948 research outputs found
Field dynamics and kink-antikink production in rapidly expanding systems
Field dynamics in a rapidly expanding system is investigated by transforming
from space-time to the rapidity - proper-time frame. The proper-time dependence
of different contributions to the total energy is established. For systems
characterized by a finite momentum cut-off, a freeze-out time can be defined
after which the field propagation in rapidity space ends and the system decays
into decoupled solitons, antisolitons and local vacuum fluctuations. Numerical
simulations of field evolutions on a lattice for the (1+1)-dimensional
model illustrate the general results and show that the freeze-out time and
average multiplicities of kinks (plus antikinks) produced in this 'phase
transition' can be obtained from simple averages over the initial ensemble of
field configurations. An extension to explicitly include additional dissipation
is discussed. The validity of an adiabatic approximation for the case of an
overdamped system is investigated. The (3+1)-dimensional generalization may
serve as model for baryon-antibaryon production after heavy-ion collisions.Comment: 18 pages, 7 figures. Two references added. New subsection III.E
added. Final version accepted for publication in PR
Characterization of a 450-km Baseline GPS Carrier-Phase Link using an Optical Fiber Link
A GPS carrier-phase frequency transfer link along a baseline of 450 km has
been established and is characterized by comparing it to a phase-stabilized
optical fiber link of 920 km length, established between the two endpoints, the
Max-Planck-Institut f\"ur Quantenoptik in Garching and the
Physikalisch-Technische Bundesanstalt in Braunschweig. The characterization is
accomplished by comparing two active hydrogen masers operated at both
institutes. The masers serve as local oscillators and cancel out when the
double differences are calculated, such that they do not constitute a
limitation for the GPS link characterization. We achieve a frequency
instability of 3 x 10^(-13) in 30 s and 5 x 10^(-16) for long averaging times.
Frequency comparison results obtained via both links show no deviation larger
than the statistical uncertainty of 6 x 10^(-16). These results can be
interpreted as a successful cross-check of the measurement uncertainty of a
truly remote end fiber link.Comment: 14 pages, 6 figure
FĂRSTER TRANSFER CALCULATIONS BASED ON CRYSTAL STRUCTURE DATA FROM Agmenellum quadruplicatum C-PHYCOCYANIN
Excitation energy transfer in C-phycocyanin is modeled using the Forster inductive resonance mechanism. Detailed calculations are carried out using coordinates and orientations of the chromophores derived from X-ray crystallographic studies of C-phycocyanin from two different species (Schirmer et al, J. Mol. Biol. 184, 257â277 (1985) and ibid., 188, 651-677 (1986)). Spectral overlap integrals are estimated from absorption and fluorescence spectra of C-phycocyanin of Mastigocladus laminosus and its separated subunits. Calculations are carried out for the ÎČ-subunit, αÎČ-monomer, (αÎČ)3-trimer and (αÎČ)0-hexamer species with the following chromophore assignments: ÎČ155 = 'sâ(sensitizer), ÎČ84 =âf (fluorescer) and α84 =âmâ(intermediate):]:. The calculations show that excitation transfer relaxation occurs to 3=98% within 200 ps in nearly every case; however, the rates increase as much as 10-fold for the higher aggregates. Comparison with experimental data on fluorescence decay and depolarization kinetics from the literature shows qualitative agreement with these calculations. We conclude that Forster transfer is sufficient to account for all of the observed fluorescence properties of C-phycocyanin in aggregation states up to the hexamer and in the absence of linker polypeptides
Extension of random-phase approximation preserving energy weighted sum rules: an application to a 3-level Lipkin model
A limitation common to all extensions of random-phase approximation including
only particle-hole configurations is that they violate to some extent the
energy weighted sum rules. Considering one such extension, the improved RPA
(IRPA), already used to study the electronic properties of metallic clusters,
we show how it can be generalized in order to eliminate this drawback. This is
achieved by enlarging the configuration space, including also elementary
excitations corresponding to the annihilation of a particle (hole) and the
creation of another particle (hole) on the correlated ground state. The
approach is tested within a solvable 3-level model.Comment: 2 figure
The Chiral Phase Transition in Dissipative Dynamics
Numerical simulations of the chiral phase transition in the (3+1)dimensional
O(4)-model are presented. The evolutions of the chiral field follow purely
dissipative dynamics, starting from random chirally symmetric initial
configurations down to the true vacuum with spontaneously broken symmetry. The
model stabilizes topological textures which are formed together with domains of
disoriented chiral condensate (DCC) during the roll-down phase. The classically
evolving field acts as source for the emission of pions and mesons.
The exponents of power laws for the growth of angular correlations and for
emission rates are extracted. Fluctuations in the abundance ratios for neutral
and charged pions are compared with those for uncorrelated sources as potential
signature for the chiral phase transition after heavy-ion collisions. It is
found that the presence of stabilizing textures (baryons and antibaryons)
prevents sufficiently rapid growth of DCC-domain size, so observability of
anomalous tails in the abundance ratios is unlikely. However, the transient
formation of growing DCC domains causes sizable broadening of the distributions
as compared to the statistical widths of generic sources.Comment: 28 pages, 8 figure
DCC Dynamics in (2+1)D-O(3) model
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.Comment: 27 pages, 17 figures; several minor insertions in the text; two
references adde
Photoionization Broadening of the 1S-2S Transition in a Beam of Atomic Hydrogen
We consider the excitation dynamics of the two-photon \sts transition in a
beam of atomic hydrogen by 243 nm laser radiation. Specifically, we study the
impact of ionization damping on the transition line shape, caused by the
possibility of ionization of the 2S level by the same laser field. Using a
Monte-Carlo simulation, we calculate the line shape of the \sts transition for
the experimental geometry used in the two latest absolute frequency
measurements (M. Niering {\it et al.}, PRL 84, 5496 (2000) and M. Fischer {\it
et al.}, PRL 92, 230802 (2004)). The calculated line shift and line width are
in excellent agreement with the experimentally observed values. From this
comparison we can verify the values of the dynamic Stark shift coefficient for
the \sts transition for the first time on a level of 15%. We show that the
ionization modifies the velocity distribution of the metastable atoms, the line
shape of the \sts transition, and has an influence on the derivation of its
absolute frequency.Comment: 10 pages, 5 figure
Quantum Corrections to Baryon Properties in Chiral Soliton Models
We present a procedure to calculate 1-loop graphs in the soliton sector of
chiral Lagrangians and use it to calculate quantum corrections to certain
baryon observables in Skyrme-type models. Results generally show an improvement
over the values obtained in tree approximation except for the case of the axial
coupling g_A.Comment: Chapters on magnetic polarizabilities and e.m. properties of Delta
isobar added, 90 pages, Latex, 9 Postscript figure
Electro-Magnetic Nucleon Form Factors and their Spectral Functions in Soliton Models
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 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 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.Comment: 24 pages, (RevTeX), 5 PS-figures; Data points in fig.2 and
corresponding references added. Final version, to be published in Z.Physik
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