundary conditions restores conservation of winding number. For N=2 in 1+1 dimensions it is illustrated that as a consequence of the angular boundary conditions nontrivial solutions exist which would be unstable in $R^2$; moving trajectories avoid crossing the origin; and time evolution of random configurations after a quench leads to quasistable soliton-antisoliton ensembles with net winding number fixed.
DCC Dynamics in (2+1)D-O(3) model
G. Holzwarth
Physics , 2001, DOI: 10.1103/PhysRevD.65.054013
Abstract: The dynamics of symmetry-breaking after a quench is numerically simulated on a lattice for the (2+1)-dimensional O(3) model. In addition to the standard sigma-model with temperature-dependent Phi^4-potential the energy functional includes a four-derivative current-current coupling to stabilize the size of the emerging extended topological textures. The total winding number can be conserved by constraint. As a model for the chiral phase transition during the cooling phase after a hadronic collision this allows to investigate the interference of 'baryon-antibaryon' production with the developing disoriented aligned domains. The growth of angular correlations, condensate, average orientation is studied in dependence of texture size, quench rate, symmetry breaking. The classical dissipative dynamics determines the rate of energy emitted from the relaxing source for each component of the 3-vector field which provides a possible signature for domains of Disoriented Chiral Condensate. We find that the 'pions' are emitted in two distinct pulses; for sufficiently small lattice size the second one carries the DCC signal, but it is strongly suppressed as compared to simultaneous 'sigma'-meson emission. We compare the resulting anomalies in the distributions of DCC pions with probabilities derived within the commonly used coherent state formalism.
Skyrmions and Bags in the 2D-O(3) model
G. Holzwarth
Physics , 1999, DOI: 10.1016/S0375-9474(99)00810-6
Abstract: Localized static solutions of the 2D-O(3) model are investigated in a representation with the 3-vector field $\vec Phi$ split into the unit vector $\hat Phi$ and the modulus $\Phi$. As in the nonlinear version of the model this allows for the definition of a topological winding number $B$, and for the separation of the complete configuration space into distinct $B$-sectors. For small values of the $\Phi^4$-coupling strength the stable energy minima in these sectors are characterized by bag formation in the modulus field which in the standard cartesian representation of the linear O(3) model would be unstable towards decay into the trivial B=0 vacuum. Stabilized by $B$-conservation they exhibit a surprising variety of very appealing features for multiply charged systems. With the total charge bound into one common deep bag opposite ways of distributing the topological charge density inside the bag can be realized: Pointlike structures which retain the individuality of single constituents (or doubly charged pairs), or a deconfined charge density spread uniformly throughout the interior of the bag. It is suggested that this extension supplies a crucial link to overcome the unsatisfactory existing mismatch between multiskyrmion configurations and nuclear structure.
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