3,916 research outputs found
Explicit generating functional for pions and virtual photons
We construct the explicit one-loop functional of chiral perturbation theory
for two light flavours, including virtual photons. We stick to contributions
where 1 or 2 mesons and at most one photon are running in the loops. With the
explicit functional at hand, the evaluation of the relevant Green functions
boils down to performing traces over the flavour matrices. For illustration, we
work out the pi+ pi- -> pi0 pi0 scattering amplitude at threshold at order p^4,
e^2p^2.Comment: 20 pages, 2 figures; version accepted for publication, minor
typographical changes, acknowledgments adde
Composite fermions in periodic and random antidot lattices
The longitudinal and Hall magnetoresistance of random and periodic arrays of artificial scatterers, imposed on a high-mobility two-dimensional electron gas, were investigated in the vicinity of Landau level filling factor ν=1/2. In periodic arrays, commensurability effects between the period of the antidot array and the cyclotron radius of composite fermions are observed. In addition, the Hall resistance shows a deviation from the anticipated linear dependence, reminiscent of quenching around zero magnetic field. Both effects are absent for random antidot lattices. The relative amplitude of the geometric resonances for opposite signs of the effective magnetic field and its dependence on illumination illustrate enhanced soft wall effects for composite fermions
Inter-molecular structure factors of macromolecules in solution: integral equation results
The inter-molecular structure of semidilute polymer solutions is studied
theoretically. The low density limit of a generalized Ornstein-Zernicke
integral equation approach to polymeric liquids is considered. Scaling laws for
the dilute-to-semidilute crossover of random phase (RPA) like structure are
derived for the inter-molecular structure factor on large distances when
inter-molecular excluded volume is incorporated at the microscopic level. This
leads to a non-linear equation for the excluded volume interaction parameter.
For macromolecular size-mass scaling exponents, , above a
spatial-dimension dependent value, , mean field like density scaling
is recovered, but for the density scaling becomes non-trivial in
agreement with field theoretic results and justifying phenomenological
extensions of RPA. The structure of the polymer mesh in semidilute solutions is
discussed in detail and comparisons with large scale Monte Carlo simulations
are added. Finally a new possibility to determine the correction to scaling
exponent is suggested.Comment: 11 pages, 5 figures; to be published in Phys. Rev. E (1999
Study of the performance and capability of the new ultra-fast 2 GSample/s FADC data acquisition system of the MAGIC telescope
In February 2007 the MAGIC Air Cherenkov Telescope for gamma-ray astronomy
was fully upgraded with an ultra fast 2 GSamples/s digitization system. Since
the Cherenkov light flashes are very short, a fast readout can minimize the
influence of the background from the light of the night sky. Also, the time
structure of the event is an additional parameter to reduce the background from
unwanted hadronic showers. An overview of the performance of the new system and
its impact on the sensitivity of the MAGIC instrument will be presented.Comment: Contribution to the 30th ICRC, Merida Mexico, July 2007 on behalf of
the MAGIC Collaboratio
Forced motion of a probe particle near the colloidal glass transition
We use confocal microscopy to study the motion of a magnetic bead in a dense
colloidal suspension, near the colloidal glass transition volume fraction
. For dense liquid-like samples near , below a threshold force
the magnetic bead exhibits only localized caged motion. Above this force, the
bead is pulled with a fluctuating velocity. The relationship between force and
velocity becomes increasingly nonlinear as is approached. The
threshold force and nonlinear drag force vary strongly with the volume
fraction, while the velocity fluctuations do not change near the transition.Comment: 7 pages, 4 figures revised version, accepted for publication in
Europhysics Letter
On the uniqueness of -gonal automorphisms of Riemann surfaces
Let be a compact Riemann surface of genus . A cyclic subgroup of
prime order of is called properly -gonal if it has a fixed
point and the quotient surface has genus . We show that if , then a
properly -gonal subgroup of is unique. We also discuss some
related results.Comment: final version, 9 pages, minor improvements, added 2 reference
Structure of Colloid-Polymer Suspensions
We discuss structural correlations in mixtures of free polymer and colloidal
particles based on a microscopic, 2-component liquid state integral equation
theory. Whereas in the case of polymers much smaller than the spherical
particles the relevant polymer degree of freedom is the center of mass, for
polymers larger than the (nano-) particles conformational rearrangements need
to be considered. They have the important consequence that the polymer
depletion layer exhibits two widely different length scales, one of the order
of the particle radius, the other of the order of the polymer radius or the
polymer density screening length in dilute or semidilute concentrations,
respectively. Their consequences on phase stability and structural correlations
are discussed extensively.Comment: 37 pages, 17 figures; topical feature articl
Cognitive Computation sans Representation
The Computational Theory of Mind (CTM) holds that cognitive processes are essentially computational, and hence computation provides the scientific key to explaining mentality. The Representational Theory of Mind (RTM) holds that representational content is the key feature in distinguishing mental from non-mental systems. I argue that there is a deep incompatibility between these two theoretical frameworks, and that the acceptance of CTM provides strong grounds for rejecting RTM. The focal point of the incompatibility is the fact that representational content is extrinsic to formal procedures as such, and the intended interpretation of syntax makes no difference to the execution of an algorithm. So the unique 'content' postulated by RTM is superfluous to the formal procedures of CTM. And once these procedures are implemented in a physical mechanism, it is exclusively the causal properties of the physical mechanism that are responsible for all aspects of the system's behaviour. So once again, postulated content is rendered superfluous. To the extent that semantic content may appear to play a role in behaviour, it must be syntactically encoded within the system, and just as in a standard computational artefact, so too with the human mind/brain - it's pure syntax all the way down to the level of physical implementation. Hence 'content' is at most a convenient meta-level gloss, projected from the outside by human theorists, which itself can play no role in cognitive processing
Pricing Options in Incomplete Equity Markets via the Instantaneous Sharpe Ratio
We use a continuous version of the standard deviation premium principle for
pricing in incomplete equity markets by assuming that the investor issuing an
unhedgeable derivative security requires compensation for this risk in the form
of a pre-specified instantaneous Sharpe ratio. First, we apply our method to
price options on non-traded assets for which there is a traded asset that is
correlated to the non-traded asset. Our main contribution to this particular
problem is to show that our seller/buyer prices are the upper/lower good deal
bounds of Cochrane and Sa\'{a}-Requejo (2000) and of Bj\"{o}rk and Slinko
(2006) and to determine the analytical properties of these prices. Second, we
apply our method to price options in the presence of stochastic volatility. Our
main contribution to this problem is to show that the instantaneous Sharpe
ratio, an integral ingredient in our methodology, is the negative of the market
price of volatility risk, as defined in Fouque, Papanicolaou, and Sircar
(2000).Comment: Keywords: Pricing derivative securities, incomplete markets, Sharpe
ratio, correlated assets, stochastic volatility, non-linear partial
differential equations, good deal bound
Chemical enrichment of the complex hot ISM of the Antennae galaxies: I. Spatial and spectral analysis of the diffuse X-ray emission
We present an analysis of the properties of the hot interstellar medium (ISM)
in the merging pair of galaxies known as The Antennae (NGC 4038/39), performed
using the deep, coadded ~411 ks Chandra ACIS-S data set. These deep X-ray
observations and Chandra's high angular resolution allow us to investigate the
properties of the hot ISM with unprecedented spatial and spectral resolution.
Through a spatially resolved spectral analysis, we find a variety of
temperatures (from 0.2 to 0.7 keV) and Nh (from Galactic to 2x10^21 cm^-2).
Metal abundances for Ne, Mg, Si, and Fe vary dramatically throughout the ISM
from sub-solar values (~0.2) up to several times solar.Comment: 33 pages, 18 figures, revised version accepted by Astrophysical
Journal Supplement Serie
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