752 research outputs found
Geant4 hadronic physics status and validation for large HEP detectors
Optimal exploitation of hadronic final states played a key role in successes
of all recent collider experiment in HEP, and the ability to use hadronic final
states will continue to be one of the decisive issues during the analysis phase
of the LHC experiments.
Monte Carlo techniques facilitate the use of hadronic final states, and have
been developed for many years. We will give a brief overview of the physics
underlying hadronic shower simulation, discussing the three basic types of
modeling; data driven, parametrization driven, and theory driven modeling at
the example of Geant4. We will confront these different types of modeling with
the stringent requirements posed by the LHC experiments on hadronic shower
simulation, and report on the current status of the validation effort for large
HEP applications. We will address robustness, and CPU and physics performance
evaluations.Comment: Computing in High Energy and Nuclear Physics, La Jolla, California,
March 24-28, 2003 1 tar fil
Axonal T<sub>2</sub> estimation using the spherical variance of the strongly diffusion-weighted MRI signal.
In magnetic resonance imaging, the application of a strong diffusion weighting suppresses the signal contributions from the less diffusion-restricted constituents of the brain's white matter, thus enabling the estimation of the transverse relaxation time T <sub>2</sub> that arises from the more diffusion-restricted constituents such as the axons. However, the presence of cell nuclei and vacuoles can confound the estimation of the axonal T <sub>2</sub> , as diffusion within those structures is also restricted, causing the corresponding signal to survive the strong diffusion weighting. We devise an estimator of the axonal T <sub>2</sub> based on the directional spherical variance of the strongly diffusion-weighted signal. The spherical variance T <sub>2</sub> estimates are insensitive to the presence of isotropic contributions to the signal like those provided by cell nuclei and vacuoles. We show that with a strong diffusion weighting these estimates differ from those obtained using the directional spherical mean of the signal which contains both axonal and isotropically-restricted contributions. Our findings hint at the presence of an MRI-visible isotropically-restricted contribution to the signal in the white matter ex vivo fixed tissue (monkey) at 7T, and do not allow us to discard such a possibility also for in vivo human data collected with a clinical 3T system
Tractable non-local correlation density functionals for flat surfaces and slabs
A systematic approach for the construction of a density functional for van
der Waals interactions that also accounts for saturation effects is described,
i.e. one that is applicable at short distances. A very efficient method to
calculate the resulting expressions in the case of flat surfaces, a method
leading to an order reduction in computational complexity, is presented.
Results for the interaction of two parallel jellium slabs are shown to agree
with those of a recent RPA calculation (J.F. Dobson and J. Wang, Phys. Rev.
Lett. 82, 2123 1999). The method is easy to use; its input consists of the
electron density of the system, and we show that it can be successfully
approximated by the electron densities of the interacting fragments. Results
for the surface correlation energy of jellium compare very well with those of
other studies. The correlation-interaction energy between two parallel jellia
is calculated for all separations d, and substantial saturation effects are
predicted.Comment: 10 pages, 6 figure
Analytical Solution of the Off-Equilibrium Dynamics of a Long Range Spin-Glass Model
We study the non-equilibrium relaxation of the spherical spin-glass model
with p-spin interactions in the limit. We analytically
solve the asymptotics of the magnetization and the correlation and response
functions for long but finite times. Even in the thermodynamic limit the system
exhibits `weak' (as well as `true') ergodicity breaking and aging effects. We
determine a functional Parisi-like order parameter which plays a
similar role for the dynamics to that played by the usual function for the
statics.Comment: 8 pages, Roma preprin
Revisiting spin alignment of heavy mesons in its inclusive production
In the heavy quark limit inclusive production rate of a heavy meson can be
factorized, in which the nonperturbative effect related to the heavy meson can
be characterized by matrix elements defined in the heavy quark effective
theory. Using this factorization, predictions for the full spin density matrix
of a spin-1 and spin-2 meson can be obtained and they are characterized only by
one coefficient representing the nonperturbative effect. Predictions for spin-1
heavy meson are compared with experiment performed at colliders in the
energy range from GeV to GeV, a complete agreement
is found for - and -meson. For meson, our prediction suffers
a large correction, as indicated by experimental data. There exists another
approach by taking heavy mesons as bound systems, in which the total angular
momentum of the light degrees of freedom is 1/2 and 3/2 for spin-1 and spin-2
meson respectively, then the diagonal parts of spin density matrices can be
obtained. However, there are distinct differences in the predictions from the
two approaches and they are discussed in detail.Comment: 14 pages with one figur
Strange Particle Production at RHIC
We report STAR measurements of mid-rapidity yields for the ,
, , , , , and
particles in Cu+Cu and Au+Au GeV
collisions. We show that at a given number of participating nucleons, bulk
strangeness production is higher in Cu+Cu collisions compared to Au+Au
collisions at the same center of mass energy, counter to predictions from the
Canonical formalism. We compare both the Cu+Cu and Au+Au yields to AMPT and
EPOS predictions, and find they reproduce key qualitative aspects of the data.
Finally, we investigate other scaling parameters and find bulk strangeness
production for both the measured data and theoretical predictions, scales
better with the number participants that undergo more than one collision.Comment: Conference proceedings for Hot Quarks 2008, 5 pages and 4 figure
Spin alignments of vector mesons in deeply inelastic lepton-nucleon scattering
We extend the calculations of the spin alignments of vector mesons in
annihilation in a recent Rapid Communication to deeply inelastic
lepton-nucleon scatterings. We present the results for different mesons in the
current fragmentation regions of at high energies
and at both high and low energies. We also present
the predictions for at NOMAD energies in the
target fragmentation region using a valence quark model.Comment: 4 pages, 6 figures; to appear in Phys. Rev.
Kasner and Mixmaster behavior in universes with equation of state w \ge 1
We consider cosmological models with a scalar field with equation of state
that contract towards a big crunch singularity, as in recent cyclic
and ekpyrotic scenarios. We show that chaotic mixmaster oscillations due to
anisotropy and curvature are suppressed, and the contraction is described by a
homogeneous and isotropic Friedmann equation if . We generalize the
results to theories where the scalar field couples to p-forms and show that
there exists a finite value of , depending on the p-forms, such that chaotic
oscillations are suppressed. We show that orbifold compactification also
contributes to suppressing chaotic behavior. In particular, chaos is avoided in
contracting heterotic M-theory models if at the crunch.Comment: 25 pages, 2 figures, minor changes, references adde
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