15,396 research outputs found
Internal stress wave measurements in solids subjected to lithotripter pulses
Semiconductor strain gauges were used to measure the internal strain along the axes of spherical and disk plaster specimens when subjected to lithotripter shock pulses. The pulses were produced by one of two lithotripters. The first source generates spherically diverging shock waves of peak pressure approximately 1 MPa at the surface of the specimen. For this source, the incident and first reflected pressure (P) waves in both sphere and disk specimens were identified. In addition, waves reflected by the disk circumference were found to contribute significantly to the strain fields along the disk axis. Experimental results compared favorably to a ray theory analysis of a spherically diverging shock wave striking either concretion. For the sphere, pressure contours for the incident P wave and caustic lines were determined theoretically for an incident spherical shock wave. These caustic lines indicate the location of the highest stresses within the sphere and therefore the areas where damage may occur. Results were also presented for a second source that uses an ellipsoidal reflector to generate a 30-MPa focused shock wave, more closely approximating the wave fields of a clinical extracorporeal lithotripter
A SUSY A4 model for fermion masses and mixings
We study a supersymmetric extension of the Standard Model based on discrete
A4xZ3xZ4 flavor symmetry. We obtain quark mixing angles as well as a realistic
fermion mass spectrum and we predict tribimaximal leptonic mixing by a
spontaneous breaking of A4. The top quark Yukawa interaction is present at the
renormalizable level in the superpotential while all the other Yukawa
interactions arise only at higher orders. We study the Higgs potential and show
that it can potentially solve the so called vacuum alignment problem. The
leading order predictions are not spoiled by subleading corrections.Comment: version accepted in JHEP, Z3xZ2 changed in Z3xZ4, typos in table
corrected, references adde
Holes in the walls: primordial black holes as a solution to the cosmological domain wall problem
We propose a scenario in which the cosmological domain wall and monopole
problems are solved without any fine tuning of the initial conditions or
parameters in the Lagrangian of an underlying filed theory. In this scenario
domain walls sweep out (unwind) the monopoles from the early universe, then the
fast primordial black holes perforate the domain walls, change their topology
and destroy them. We find further that the (old vacuum) energy density released
from the domain walls could alleviate but not solve the cosmological flatness
problem.Comment: References added; Published in Phys. Rev.
Bose-Einstein condensates with attractive interactions on a ring
Considering an effectively attractive quasi-one-dimensional Bose-Einstein
condensate of atoms confined in a toroidal trap, we find that the system
undergoes a phase transition from a uniform to a localized state, as the
magnitude of the coupling constant increases. Both the mean-field
approximation, as well as a diagonalization scheme are used to attack the
problem.Comment: 4 pages, 4 ps figures, RevTex, typographic errors correcte
Near-Critical Gravitational Collapse and the Initial Mass Function of Primordial Black Holes
The recent discovery of critical phenomena arising in gravitational collapse
near the threshold of black hole formation is used to estimate the initial mass
function of primordial black holes (PBHs). It is argued that the universal
scaling relation between black hole mass and initial perturbation found for a
variety of collapsing space-times also applies to PBH formation, indicating the
possibility of the formation of PBHs with masses much smaller than one horizon
mass. Owing to the natural fine-tuning of initial conditions by the exponential
decline of the probability distribution for primordial density fluctuations,
sub-horizon mass PBHs are expected to form at all epochs. This result suggests
that the constraints on the primordial fluctuation spectrum based on the
abundance of PBHs at different mass scales may have to be revisited.Comment: 4 pages, uses revtex, also available at
http://bigwhirl.uchicago.edu/jcn/pub_pbh.html . To appear in Phys. Rev. Let
Cosmological expansion and local physics
The interplay between cosmological expansion and local attraction in a
gravitationally bound system is revisited in various regimes. First, weakly
gravitating Newtonian systems are considered, followed by various exact
solutions describing a relativistic central object embedded in a Friedmann
universe. It is shown that the ``all or nothing'' behaviour recently discovered
(i.e., weakly coupled systems are comoving while strongly coupled ones resist
the cosmic expansion) is limited to the de Sitter background. New exact
solutions are presented which describe black holes perfectly comoving with a
generic Friedmann universe. The possibility of violating cosmic censorship for
a black hole approaching the Big Rip is also discussed.Comment: 17 pages, LaTeX, to appear in Phys. Rev.
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