1,592 research outputs found
Dependence on the Identification of the Scale Energy Parameter Q 2 in the Quark Distribution Functions for a DIS Production of Za
We discuss the Z-production in a DIS (Deep Inelastic Scattering) process e + p â e + Z + X using the Parton Model, within the context of the Standard Model. In contrast with deep inelastic eP-scattering (e + p â e + X), where the choice of Q2, as the transferred momentum squared, is unambiguous; whereas in the case of boson production , the transferred momentum squared, at quark level, depends on the reaction mechanism (where is the EW interaction taking place). We suggest a proposal based on kinematics of the process considered and the usual criterion for Q2 , which leads to a simple and practical prescription to calculate Z-production via ep-DIS. We also introduce different options in order o perform the convolution of the parton distribution functions (PDFs) and the scattering amplitude of he quark processes. Our aim in this work is to analyze and show how large could be the dependence of the total cross section rates on different possible prescriptions used for the identification of the scale energy parameter Q2 . We present results for the total cross section as a function of the total energy âs of the system ep, in the range 300 <âs †1300 Ge
Gravitational wave background from binary systems
Basic aspects of the background of gravitational waves and its mathematical
characterization are reviewed. The spectral energy density parameter
, commonly used as a quantifier of the background, is derived for an
ensemble of many identical sources emitting at different times and locations.
For such an ensemble, is generalized to account for the duration of
the signals and of the observation, so that one can distinguish the resolvable
and unresolvable parts of the background. The unresolvable part, often called
confusion noise or stochastic background, is made by signals that cannot be
either individually identified or subtracted out of the data. To account for
the resolvability of the background, the overlap function is introduced. This
function is a generalization of the duty cycle, which has been commonly used in
the literature, in some cases leading to incorrect results. The spectra
produced by binary systems (stellar binaries and massive black hole binaries)
are presented over the frequencies of all existing and planned detectors. A
semi-analytical formula for is derived in the case of stellar
binaries (containing white dwarfs, neutron stars or stellar-mass black holes).
Besides a realistic expectation of the level of background, upper and lower
limits are given, to account for the uncertainties in some astrophysical
parameters such as binary coalescence rates. One interesting result concerns
all current and planned ground-based detectors (including the Einstein
Telescope). In their frequency range, the background of binaries is resolvable
and only sporadically present. In other words, there is no stochastic
background of binaries for ground-based detectors.Comment: 30 pages, 16 figure
Superbubble evolution including the star-forming clouds: Is it possible to reconcile LMC observations with model predictions?
Here we present a possible solution to the apparent discrepancy between the
observed properties of LMC bubbles and the standard, constant density bubble
model. A two-dimensional model of a wind-driven bubble expanding from a
flattened giant molecular cloud is examined. We conclude that the expansion
velocities derived from spherically symmetric models are not always applicable
to elongated young bubbles seen almost face-on due to the LMC orientation. In
addition, an observational test to differentiate between spherical and
elongated bubbles seen face-on is discussed.Comment: 25 pages, 7 figures, accepted to ApJ (September, 1999 issue
Surfactant-free purification of membrane protein complexes from bacteria: application to the staphylococcal penicillin-binding protein complex PBP2/PBP2a
Surfactant-mediated removal of proteins from biomembranes invariably results in partial or complete loss of function and disassembly of multi-protein complexes. We determined the capacity of styrene-co-maleic acid (SMA) co-polymer to remove components of the cell division machinery from the membrane of drug-resistant staphylococcal cells. SMA-lipid nanoparticles solubilized FtsZ-PBP2-PBP2a complexes from intact cells, demonstrating the close physical proximity of these proteins within the lipid bilayer. Exposure of bacteria to (-)-epicatechin gallate, a polyphenolic agent that abolishes ÎČ-lactam resistance in staphylococci, disrupted the association between PBP2 and PBP2a. Thus, SMA purification provides a means to remove native integral membrane protein assemblages with minimal physical disruption and shows promise as a tool for the interrogation of molecular aspects of bacterial membrane protein structure and function
Seyfert's Sextet: A Slowly Dissolving Stephan's Quintet?
We present a multiwavelength study of the highly evolved compact galaxy group
known as Seyfert's Sextet (HCG79: SS). We interpret SS as a 2-3 Gyr more
evolved analog of Stephan's Quintet (HCG92: SQ). We postulate that SS formed by
sequential acquisition of 4-5 primarily late-type field galaxies. Four of the
five galaxies show an early-type morphology which is likely the result of
secular evolution driven by gas stripping. Stellar stripping has produced a
massive/luminous halo and embedded galaxies that are overluminous for their
size. These are interpreted as remnant bulges of the accreted spirals. H79d
could be interpreted as the most recent intruder being the only galaxy with an
intact ISM and uncertain evidence for tidal perturbation. In addition to
stripping activity we find evidence for past accretion events. H79b (NGC6027)
shows a strong counter-rotating emission line component interpreted as an
accreted dwarf spiral. H79a shows evidence for an infalling component of gas
representing feedback or possible cross fueling by H79d. The biggest challenge
to this scenario involves the low gas fraction in the group. If SS formed from
normal field spirals then much of the gas is missing. Finally, despite its
advanced stage of evolution, we find no evidence for major mergers and infer
that SS (and SQ) are telling us that such groups coalesce via slow dissolution.Comment: 70 pages, 19 figures, 15 tables - accepted for publication in the
Astronomical Journa
24-hour Heart Rate is Related to Lower Extremity Venous Vascular Function in Persons with Paraplegia
Please refer to the pdf version of the abstract located adjacent to the title
Are the Models for Type Ia Supernova Progenitors Consistent with the Properties of Supernova Remnants?
We explore the relationship between the models for progenitor systems of Type
Ia supernovae and the properties of the supernova remnants that evolve after
the explosion. Most models for Type Ia progenitors in the single degenerate
scenario predict substantial outflows during the presupernova evolution.
Expanding on previous work, we estimate the imprint of these outflows on the
structure of the circumstellar medium at the time of the supernova explosion,
and the effect that this modified circumstellar medium has on the evolution of
the ensuing supernova remnant. We compare our simulations with the
observational properties of known Type Ia supernova remnants in the Galaxy
(Kepler, Tycho, SN 1006), the Large Magellanic Cloud (0509-67.5, 0519-69.0,
N103B), and M31 (SN 1885). We find that optically thick outflows from the white
dwarf surface (sometimes known as accretion winds) with velocities above 200
km/s excavate large low-density cavities around the progenitors. Such large
cavities are incompatible with the dynamics of the forward shock and the X-ray
emission from the shocked ejecta in all the Type Ia remnants that we have
examined.Comment: To appear in ApJ. 17 pages, 10 figures, emulateap
The UTMOST pulsar timing programme I: overview and first results
We present an overview and the first results from a large-scale pulsar timing
programme that is part of the UTMOST project at the refurbished Molonglo
Observatory Synthesis Radio Telescope (MOST) near Canberra, Australia. We
currently observe more than 400 mainly bright southern radio pulsars with up to
daily cadences. For 205 (8 in binaries, 4 millisecond pulsars) we publish
updated timing models, together with their flux densities, flux density
variability, and pulse widths at 843 MHz, derived from observations spanning
between 1.4 and 3 yr. In comparison with the ATNF pulsar catalogue, we improve
the precision of the rotational and astrometric parameters for 123 pulsars, for
47 by at least an order of magnitude. The time spans between our measurements
and those in the literature are up to 48 yr, which allows us to investigate
their long-term spin-down history and to estimate proper motions for 60
pulsars, of which 24 are newly determined and most are major improvements. The
results are consistent with interferometric measurements from the literature. A
model with two Gaussian components centred at 139 and fits the transverse velocity distribution best. The pulse duty
cycle distributions at 50 and 10 per cent maximum are best described by
log-normal distributions with medians of 2.3 and 4.4 per cent, respectively. We
discuss two pulsars that exhibit spin-down rate changes and drifting subpulses.
Finally, we describe the autonomous observing system and the dynamic scheduler
that has increased the observing efficiency by a factor of 2-3 in comparison
with static scheduling.Comment: 31 pages, 14 figures, 6 tables, accepted for publication in MNRA
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