5,288 research outputs found
Premicellar aggregation of amphiphilic molecules: Aggregate lifetime and polydispersity
A recently introduced thermodynamic model of amphiphilic molecules in
solution has yielded, under certain realistic conditions, a significant
presence of metastable aggregates well below the critical micelle concentration
-- a phenomenon that has been reported also experimentally. The theory is
extended in two directions pertaining to the experimental and technological
relevance of such premicellar aggregates. (a) Combining the thermodynamic model
with reaction rate theory, we calculate the lifetime of the metastable
aggregates. (b) Aggregation number fluctuations are examined. We demonstrate
that, over most of the metastable concentration range, the premicellar
aggregates should have macroscopic lifetimes and small polydispersity.Comment: 7 pages, 2 figure
Probing Split Supersymmetry with Cosmic Rays
A striking aspect of the recently proposed split supersymmetry is the
existence of heavy gluinos which are metastable because of the very heavy
squarks which mediate their decay. In this paper we correlate the expected flux
of these particles with the accompanying neutrino flux produced in inelastic
collisions in distant astrophysical sources. We show that an event rate at
the Pierre Auger Observatory of approximately 1 yr for gluino masses of
about 500 GeV is consistent with existing limits on neutrino fluxes. The
extremely low inelasticity of the gluino-containing hadrons in their collisions
with the air molecules makes possible a distinct characterization of the
showers induced in the atmosphere. Should such anomalous events be observed, we
show that their cosmogenic origin, in concert with the requirement that they
reach the Earth before decay, leads to a lower bound on their proper lifetime
of the order of 100 years, and consequently, to a lower bound on the scale of
supersymmetry breaking, GeV. Obtaining
such a bound is not possible in collider experiments.Comment: Version to be published in Phys. Rev.
Nanoscale surface relaxation of a membrane stack
Recent measurements of the short-wavelength (~ 1--100 nm) fluctuations in
stacks of lipid membranes have revealed two distinct relaxations: a fast one
(decay rate of ~ 0.1 ns^{-1}), which fits the known baroclinic mode of bulk
lamellar phases, and a slower one (~ 1--10 \mu s^{-1}) of unknown origin. We
show that the latter is accounted for by an overdamped capillary mode,
depending on the surface tension of the stack and its anisotropic viscosity. We
thereby demonstrate how the dynamic surface tension of membrane stacks could be
extracted from such measurements.Comment: 4 page
TeV gamma-rays from photo-disintegration/de-excitation of cosmic-ray nuclei
It is commonly assumed that high-energy gamma-rays are made via either purely
electromagnetic processes or the hadronic process of pion production, followed
by decay. We investigate astrophysical contexts where a third process (A*) may
dominate, namely the photo-disintegration of highly boosted nuclei followed by
daughter de-excitation. Starbust regions such as Cygnus OB2 appear to be
promising sites for TeV gamma-ray emission via this mechanism. A unique feature
of the A* process is a sharp energy minimum ~ 10 TeV/(T/eV) for gamma-ray
emission from a thermal region of temperature T. We also check that a diffuse
gamma-ray component resulting from the interaction of a possible extreme-energy
cosmic-ray nuclei with background radiation is well below the observed EGRET
data. The A* mechanism described herein offers an important contribution to
gamma-ray astronomy in the era of intense observational activity.Comment: To be published in Phys. Rev. Let
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