14,591 research outputs found
Are Simple Real Pole Solutions Physical?
We consider exact solutions generated by the inverse scattering technique,
also known as the soliton transformation. In particular, we study the class of
simple real pole solutions. For quite some time, those solutions have been
considered interesting as models of cosmological shock waves. A coordinate
singularity on the wave fronts was removed by a transformation which induces a
null fluid with negative energy density on the wave front. This null fluid is
usually seen as another coordinate artifact, since there seems to be a general
belief that that this kind of solution can be seen as the real pole limit of
the smooth solution generated with a pair of complex conjugate poles in the
transformation. We perform this limit explicitly, and find that the belief is
unfounded: two coalescing complex conjugate poles cannot yield a solution with
one real pole. Instead, the two complex conjugate poles go to a different
limit, what we call a ``pole on a pole''. The limiting procedure is not unique;
it is sensitive to how quickly some parameters approach zero. We also show that
there exists no improved coordinate transformation which would remove the
negative energy density. We conclude that negative energy is an intrinsic part
of this class of solutions.Comment: 13 pages, 3 figure
Particles held by springs in a linear shear flow exhibit oscillatory motion
The dynamics of small spheres, which are held by linear springs in a low
Reynolds number shear flow at neighboring locations is investigated. The flow
elongates the beads and the interplay of the shear gradient with the nonlinear
behavior of the hydrodynamic interaction among the spheres causes in a large
range of parameters a bifurcation to a surprising oscillatory bead motion. The
parameter ranges, wherein this bifurcation is either super- or subcritical, are
determined.Comment: 4 pages, 5 figure
Thermodynamics of two lattice ice models in three dimensions
In a recent paper we introduced two Potts-like models in three dimensions,
which share the following properties: (A) One of the ice rules is always
fulfilled (in particular also at infinite temperature). (B) Both ice rules hold
for groundstate configurations. This allowed for an efficient calculation of
the residual entropy of ice I (ordinary ice) by means of multicanonical
simulations. Here we present the thermodynamics of these models. Despite their
similarities with Potts models, no sign of a disorder-order phase transition is
found.Comment: 5 pages, 7 figure
The true reinforced random walk with bias
We consider a self-attracting random walk in dimension d=1, in presence of a
field of strength s, which biases the walker toward a target site. We focus on
the dynamic case (true reinforced random walk), where memory effects are
implemented at each time step, differently from the static case, where memory
effects are accounted for globally. We analyze in details the asymptotic
long-time behavior of the walker through the main statistical quantities (e.g.
distinct sites visited, end-to-end distance) and we discuss a possible mapping
between such dynamic self-attracting model and the trapping problem for a
simple random walk, in analogy with the static model. Moreover, we find that,
for any s>0, the random walk behavior switches to ballistic and that field
effects always prevail on memory effects without any singularity, already in
d=1; this is in contrast with the behavior observed in the static model.Comment: to appear on New J. Phy
The RFOFO Ionization Cooling Ring for Muons
Practical ionization cooling rings could lead to lower cost or improved
performance in neutrino factory or muon collider designs. The ring modeled here
uses realistic three-dimensional fields. The performance of the ring compares
favorably with the linear cooling channel used in the second US Neutrino
Factory Study. The normalized 6D emittance of an ideal ring is decreased by a
factor of approximately 240, compared with a factor of only 15 for the linear
channel. We also examine such \textit{real-world} effects as windows on the
absorbers and rf cavities and leaving empty lattice cells for injection and
extraction. For realistic conditions the ring decreases the normalized 6D
emittance by a factor of 49.Comment: 27 pages, 18 figures and 5 tables. Submitted to Phys. Rev. ST-A
Gravitational diffraction radiation
We show that if the visible universe is a membrane embedded in a
higher-dimensional space, particles in uniform motion radiate gravitational
waves because of spacetime lumpiness. This phenomenon is analogous to the
electromagnetic diffraction radiation of a charge moving near to a metallic
grating. In the gravitational case, the role of the metallic grating is played
by the inhomogeneities of the extra-dimensional space, such as a hidden brane.
We derive a general formula for gravitational diffraction radiation and apply
it to a higher-dimensional scenario with flat compact extra dimensions.
Gravitational diffraction radiation may carry away a significant portion of the
particle's initial energy. This allows to set stringent limits on the scale of
brane perturbations. Physical effects of gravitational diffraction radiation
are briefly discussed.Comment: 5 pages, 2 figures, RevTeX4. v2: References added. Version to appear
in Phys. Rev.
Theta dependence of CP^9 model
We apply to the model two recently proposed numerical techniques for
simulation of systems with a theta term. The algorithms, successfully tested in
the strong coupling limit, are applied to the weak coupling region. The results
agree and errors have been evaluated and are at % level. The results scale well
with the renormalization group equation and show that, for in presence
of a theta term, CP symmetry is spontaneously broken at in the
continuum limit.Comment: 4 pages, 4 figure
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