1,323 research outputs found
Time-Dependent Quasiparticle Current Density Functional Theory of X-Ray Nonlinear Response Functions
A real-space representation of the current response of many-electron systems
with possible applications to x-ray nonlinear spectroscopy and magnetic
susceptibilities is developed. Closed expressions for the linear, quadratic and
third-order response functions are derived by solving the adiabatic Time
Dependent Current Density Functional (TDCDFT) equations for the single-electron
density matrix in Liouville space.Comment: 11 page
Enhanced Perturbative Continuous Unitary Transformations
Unitary transformations are an essential tool for the theoretical
understanding of many systems by mapping them to simpler effective models. A
systematically controlled variant to perform such a mapping is a perturbative
continuous unitary transformation (pCUT) among others. So far, this approach
required an equidistant unperturbed spectrum. Here, we pursue two goals: First,
we extend its applicability to non-equidistant spectra with the particular
focus on an efficient derivation of the differential flow equations, which
define the enhanced perturbative continuous unitary transformation (epCUT).
Second, we show that the numerical integration of the flow equations yields a
robust scheme to extract data from the epCUT. The method is illustrated by the
perturbation of the harmonic oscillator with a quartic term and of the two-leg
spin ladders in the strong-rung-coupling limit for uniform and alternating rung
couplings. The latter case provides an example of perturbation around a
non-equidistant spectrum.Comment: 27 pages, 18 figures; separated methodological background from
introduction, added perturbed harmonic oscillator for additional
illustration, added explicit solution of deepCUT equation
New Measurements and Quantitative Analysis of Electron Backscattering in the Energy Range of Neutron Beta-Decay
We report on the first detailed measurements of electron backscattering from
plastic scintillator targets, extending our previous work on beryllium and
silicon targets. The scintillator experiment posed several additional
experimental challenges associated with charging of the scintillator target,
and those challenges are addressed in detail. In addition, we quantitatively
compare the energy and angular distributions of this data, and our previous
data, with electron transport simulations based on the Geant4 and Penelope
Monte Carlo simulation codes. The Penelope simulation is found globally to give
a superior description of the data. Such information is crucial for a broad
array of weak-interaction physics experiments, where electron backscattering
can give rise to the dominant detector-related systematic uncertainty.Comment: 7 pages, 3 figure
Strangeness Suppression in Proton-Proton Collisions
We analyse strangeness production in proton-proton (pp) collisions at SPS and
RHIC energies, using the recently advanced NeXus approach. After having
verified that the model reproduces well the existing data, we interpret the
results: strangeness is suppressed in proton-proton collisions at SPS energy as
compared to electron-positron (e+e-) annihilation due to the limited masses of
the strings produced in the reaction, whereas high energy pp and e+e-
collisions agree quantitatively . Thus strangeness suppression at SPS energies
is a consequence of the limited phase-space available in string fragmentation.Comment: 7 Figures, 4 Page
Communication: XUV transient absorption spectroscopy of iodomethane and iodobenzene photodissociation
Time-resolved extreme ultraviolet (XUV) transient absorption spectroscopy of
iodomethane and iodobenzene photodissociation at the iodine pre-N4,5 edge is
presented, using femtosecond UV pump pulses and XUV probe pulses from high
harmonic generation. For both molecules the molecular core-to-valence
absorption lines fade immediately, within the pump-probe time-resolution.
Absorption lines converging to the atomic iodine product emerge promptly in
CH3I but are time-delayed in C6H5I. We attribute this delay to the initial π →
σ* excitation in iodobenzene, which is distant from the iodine reporter atom.
We measure a continuous shift in energy of the emerging atomic absorption
lines in CH3I, attributed to relaxation of the excited valence shell. An
independent particle model is used to rationalize the observed experimental
findings
Consistent Treatment of Soft and Hard Processes in Hadronic Interactions
The QCD improved parton model is a very successful concept to treat processes
in hadronic interactions, whenever large partonic transverse momenta are
involved. However, cross sections diverge in the limit p_T -> 0, and the usual
treatment is the definition of a lower cutoff p_T_min, such that processes with
a smaller p_T -- so-called soft processes -- are simply ignored, which is
certainly not correct for example at RHIC energies. A more consistent procedure
amounts to introduce a technical parameter Q_0^2, referred to as soft
virtuality scale, which is nothing but an artificial borderline between soft
and hard physics. We will discuss such a formalism, which coincides with the
improved parton model for high p_T processes and with the phenomenological
treatment of soft scattering, when only small virtualities are involved. The
most important aspect of our approach is that it allows to obtain a smooth
transition between soft and hard scattering, and therefore no artificial
dependence on a cutoff parameter should appear.Comment: 19 pages, 19 figure
Muon production in extensive air showers and its relation to hadronic interactions
In this work, the relation between muon production in extensive air showers
and features of hadronic multiparticle production at low energies is studied.
Using CORSIKA, we determine typical energies and phase space regions of
secondary particles which are important for muon production in extensive air
showers and confront the results with existing fixed target measurements.
Furthermore possibilities to measure relevant quantities of hadron production
in existing and planned accelerator experiments are discussed.Comment: To appear in the proceedings of international conference on
interconnection between high energy physics and astroparticle physics "From
Colliders to Cosmic Rays" (C2CR 2005), Prague, Czech Republic, 7-13 September
200
The KATRIN Pre-Spectrometer at reduced Filter Energy
The KArlsruhe TRItium Neutrino experiment, KATRIN, will determine the mass of
the electron neutrino with a sensitivity of 0.2 eV (90% C.L.) via a measurement
of the beta-spectrum of gaseous tritium near its endpoint of E_0 =18.57 keV. An
ultra-low background of about b = 10 mHz is among the requirements to reach
this sensitivity. In the KATRIN main beam-line two spectrometers of MAC-E
filter type are used in a tandem configuration. This setup, however, produces a
Penning trap which could lead to increased background. We have performed test
measurements showing that the filter energy of the pre-spectrometer can be
reduced by several keV in order to diminish this trap. These measurements were
analyzed with the help of a complex computer simulation, modeling multiple
electron reflections both from the detector and the photoelectric electron
source used in our test setup.Comment: 22 pages, 12 figure
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