24,627 research outputs found
QCD sum rules in the effective heavy quark theory
We derive sum rules for the leptonic decay constant of a heavy-light meson in the effective heavy quark theory. We show that the summation of logarithms in the heavy quark mass by the renormalization group technique enhances considerably radiative corrections. Our result for the decay constant in the static limit agrees well with recent lattice calculations. Finite quark mass corrections are estimated
Structural lubricity: Role of dimension and symmetry
When two chemically passivated solids are brought into contact, interfacial
interactions between the solids compete with intrabulk elastic forces. The
relative importance of these interactions, which are length-scale dependent,
will be estimated using scaling arguments. If elastic interactions dominate on
all length scales, solids will move as essentially rigid objects. This would
imply superlow kinetic friction in UHV, provided wear was absent. The results
of the scaling study depend on the symmetry of the surfaces and the
dimensionalities of interface and solids. Some examples are discussed
explicitly such as contacts between disordered three-dimensional solids and
linear bearings realized from multiwall carbon nanotubes.Comment: 7 pages, 1 figur
Fluctuations of the number of participants and binary collisions in AA interactions at fixed centrality in the Glauber approach
In the framework of the classical Glauber approach, the analytical
expressions for the variance of the number of wounded nucleons and binary
collisions in AA interactions at a given centrality are presented. Along with
the optical approximation term, they contain additional contact terms arising
only in the case of nucleus-nucleus collisions. The magnitude of the additional
contributions, e.g., for PbPb collisions at SPS energies, is larger than the
contribution of the optical approximation at some values of the impact
parameter. The sum of the additional contributions is in good agreement with
the results of independent Monte Carlo simulations of this process. Due to
these additional terms, the variance of the total number of participants for
peripheral PbPb collisions and the variance of the number of collisions at all
values of the impact parameter exceed several multiples of the Poisson
variances. The correlator between the numbers of participants in colliding
nuclei at fixed centrality is also analytically calculated.Comment: updated version; as published by Phys. Rev.
String Method for the Study of Rare Events
We present a new and efficient method for computing the transition pathways,
free energy barriers, and transition rates in complex systems with relatively
smooth energy landscapes. The method proceeds by evolving strings, i.e. smooth
curves with intrinsic parametrization whose dynamics takes them to the most
probable transition path between two metastable regions in the configuration
space. Free energy barriers and transition rates can then be determined by
standard umbrella sampling technique around the string. Applications to
Lennard-Jones cluster rearrangement and thermally induced switching of a
magnetic film are presented.Comment: 4 pages, 4 figure
TDC Chip and Readout Driver Developments for COMPASS and LHC-Experiments
A new TDC-chip is under development for the COMPASS experiment at CERN. The
ASIC, which exploits the 0.6 micrometer CMOS sea-of-gate technology, will allow
high resolution time measurements with digitization of 75 ps, and an
unprecedented degree of flexibility accompanied by high rate capability and low
power consumption. Preliminary specifications of this new TDC chip are
presented.
Furthermore a FPGA based readout-driver and buffer-module as an interface
between the front-end of the COMPASS detector systems and an optical S-LINK is
in development. The same module serves also as remote fan-out for the COMPASS
trigger distribution and time synchronization system. This readout-driver
monitors the trigger and data flow to and from front-ends. In addition, a
specific data buffer structure and sophisticated data flow control is used to
pursue local pre-event building. At start-up the module controls all necessary
front-end initializations.Comment: 5 pages, 4 figure
Parallel updating cellular automaton models of driven diffusive Frenkel-Kontorova-type systems
Three cellular automaton (CA) models of increasing complexity are introduced
to model driven diffusive systems related to the generalized Frenkel-Kontorova
(FK) models recently proposed by Braun [Phys.Rev.E58, 1311 (1998)]. The models
are defined in terms of parallel updating rules. Simulation results are
presented for these models. The features are qualitatively similar to those
models defined previously in terms of sequentially updating rules. Essential
features of the FK model such as phase transitions, jamming due to atoms in the
immobile state, and hysteresis in the relationship between the fraction of
atoms in the running state and the bias field are captured. Formulating in
terms of parallel updating rules has the advantage that the models can be
treated analytically by following the time evolution of the occupation on every
site of the lattice. Results of this analytical approach are given for the two
simpler models. The steady state properties are found by studying the stable
fixed points of a closed set of dynamical equations obtained within the
approximation of retaining spatial correlations only upto two nearest
neighboring sites. Results are found to be in good agreement with numerical
data.Comment: 26 pages, 4 eps figure
Cold Dust in Kepler's Supernova Remnant
The timescales to replenish dust from the cool, dense winds of Asymptotic
Giant Branch stars are believed to be greater than the timescales for dust
destruction. In high redshift galaxies, this problem is further compounded as
the stars take longer than the age of the Universe to evolve into the dust
production stages. To explain these discrepancies, dust formation in supernovae
(SNe) is required to be an important process but until very recently dust in
supernova remnants has only been detected in very small quantities. We present
the first submillimeter observations of cold dust in Kepler's supernova remnant
(SNR) using SCUBA. A two component dust temperature model is required to fit
the Spectral Energy Distribution (SED) with K and K. The total mass of dust implied for Kepler is -
1000 times greater than previous estimates. Thus SNe, or their progenitors may
be important dust formation sites.Comment: 12 pages, 2 figures, accepted to ApJL, corrected proof
On Free Quotients of Complete Intersection Calabi-Yau Manifolds
In order to find novel examples of non-simply connected Calabi-Yau
threefolds, free quotients of complete intersections in products of projective
spaces are classified by means of a computer search. More precisely, all
automorphisms of the product of projective spaces that descend to a free action
on the Calabi-Yau manifold are identified.Comment: 39 pages, 3 tables, LaTe
- âŠ