2,033 research outputs found
Electroproduction of hadrons on nuclei at GeV energies
We investigate coherence length effects and hadron attenuation in lepton
scattering off nuclei in the kinematic regime of the HERMES experiment. The
elementary electron-nucleon interaction is described within the event generator
PYTHIA while a full coupled-channel treatment of the final state interactions
is included by means of a BUU transport model. The results of our calculations
are in good agreement with the experimentally measured transparency ratio of
incoherent rho0 electroproduction off Nitrogen and the multiplicity ratio R_M^h
for charged hadrons, pions, kaons, protons and anti-protons in deep inelastic
scattering off Nitrogen and Krypton targets.Comment: 17 pages, 4 figures. Proceedings of 2nd Int. Conf. on Nuclear and
Particle Physics with CEBAF at Jefferson Lab, Dubrovnik, 26-31 May 2003, to
be published in Fizika
Phonons, electronic charge response and electron-phonon interaction in the high-temperature superconductors
We investigate in the framework of linear response theory the complete phonon
dispersion, phonon induced electronic charge response, electron-phonon
interaction and dielectric and infrared properties of the high-temperature
superconductors (HTSC's). In particular the experimentally observed strong
renormalization of the in-plane oxygen bond-stretching modes (OBSM) which
appear upon doping in the HTSC's is discussed. It is shown that the
characteristic softening, indicating a strong EPI, is most likely a generic
effect of the CuO plane and is driven by a nonlocal coupling of the displaced
ions to the localized charge-fluctuations (CF's) at the Cu and O ions. The
different behaviour of the OBSM during the insulator-metal transition via the
underdoped phase is calculated and from a comparison of these modes conclusions
about the electronic state in the HTSC's are drawn. The underdoped state is
modelled in terms of a charge response which is insulator-like at the Cu and is
competing with a metallic charge response at the O-network in the CuO plane.
For the non-cuprate HTSC Ba-Bi-O also a strong renormalization of the OBSM is
predicted. C-axis polarized infrared and Raman-active modes of the HTSC's are
calculated in terms of CF's and anisotropic dipole-fluctuations and the problem
of a metallic character of the BiO planes is studied.Interlayer phonons and
their accompanying charge response are investigated. Depending on the
interlayer coupling calculations are performed from the static, adiabatic- to
the non-adiabatic regime.It is shown that phonon-plasmon mixing and a strong
long-ranged non-adiabatic EPI becomes evident within a certain region around
the c-axis. Both the OBSM and the non-adiabatic coupled c-axis phonon-plasmon
modes are found to be important for pairing in the HTSC's.Comment: 65 pages,20 figures. Extended version to appear in Physica Status
Solidi (b) 2004; figure 20 has been corrected; references have been adde
Nonlocal electron-phonon interaction as a source of dynamic charge stripes in the cuprates
We calculate for La2CuO4 the phonon-induced redistribution of the electronic
charge density in the insulating, the underdoped pseudogap and the more
conventional metallic state as obtained for optimal and overdoping,
respectively. The investigation is performed for the anomalous
high-frequency-oxygen-bond stretching modes (OBSM) which experimentally are
known to display a strong softening of the frequencies upon doping in the
cuprates. This most likely generic anomalous behaviour of the OBSM has been
shown to be due to a strong nonlocal electron-phonon interaction (EPI) mediated
by charge fluctuations on the ions. The modeling of the competing electronic
states of the cuprates is achieved in terms of consecutive orbital selective
incompressibility-compressibility transitions for the charge response. We
demonstrate that the softening of the OBSM in these states is due to nonlocally
induced dynamic charge inhomogenities in form of charge stripes along the CuO
bonds with different orbital character. Thus, a multi-orbital approach is
essential for the CuO plane. The dynamic charge inhomogeneities may in turn be
considered as precursors of static charge stripe order as recently observed in
LaBaCuO in a broad range of doping around x=1/8. The latter
may trigger a reconstruction of the Fermi surface into small pockets with
reduced doping. We argue that the incompressibility of the Cu3d orbital and
simultaneously the compressibility of the O2p orbital in the pseudogap state
seems to be required to nucleate dynamic stripes.Comment: 10 pages, 4 figures, to be published in "Advances in Condensed Matter
Physics
Modeling of the electronic state of the High-Temperature Superconductor LaCuO: Phonon dynamics and charge response
A modeling of the normal state of the p-doped high-temperature
superconductors (HTSC's) is presented. This is achieved starting from a more
conventional metallic phase for optimal- and overdoping and passing via the
underdoped to the insulating state by consecutive orbital selective
compressibility-incompressibility transitions in terms of sum rules for the
charge response. The modeling is substantiated by corresponding phonon
calculations. Extending investigations of the full dispersion and in particular
of the strongly doping dependent anomalous phonon modes in LaCuO, which so far
underpin our treatment of the density response of the electrons in the p-doped
HTSC's, gives additional support for the modeling of the electronic state,
compares well with recent experimental data and predicts the dispersion for the
overdoped regime. Moreover, phonon densities of states have been calculated and
compared for the insulating, underdoped, optimally doped and overdoped state of
LaCuO. From our modeling of the normal state a consistent picture of the
superconducting phase also can be extracted qualitatively pointing in the
underdoped regime to a phase ordering transition. On the other hand, the
modeling of the optimal and overdoped state is consistent with a quasi-particle
picture with a well defined Fermi surface. Thus, in the latter case a Fermi
surface instability with an evolution of pairs of well defined quasiparticles
is possible and can lead to a BCS-type ordering. So, it is tempting to
speculate that optimal in the HTSC's marks a crossover region between
these two forms of ordering.Comment: 18 RevTex pages, 10 figures, revised version, references updated,
accepted for publication in Physical Review
Space-Time Picture of Fragmentation in PYTHIA/JETSET for HERMES and RHIC
We examine the space-time evolution of (pre-)hadron production within the
Lund string fragmentation model. The complete four-dimensional information of
the string breaking vertices and the meeting points of the prehadron
constituents are extracted for each single event in Monte Carlo simulations
using the Jetset-part of Pythia. We discuss the implication on the deep
inelastic lepton scattering experiments at HERMES as well as on observables in
ultra-relativistic heavy ion collisions at RHIC, using Pythia also for modeling
the hard part of the interaction.Comment: 15 pages, 7 figures, final version as accepted by Phys Lett
In-Medium Properties of Hadrons - Observables II
In this review we discuss the observable consequences of in-medium changes of
hadronic properties in reactions with elementary probes, and in particular
photons, on nuclei. After an outline of the theoretical method used we focus on
a discussion of actual observables in photonuclear reactions; we discuss in
detail - and vector-meson production. We show that the
photoproduction data can be well described by final state interactions of the
pions produced whereas the semi-charged channel exhibits a major
discrepancy with theory. For production on nuclei in the TAPS/CB@ELSA
experiment we analyse the decay channel, and illustrate the
strength of the method by simulating experimental acceptance problems.
Completely free of final state interactions is dilepton production in the few
GeV range. We show that the sensitivity of this decay channel to changes of
hadronic properties in medium in photonuclear reactions on nuclei is as large
as in ultrarelativistic heavy ion collisions and make predictions for the
on-going G7 experiment at JLAB. Finally we discuss that hadron production in
nuclei at 10 -- 20 GeV photon energies can give important information on the
hadronization process, and in particular on the time-scales involved. We show
here detailed calculations for the low-energy (12 GeV) run at HERMES and
predictions for planned experiments at JLAB.Comment: Invited Talk by U. Mosel, Proceedings of the Int. School on Nuclear
Physics, 26th Course, "Lepton scattering and the structure of hadrons and
nuclei", Erice (Sicily), September 16th-24th, 2004, short piece of text adde
Nuclear shadowing and in-medium properties of the rho^0
We explain the early onset of shadowing in nuclear photoabsorption within a
multiple scattering approach and discuss its relation to in-medium
modifications of the rho^0.Comment: 4 pages, 2 figures, to appear in the proceedings of the 9th
International Conference on the Structure of Baryons, Newport News, VA, USA,
3-8 Mar 200
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