167 research outputs found
Twist-4 contributions to the azimuthal asymmetry in SIDIS
We calculate the differential cross section for the unpolarized
semi-inclusive deeply inelastic scattering (SIDIS) process
in leading order (LO) of perturbative QCD and up to twist-4 in power
corrections and study in particular the azimuthal asymmetry . The
final results are expressed in terms of transverse momentum dependent (TMD)
parton matrix elements of the target nucleon up to twist-4. %Under the maximal
two-gluon correlation approximation, these TMD parton matrix elements in a
nucleus %can be expressed terms of a Gaussian convolution of that in a nucleon
with the width given by the jet transport %parameter inside cold nuclei. We
also apply it to $e^-+A \to e^-+q+X$ and illustrate numerically the nuclear
dependence of the azimuthal asymmetry by using a Gaussian ansatz
for the TMD parton matrix elements.Comment: 9 pages, afigur
Quantum-mechanical description of in-medium fragmentation
We present a quantum-mechanical description of quark-hadron fragmentation in
a nuclear environment. It employs the path-integral formulation of quantum
mechanics, which takes care of all phases and interferences, and which contains
all relevant time scales, like production, coherence, formation, etc. The cross
section includes the probability of pre-hadron (colorless dipole) production
both inside and outside the medium. Moreover, it also includes inside-outside
production, which is a typical quantum-mechanical interference effect (like
twin-slit electron propagation). We observe a substantial suppression caused by
the medium, even if the pre-hadron is produced outside the medium and no energy
loss is involved. This important source of suppression is missed in the usual
energy-loss scenario interpreting the effect of jet quenching observed in heavy
ion collisions. This may be one of the reasons of a too large gluon density,
reported by such analyzes.Comment: 20 pages, 7 figure
RXJ1716.6+6708: a young cluster at z=0.81
Clusters of galaxies at redshifts nearing one are of special importance since
they may be caught at the epoch of formation. At these high redshifts there are
very few known clusters. We present follow-up ASCA, ROSAT HRI and Keck LRIS
observations of the cluster RXJ1716.6+6708 which was discovered during the
optical identification of X-ray sources in the North Ecliptic Pole region of
the ROSAT All-Sky Survey. At z=0.809, RXJ1716.6+6708 is the second most distant
X-ray selected cluster so far published and the only one with a large number of
spectroscopically determined cluster member velocities. The optical morphology
of RXJ1716.6+6708 resembles an inverted S-shape filament with the X-rays coming
from the midpoint of the filament. The X-ray contours have an elongated shape
that roughly coincide with the weak lensing contours. The cluster has a low
temperature, kT=5.66{+1.37 -0.58} keV, and a very high velocity dispersion
sigma_{los}=1522{+215 -150} km s^{-1}. While the temperature is commensurate
with its X-ray luminosity of (8.19 +/- 0.43)x10^{44} h_{50}^{-2} erg s^{-1}
(2-10 keV rest frame), its velocity dispersion is much higher than expected
from the sigma-T_X relationship of present-day clusters with comparable X-ray
luminosity. RXJ1716.6+6708 could be an example of a protocluster, where matter
is flowing along filaments and the X-ray flux is maximum at the impact point of
the colliding streams of matter.Comment: Latex file, 18 pages, 4 figures, accepted for publication in the
Astronomical Journa
A Serendipitous Galaxy Cluster Survey with XMM: Expected Catalogue Properties and Scientific Applications
This paper describes a serendipitous galaxy cluster survey that we plan to
conduct with the XMM X-ray satellite. We have modeled the expected properties
of such a survey for three different cosmological models, using an extended
Press-Schechter (Press & Schechter 1974) formalism, combined with a detailed
characterization of the expected capabilities of the EPIC camera on board XMM.
We estimate that, over the ten year design lifetime of XMM, the EPIC camera
will image a total of ~800 square degrees in fields suitable for the
serendipitous detection of clusters of galaxies. For the presently-favored
low-density model with a cosmological constant, our simulations predict that
this survey area would yield a catalogue of more than 8000 clusters, ranging
from poor to very rich systems, with around 750 detections above z=1. A
low-density open Universe yields similar numbers, though with a different
redshift distribution, while a critical-density Universe gives considerably
fewer clusters. This dependence of catalogue properties on cosmology means that
the proposed survey will place strong constraints on the values of Omega-Matter
and Omega-Lambda. The survey would also facilitate a variety of follow-up
projects, including the quantification of evolution in the cluster X-ray
luminosity-temperature relation, the study of high-redshift galaxies via
gravitational lensing, follow-up observations of the Sunyaev-Zel'dovich effect
and foreground analyses of cosmic microwave background maps.Comment: Accepted to ApJ. Minor changes, e.g. presentation of temperature
errors as a figure (rather than as a table). Latex (20 pages, 6 figures, uses
emulateapj.sty
Infall Regions of Galaxy Clusters
In hierarchical clustering, galaxy clusters accrete mass through the
aggregation of smaller systems. Thus, the velocity field of the infall regions
of clusters contains significant random motion superimposed on radial infall.
Because the purely spherical infall model does not predict the amplitude of the
velocity field correctly, methods estimating the cosmological density parameter
Omega_0 based on this model yield unreliable biased results. In fact, the
amplitude of the velocity field depends on local dynamics and only very weakly
on the global properties of the universe. We use N-body simulations of flat and
open universes to show that the amplitude of the velocity field of the infall
regions of dark matter halos is a direct measure of the escape velocity within
these regions. We can use this amplitude to estimate the mass of dark matter
halos within a few megaparsecs from the halo center. In this region dynamical
equilibrium assumptions do not hold. The method yields a mass estimate with
better than 30% accuracy. If galaxies trace the velocity field of the infall
regions of clusters reliably, this method provides a straightforward way to
estimate the amount of mass surrounding rich galaxy clusters from redshift data
alone.Comment: 26 pages, AAS Latex macros v4.0, to appear in The Astrophysical
Journal, June 1, 1997 issue, Vol. 48
Optimized Trigger for Ultra-High-Energy Cosmic-Ray and Neutrino Observations with the Low Frequency Radio Array
When an ultra-high energy neutrino or cosmic ray strikes the Lunar surface a
radio-frequency pulse is emitted. We plan to use the LOFAR radio telescope to
detect these pulses. In this work we propose an efficient trigger
implementation for LOFAR optimized for the observation of short radio pulses.Comment: Submitted to Nuclear Instruments and Methods in Physics Research
Section
Internal properties and environments of dark matter halos
We use seven high-resolution -body simulations to study the correlations
among different halo properties (assembly time, spin, shape and substructure),
and how these halo properties are correlated with the large-scale environment
in which halos reside. The large-scale tidal field estimated from halos above a
mass threshold is used as our primary quantity to characterize the large-scale
environment, while other parameters, such as the local overdensity and the
morphology of large-scale structure, are used for comparison. For halos at a
fixed mass, all the halo properties depend significantly on environment,
particularly the tidal field. The environmental dependence of halo assembly
time is primarily driven by local tidal field. The mass of the unbound fraction
in substructure is boosted in strong tidal force region, while the bound
fraction is suppressed. Halos have a tendency to spin faster in stronger tidal
field and the trend is stronger for more massive halos. The spin vectors show
significant alignment with the intermediate axis of the tidal field, as
expected from the tidal torque theory. Both the major and minor axes of halos
are strongly aligned with the corresponding principal axes of the tidal field.
In general, a halo that can accrete more material after the formation of its
main halo on average is younger, is more elongated, spins faster, and contains
a larger amount of substructure. Higher density environments not only provide
more material for halo to accrete, but also are places of stronger tidal field
that tends to suppress halo accretion. The environmental dependencies are the
results of these two competing effects. The tidal field based on halos can be
estimated from observation, and we discuss the implications of our results for
the environmental dependence of galaxy properties.Comment: Accepted for publication in MNRA
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