684 research outputs found
Off-shell effects on particle production
We investigate the observable effects of off-shell propagation of nucleons in
heavy-ion collisions at SIS energies. Within a semi-classical BUU transport
model we find a strong enhancement of subthreshold particle production when
off-shell nucleons are propagated.Comment: 11 pages, 3 figure
Transport Theoretical Approach to the Nucleon Spectral Function in Nuclear Matter
The nucleon spectral function in infinite nuclear matter is calculated in a
quantum transport theoretical approach. Exploiting the known relation between
collision rates and correlation functions the spectral function is derived
self-consistently. By re-inserting the spectral functions into the collision
integrals the description of hard processes from the high-momentum components
of wave functions and interactions is improved iteratively until convergence is
achieved. The momentum and energy distributions and the nuclear matter
occupation probabilities are in very good agreement with the results obtained
from many-body theory.Comment: minor changes in the text, additional curves in fig.
Hadron formation in high energy photonuclear reactions
We present a new method to account for coherence length effects in a
semi-classical transport model. This allows us to describe photo- and
electroproduction at large nuclei (A>12) and high energies using a realistic
coupled channel description of the final state interactions that goes beyond
simple Glauber theory. We show that the purely absorptive treatment of the
final state interactions can lead to wrong estimates of color transparency and
formation time effects in particle production. As an example, we discuss
exclusive rho^0 photoproduction on Pb at a photon energy of 7 GeV as well as
K^+ production in the photon energy range 1-7 GeV.Comment: 14 pages, 6 figures, version published in Phys. Rev.
e^+e^- Pair Production from A Reactions
We present a calculation of e^+e^- production in reactions at MAMI
and TJNAF energies within a semi-classical BUU transport model. Dilepton
invariant mass spectra for C, Ca and Pb are calculated
at 0.8, 1.5 and 2.2 GeV. We focus on observable effects of medium modifications
of the and mesons. The in-medium widths of these mesons are
taken into account in a dynamical, consistent way. We discuss the transport
theoretical treatment of broad resonances.Comment: 42 pages including 16 figure
Photoproduction of mesons in nuclei at GeV energies
In a transport model that combines initial state interactions of the photon
with final state interactions of the produced particles we present a
calculation of inclusive photoproduction of mesons in nuclei in the energy
range from 1 to 7 GeV. We give predictions for the photoproduction cross
sections of pions, etas, kaons, antikaons, and invariant mass
spectra in ^{12}C and ^{208}Pb. The effects of nuclear shadowing and final
state interaction of the produced particles are discussed in detail.Comment: Text added in summary in general reliability of the method,
references updated. Phys. Rev. C (2000) in pres
Equilibration within a semiclassical off-shell transport approach
Equilibration times for nuclear matter configurations -- modelling
intermediate and high energy nucleus-nucleus collisions -- are evaluated within
the semiclassical off-shell transport approach developed recently. The
transport equations are solved for a finite box in coordinate space employing
periodic boundary conditions. The off-shell transport model is shown to give
proper off-shell equilibrium distributions in the limit for the
nucleon and -resonance spectral functions. We find that equilibration
times within the off-shell approach are only slightly enhanced as compared to
the on-shell limit for the momentum configurations considered.Comment: 19 pages, LaTeX, including 4 postscript figures, submitted to Nucl.
Phys.
Photoproduction of pions and etas in nuclei
We calculate the cross sections for inclusive one-pion, two-pion and eta
photoproduction in nuclei in the photon energy range from 300 MeV to 900 MeV
within the framework of a semi-classical BUU transport model. Our results are
compared with existing experimental data and discussed with respect to a
calculation of the total photoabsorption cross section.Comment: 30 pages LaTeX including 13 postscript figure
Electron Power-Law Spectra in Solar and Space Plasmas
Particles are accelerated to very high, non-thermal energies in solar and
space plasma environments. While energy spectra of accelerated electrons often
exhibit a power law, it remains unclear how electrons are accelerated to high
energies and what processes determine the power-law index . Here, we
review previous observations of the power-law index in a variety of
different plasma environments with a particular focus on sub-relativistic
electrons. It appears that in regions more closely related to magnetic
reconnection (such as the `above-the-looptop' solar hard X-ray source and the
plasma sheet in Earth's magnetotail), the spectra are typically soft ( 4). This is in contrast to the typically hard spectra ( 4) that are observed in coincidence with shocks. The difference
implies that shocks are more efficient in producing a larger non-thermal
fraction of electron energies when compared to magnetic reconnection. A caveat
is that during active times in Earth's magnetotail, values seem
spatially uniform in the plasma sheet, while power-law distributions still
exist even in quiet times. The role of magnetotail reconnection in the electron
power-law formation could therefore be confounded with these background
conditions. Because different regions have been studied with different
instrumentations and methodologies, we point out a need for more systematic and
coordinated studies of power-law distributions for a better understanding of
possible scaling laws in particle acceleration as well as their universality.Comment: 67 pages, 15 figures; submitted to Space Science Reviews; comments
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Photoproduction of pi0-mesons from nuclei
Photoproduction of neutral pions from nuclei (carbon, calcium, niobium, lead)
has been studied for incident photon energies from 200 MeV to 800 MeV with the
TAPS detector using the Glasgow photon tagging spectrometer at the Mainz MAMI
accelerator. Data were obtained for the inclusive photoproduction of neutral
pions and the partial channels of quasifree single pi0, double pi0, and
pi0pi+/- photoproduction. They have been analyzed in terms of the in-medium
behavior of nucleon resonances and the pion - nucleus interaction. They are
compared to earlier measurements from the deuteron and to the predictions of a
Boltzmann-Uehling-Uhlenbeck (BUU) transport model for photon induced pion
production from nuclei.Comment: 15 pages, 22 figures, accepted for publication in EPJ
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