26,981 research outputs found
Spherical Colloids: Effect of Discrete Macroion Charge Distribution and Counterion Valence
We report the coupled effects of macroion charge discretization and
counterion valence in the primitive model for spherical colloids. Instead of
considering a uniformly charged surface, as it is traditionally done, we
consider a more realistic situation where \textit{discrete monovalent
microscopic charges} are randomly distributed over the sphere. Monovalent or
multivalent counterions ensure global electroneutrality. We use molecular
dynamics simulations to study these effects at the ground state and for finite
temperature. The ground state analysis concerns the counterion structure and
\textit{charge inversion}. Results are discussed in terms of simple analytical
models. For finite temperature, strong and weak Coulomb couplings are treated.
In this situation of finite temperature, we considered and discussed the
phenomena of ionic pairing (pinning) and unpairing (unpinning).Comment: 24 pages, 12 (main) figures (28 EPS files). To appear in Physica
Image charges in spherical geometry: Application to colloidal systems
The effects of image charges (i.e., induced surface charges of polarization)
in spherical geometry and their implication for charged colloidal systems are
investigated. We study analytically and exactly a single microion interacting
with a dielectric sphere and discuss the similarities and discrepancies with
the case of a planar interface. By means of extensive Monte Carlo (MC)
simulations, we study within the framework of the primitive model the effects
of image charges on the structure of the electrical double layer. Salt-free
environment as well as salty solutions are considered. A remarkable finding of
this study is that the position of the maximum in the counterion density
(appearing at moderately surface charge density) remains quasi-identical,
regardless of the counterion valence and the salt content, to that obtained
within the \textit{single}-counterion system.Comment: 14 pages, 12 figures, RevTeX4, to appear in J. Chem. Phy
W + Jet Production at CDF
The cross section for the inclusive production of W bosons in association
with jets in ppbar collisions at sqrt(s)=1.96 TeV using the Collider Detector
at Fermilab (CDF II) is presented. The measurement is based on an integrated
luminosity of 320pb^-1, and includes events with up to 4 or more jets. In each
jet multiplicity sample the differential and cumulative cross sections with
respect to the transverse energy of the i^th-jet are measured. For W+>=2 jets
the differential cross section with respect to the 2-leading jets invariant
mass m_j1j2 and angular separation Delta R_j1j2 is also reported. The data are
compared to predictions from Monte Carlo simulations.Comment: 4 pages, FERMILAB-CONF-06-377-E, to appear in the proceedings of the
14th International Workshop on Deep Inelastic Scattering (DIS2006
Latest Jet Results from Tevatron
This contribution reports preliminary jet results in ppbar collisions at
sqrt(s)=1.96 TeV from the CDF and D0 experiments. The jet inclusive cross
section, measured using both the Midpoint and the K_T jet clustering algorithm,
is compared to next-to-leading order QCD prediction in different rapidity
regions. The b-jet inclusive cross section measured exploiting the long
lifetime and large mass of B hadrons is presented and compared to QCD
prediction. A complementary measurement, using the large branching fraction of
B hadrons into muons, is also described. The measurement of two-particle
momentum correlation in jets is presented and compared to predictions.Comment: 4 pages, Moriond QCD 2006 conference proceedin
A single atom-based generation of Bell states of two cavities
A new conditional scheme for generating Bell states of two spatially
separated high-Q cavities is reported. Our method is based on the passage of
one atom only through the two cavities. A distinctive feature of our treatment
is that it incorporates from the very beginning the unavoidable presence of
fluctuations in the atom-cavity interaction times. The possibility of
successfully implementing our proposal against cavity losses and atomic
spontaneous decay is carefully discussed
Determination of rotation periods in solar-like stars with irregular sampling: the Gaia case
We present a study on the determination of rotation periods (P) of solar-like
stars from the photometric irregular time-sampling of the ESA Gaia mission,
currently scheduled for launch in 2013, taking into account its dependence on
ecliptic coordinates. We examine the case of solar-twins as well as thousands
of synthetic time-series of solar-like stars rotating faster than the Sun. In
the case of solar twins we assume that the Gaia unfiltered photometric passband
G will mimic the variability of the total solar irradiance (TSI) as measured by
the VIRGO experiment. For stars rotating faster than the Sun, light-curves are
simulated using synthetic spectra for the quiet atmosphere, the spots, and the
faculae combined by applying semi-empirical relationships relating the level of
photospheric magnetic activity to the stellar rotation and the Gaia
instrumental response. The capabilities of the Deeming, Lomb-Scargle, and Phase
Dispersion Minimisation methods in recovering the correct rotation periods are
tested and compared. The false alarm probability (FAP) is computed using Monte
Carlo simulations and compared with analytical formulae. The Gaia scanning law
makes the rate of correct detection of rotation periods strongly dependent on
the ecliptic latitude (beta). We find that for P ~ 1 d, the rate of correct
detection increases with ecliptic latitude from 20-30 per cent at beta ~
0{\deg} to a peak of 70 per cent at beta=45{\deg}, then it abruptly falls below
10 per cent at beta > 45{\deg}. For P > 5 d, the rate of correct detection is
quite low and for solar twins is only 5 per cent on average.Comment: 12 pages, 18 figures, accepted by MNRA
Measurement of the W + Jet Cross Section at CDF
A measurement of W->e nu + n-jet cross sections in ppbar collisions at
sqrt(s)=1.96 TeV using the Collider Detector at Fermilab in Run II is
presented. The measurement is based on an integrated luminosity of 320 pb-1,
and includes events with jet multiplicity from >= 1 to >= 4. In each jet
multiplicity sample the differential and cumulative cross sections with respect
to the transverse energy of the n^(th)-leading jet are measured. For W+>=2 jets
the differential cross section with respect to the 2-leading jets invariant
mass m_(j_(1)j_(2)) and angular separation Delta R_(j_(1)j_(2)) is also
reported. The data are compared to predictions from Monte Carlo simulations.Comment: 3 pages, Presented at the XXXVI international Symposium on
Multiparticle Dynamics (ISMD 2006), Rio De Janeiro, Brazil, 2-8 Sept 200
Adsorption of Oppositely Charged Polyelectrolytes onto a Charged Rod
The adsorption of highly charged flexible polycations and polyanions on a
charged cylindrical substrate is investigated by means of Monte Carlo (MC)
simulations. A detailed structural study, including monomer and fluid charge
distributions, is provided. The influence of a short range attraction between
the polycations and the negatively charged substrate is also considered. We
demonstrate that the building up of multilayer structures is highly prohibited
mainly due to the high entropy penalty stemming from the low dimensionality of
the substrate at strong curvature.Comment: 18 pages - 7 (main) EPS figs - revtex4 - to appear in J. Chem. Phy
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