3,026 research outputs found
Entropy considerations in constraining the mSUGRA parameter space
We explore the use of two criteria to constraint the allowed parameter space
in mSUGRA models. Both criteria are based in the calculation of the present
density of neutralinos as dark matter in the Universe. The first one is the
usual ``abundance'' criterion which is used to calculate the relic density
after the ``freeze-out'' era. To compute the relic density we used the
numerical public code micrOMEGAs. The second criterion applies the
microcanonical definition of entropy to a weakly interacting and
self-gravitating gas evaluating then the change in the entropy per particle of
this gas between the ``freeze-out'' era and present day virialized structures.
An ``entropy-consistency'' criterion emerges by comparing theoretical and
empirical estimates of this entropy. The main objective of our work is to
determine for which regions of the parameter space in the mSUGRA model are both
criteria consistent with the 2 bounds according to WMAP for the relic
density: . As a first result, we found that for
, sgn, small values of tan are not favored; only for
tan are both criteria significantly consistent.Comment: 5 pages, 1 figure. To appear in the Proceedings of X Mexican Workshop
on Particles and Fields, Morelia Michoac\'an, M\'exico, November 7-12, 200
Axial Couplings on the World-Line
We construct a world-line representation for the fermionic one-loop effective
action with axial and also vector, scalar, and pseudo-scalar couplings. We use
this expression to compute a few selected scattering amplitudes. These allow us
to verify that our method yields the same results as standard field theory. In
particular, we are able to reproduce the chiral anomaly. Our starting point is
the second-order formulation for the Dirac fermion. We translate the second
order expressions into a world-line action.Comment: 12 pages, LaTeX 2e with array and epsf packages, Postscript figures.
Submitted to Phys. Lett. B. Minor corrections, fixed a number of typo
Yukawa Couplings for the Spinning Particle and the World Line Formalism
We construct the world-line action for a Dirac particle coupled to a
classical scalar or pseudo-scalar background field. This action can be used to
compute loop diagrams and the effective action in the Yukawa model using the
world-line path-integral formalism for spinning particles.Comment: 10 pages Latex, two uuencoded postscript figures. Note added at the
en
Direct and long-range action of a DPP morphogen gradient.
During development of the Drosophila wing, the decapentaplegic (dpp) gene is expressed in a stripe of cells along the anteroposterior compartment boundary and gives rise to a secreted protein that exerts a long-range organizing influence on both compartments. Using clones of cells that express DPP, or in which DPP receptor activity has been constitutively activated or abolished, we show that DPP acts directly and at long range on responding cells, rather than by proxy through the short-range induction of other signaling molecules. Further, we show that two genes, optomotor-blind and spalt are transcriptionally activated at different distances from DPP-secreting cells and provide evidence that these genes respond to different threshold concentrations of DPP protein. We propose that DPP acts as a gradient morphogen during wing development
Critical Casimir effect in classical binary liquid mixtures
If a fluctuating medium is confined, the ensuing perturbation of its
fluctuation spectrum generates Casimir-like effective forces acting on its
confining surfaces. Near a continuous phase transition of such a medium the
corresponding order parameter fluctuations occur on all length scales and
therefore close to the critical point this effect acquires a universal
character, i.e., to a large extent it is independent of the microscopic details
of the actual system. Accordingly it can be calculated theoretically by
studying suitable representative model systems.
We report on the direct measurement of critical Casimir forces by total
internal reflection microscopy (TIRM), with femto-Newton resolution. The
corresponding potentials are determined for individual colloidal particles
floating above a substrate under the action of the critical thermal noise in
the solvent medium, constituted by a binary liquid mixture of water and
2,6-lutidine near its lower consolute point. Depending on the relative
adsorption preferences of the colloid and substrate surfaces with respect to
the two components of the binary liquid mixture, we observe that, upon
approaching the critical point of the solvent, attractive or repulsive forces
emerge and supersede those prevailing away from it. Based on the knowledge of
the critical Casimir forces acting in film geometries within the Ising
universality class and with equal or opposing boundary conditions, we provide
the corresponding theoretical predictions for the sphere-planar wall geometry
of the experiment. The experimental data for the effective potential can be
interpreted consistently in terms of these predictions and a remarkable
quantitative agreement is observed.Comment: 30 pages, 17 figure
Observing muon decays in water Cherenkov detectors at the Pierre Auger Observatory
Muons decaying in the water volume of a Cherenkov detector of the Pierre
Auger Observatory provide a useful calibration point at low energy. Using the
digitized waveform continuously recorded by the electronics of each tank, we
have devised a simple method to extract the charge spectrum of the Michel
electrons, whose typical signal is about 1/8 of a crossing vertical muon. This
procedure, moreover, allows continuous monitoring of the detector operation and
of its water level. We have checked the procedure with high statistics on a
test tank at the Observatory base and applied with success on the whole array.Comment: 4 pages, 2 figures, proceedings of the 29th ICRC Pune, Indi
Normal and lateral critical Casimir forces between colloids and patterned substrates
We study the normal and lateral effective critical Casimir forces acting on a
spherical colloid immersed in a critical binary solvent and close to a
chemically structured substrate with alternating adsorption preference. We
calculate the universal scaling function for the corresponding potential and
compare our results with recent experimental data [Soyka F., Zvyagolskaya O.,
Hertlein C., Helden L., and Bechinger C., Phys. Rev. Lett., 101, 208301
(2008)]. The experimental potentials are properly captured by our predictions
only by accounting for geometrical details of the substrate pattern for which,
according to our theory, critical Casimir forces turn out to be a sensitive
probe.Comment: 6 pages, 3 figure
Lower Energy Consequences of an Anomalous High-Energy Neutrino Cross-Section
A new strong-interaction has been postulated for neutrinos above ~10^{19} eV
to explain the production of highest-energy cosmic ray events. We derive a
dispersion relation relating the hypothesized high-energy cross-section to the
lower-energy neutrino-nucleon elastic amplitude. Remarkably, we find that the
real forward amplitude becomes anomalous seven orders of magnitude lower in
energy than does the total cross-section. We discuss possible measurable
consequences of this early onset of new neutrino physics, and conclude that a
significantly enhanced elastic \nu-N scattering rate may occur for the neutrino
beams available at Fermilab and CERN.Comment: 13 pages, LaTe
Critical Casimir effect in films for generic non-symmetry-breaking boundary conditions
Systems described by an O(n) symmetrical Hamiltonian are considered
in a -dimensional film geometry at their bulk critical points. A detailed
renormalization-group (RG) study of the critical Casimir forces induced between
the film's boundary planes by thermal fluctuations is presented for the case
where the O(n) symmetry remains unbroken by the surfaces. The boundary planes
are assumed to cause short-ranged disturbances of the interactions that can be
modelled by standard surface contributions corresponding
to subcritical or critical enhancement of the surface interactions. This
translates into mesoscopic boundary conditions of the generic
symmetry-preserving Robin type .
RG-improved perturbation theory and Abel-Plana techniques are used to compute
the -dependent part of the reduced excess free energy per
film area to two-loop order. When , it takes the scaling
form as
, where are scaling fields associated with the
surface-enhancement variables , while is a standard
surface crossover exponent. The scaling function
and its analogue for the Casimir force
are determined via expansion in and extrapolated to
dimensions. In the special case , the expansion
becomes fractional. Consistency with the known fractional expansions of D(0,0)
and to order is achieved by appropriate
reorganisation of RG-improved perturbation theory. For appropriate choices of
and , the Casimir forces can have either sign. Furthermore,
crossovers from attraction to repulsion and vice versa may occur as
increases.Comment: Latex source file, 40 pages, 9 figure
- …