118 research outputs found
Chaotic scalar fields as models for dark energy
We consider stochastically quantized self-interacting scalar fields as
suitable models to generate dark energy in the universe. Second quantization
effects lead to new and unexpected phenomena is the self interaction strength
is strong. The stochastically quantized dynamics can degenerate to a chaotic
dynamics conjugated to a Bernoulli shift in fictitious time, and the right
amount of vacuum energy density can be generated without fine tuning. It is
numerically observed that the scalar field dynamics distinguishes fundamental
parameters such as the electroweak and strong coupling constants as
corresponding to local minima in the dark energy landscape. Chaotic fields can
offer possible solutions to the cosmological coincidence problem, as well as to
the problem of uniqueness of vacua.Comment: 30 pages, 3 figures. Replaced by final version accepted by Phys. Rev.
Double Photoproduction off the Proton at Threshold
The reaction has been measured using the TAPS
BaF calorimeter at the tagged photon facility of the Mainz Microtron
accelerator. Chiral perturbation theory (ChPT) predicts that close to threshold
this channel is significantly enhanced compared to double pion final states
with charged pions. In contrast to other reaction channels, the lower order
tree terms are strongly suppressed in 2 photoproduction. The consequence
is the dominance of pion loops in the 2 channel close to threshold - a
result that opens new prospects for the test of ChPT and in particular its
inherent loop terms. The present measurement is the first which is sensitive
enough for a conclusive comparison with the ChPT calculation and is in
agreement with its prediction. The data also show good agreement with a
calculation in the unitary chiral approach.Comment: Submitted to PL
Three-nucleon mechanisms in photoreactions
The C reaction has been measured for
E=150-800 MeV in the first study of this reaction in a target
heavier than He. The experimental data are compared to a microscopic many
body calculation. The model, which predicts that the largest contribution to
the reaction arises from final state interactions following an initial pion
production process, overestimates the measured cross sections and there are
strong indications that the overestimate arises in this two-step process. The
selection of suitable kinematic conditions strongly suppresses this two-step
contribution leaving cross sections in which up to half the yield is predicted
to arise from the absorption of the photon on three interacting nucleons and
which agree with the model. The results indicate measurements on
nuclei may be a valuable tool for obtaining information on the nuclear
three-body interaction.Comment: 5 pages, 3 figure
Deuteron photodisintegration cross-sections 50-155 MeV
The study of the photodisintegration of the deuteron should be a rewarding area over the next few years both as a result of the increased sophistication of recent theoretical treatments and also because of the potential, which has only just begun to be realised, of much more accurate and reliable photoreaction data
Convergence of the critical attractor of dissipative maps: Log-periodic oscillations, fractality and nonextensivity
For a family of logistic-like maps, we investigate the rate of convergence to
the critical attractor when an ensemble of initial conditions is uniformly
spread over the entire phase space. We found that the phase space volume
occupied by the ensemble W(t) depicts a power-law decay with log-periodic
oscillations reflecting the multifractal character of the critical attractor.
We explore the parametric dependence of the power-law exponent and the
amplitude of the log-periodic oscillations with the attractor's fractal
dimension governed by the inflexion of the map near its extremal point.
Further, we investigate the temporal evolution of W(t) for the circle map whose
critical attractor is dense. In this case, we found W(t) to exhibit a rich
pattern with a slow logarithmic decay of the lower bounds. These results are
discussed in the context of nonextensive Tsallis entropies.Comment: 8 pages and 8 fig
(gamma,np) reactions in <sup>12</sup>C , <sup>6</sup>Li and <sup>3,4</sup>He
The emission of neutron-proton pairs is the most probable outcome of photon absorbtion in the energy region above the giant resonance at least up to the pion threshold, but little detailed information on the process has been obtained due to experimental difficulties. Two nucleon emission following photon absorbtion by a correlated pair is favoured
compared to direct knockout of a single nucleon, which is suppressed by the large momentum mismatch between the ingoing photon and a single outgoing fast nucleon. Studies of the (gamma,np) process seek firstly to obtain a quantitative
understanding of the photon interaction mechanism, and through this to open the door to investigations of nucleon
correlations in nuclei [1], information about which is long sought but not readily obtainable
Coherent pi0-photoproduction from atomic nuclei
Differential and total cross sections for coherent pi0-photoproduction from
12C, 40Ca, 93Nb and Pb targets have been measured throughout the region of the
Delta(1232)-resonance. The experiments were performed with the TAPS-detector at
the Mainz accelerator MAMI. The characteristic proportionality of the cross
section to the square of the atomic mass number and to the nuclear mass form
factor is clearly demonstrated. The data allow for the first time detailed
tests of model predictions for this reaction. The comparison of the data to
model predictions shows that the Delta-nucleus interaction saturates: it is
described for heavy nuclei with the same potential parameters as for 4He.Comment: 10 pages, 6 figure
Dependence of the C(,pd) reaction on photon linear polarisation
The sensitivity of the C reaction to photon linear
polarisation has been determined at MAMI, giving the first measurement of the
reaction for a nucleus heavier than He. Photon asymmetries and cross
sections were measured for =170 to 350 MeV. For below
the resonance, reactions leaving the residual Be near its ground
state show a positive asymmetry of up to 0.3, similar to that observed for
He suggesting a similar reaction mechanism for the two nuclei.Comment: 4 pages, 2 figure
Photoproduction of pions and properties of baryon resonances from a Bonn-Gatchina partial wave analysis
Masses, widths and photocouplings of baryon resonances are determined in a
coupled-channel partial wave analysis of a large variety of data. The
Bonn-Gatchina partial wave formalism is extended to include a decomposition of
t- and u-exchange amplitudes into individual partial waves. The multipole
transition amplitudes for and are
given and compared to results from other analyses.Comment: 18 pages, 14 figure
P-wave excited baryons from pion- and photo-induced hyperon production
We report evidence for , , ,
, , and , and find
indications that might have a companion state at 1970\,MeV. The
controversial is not seen. The evidence is derived from a
study of data on pion- and photo-induced hyperon production, but other data are
included as well. Most of the resonances reported here were found in the
Karlsruhe-Helsinki (KH84) and the Carnegie-Mellon (CM) analyses but were
challenged recently by the Data Analysis Center at GWU. Our analysis is
constrained by the energy independent scattering amplitudes from either
KH84 or GWU. The two amplitudes from KH84 or GWU, respectively, lead to
slightly different branching ratios of contributing resonances but the
debated resonances are required in both series of fits.Comment: 22 pages, 28 figures. Some additional sets of data are adde
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