14,759 research outputs found
Theoretical study of turbulent channel flow: Bulk properties, pressure fluctuations, and propagation of electromagnetic waves
In this paper, we apply two theoretical turbulence models, DIA and the recent GISS model, to study properties of a turbulent channel flow. Both models provide a turbulent kinetic energy spectral function E(k) as the solution of a non-linear equation; the two models employ the same source function but different closures. The source function is characterized by a rate n sub s (k) which is derived from the complex eigenvalues of the Orr--Sommerfeld (OS) equation in which the basic flow is taken to be of a Poiseuille type. The O--S equation is solved for a variety of Reynolds numbers corresponding to available experimental data. A physical argument is presented whereby the central line velocity characterizing the basic flow, U0 sup L, is not to be identified with the U0 appearing in the experimental Reynolds number. The theoretical results are compared with two types of experimental data: (1) turbulence bulk properties, and (2) properties that depend stongly on the structure of the turbulence spectrun at low wave numbers. The only existing analytical expression for Pi (k) cannot be used in the present case because it applies to the case of a flat plate, not a finite channel
Identification of Neutral B Mesons Using Correlated Hadrons
The identification of the flavor of a neutral meson can make use of
hadrons produced nearby in phase space. Examples include the decay of
``'' resonances or the production of hadrons as a result of the
fragmentation process. Some aspects of this method are discussed, including
time-dependent effects in neutral decays to flavor states, to eigenstates
of CP and to other states, and the effects of possible coherence between
and in the initial state. We study the behavior of the leading
hadrons in -quark jets and the expected properties of resonances.
These are extrapolated from the corresponding resonances, of whose
properties we suggest further studies.Comment: To be submitted to Phys. Rev. D. 26 pages, LaTeX, figures not
included (available upon request). Technion-PH-93-32 / EFI 93-4
Brief review of the searches for the rare decays and
The current experimental status of the searches for the very rare decays
and is discussed.
These channels are highly sensitive to various extensions of the Standard
Model, specially in the scalar and pseudoscalar sector. The recent, most
sensitive measurements from the CDF, ATLAS, CMS and LHCb collaborations are
discussed and the combined upper exclusion limit on the branching fractions
determined by the LHC experiments is shown to be for and for . The implications of these tight bounds on a selected set of New Physics
models is sketched.Comment: 20 pages, 15 figures, invited review for Modern Physics Letters
Doppler peaks from active perturbations
We examine how the qualitative structure of the Doppler peaks in the angular
power spectrum of the cosmic microwave anisotropy depends on the fundamental
nature of the perturbations which produced them. The formalism of Hu and
Sugiyama is extended to treat models with cosmic defects. We discuss how
perturbations can be ``active'' or ``passive'' and ``incoherent'' or
``coherent'', and show how causality and scale invariance play rather different
roles in these various cases. We find that the existence of secondary Doppler
peaks and the rough placing of the primary peak unambiguously reflect these
basic properties.Comment: uufile, 8pages, 3 figures. Now available at
http://euclid.tp.ph/Papers/index.html; Changes: URL added, Eqn. (8) expanded,
grant numbers include
Transport signatures of quasiparticle poisoning in a Majorana island
We investigate effects of quasiparticle poisoning in a Majorana island with
strong tunnel coupling to normal-metal leads. In addition to the main Coulomb
blockade diamonds, "shadow" diamonds appear, shifted by 1e in gate voltage,
consistent with transport through an excited (poisoned) state of the island.
Comparison to a simple model yields an estimate of parity lifetime for the
strongly coupled island (~ 1 {\mu}s) and sets a bound for a weakly coupled
island (> 10 {\mu}s). Fluctuations in the gate-voltage spacing of Coulomb peaks
at high field, reflecting Majorana hybridization, are enhanced by the reduced
lever arm at strong coupling. In energy units, fluctuations are consistent with
previous measurements.Comment: includes supplementary materia
Initial Conditions for Bubble Universes
The "bubble universes" of Coleman and De Luccia play a crucial role in string
cosmology. Since our own Universe is supposed to be of this kind, bubble
cosmology should supply definite answers to the long-standing questions
regarding cosmological initial conditions. In particular, it must explain how
an initial singularity is avoided, and also how the initial conditions for
Inflation were established. We argue that the simplest non-anthropic approach
to these problems involves a requirement that the spatial sections defined by
distinguished bubble observers should not be allowed to have arbitrarily small
volumes. Casimir energy is a popular candidate for a quantum effect which can
ensure this, but [because it violates energy conditions] there is a danger that
it could lead to non-perturbative instabilities in string theory. We make a
simple proposal for the initial conditions of a bubble universe, and show that
our proposal ensures that the system is non-perturbatively stable. Thus,
low-entropy conditions can be established at the beginning of a bubble universe
without violating the Second Law of thermodynamics and without leading to
instability in string theory. These conditions are inherited from the ambient
spacetime.Comment: Further clarifications; 28 pages including three eps files. This is
the final [accepted for publication] versio
Parity lifetime of bound states in a proximitized semiconductor nanowire
Quasiparticle excitations can compromise the performance of superconducting
devices, causing high frequency dissipation, decoherence in Josephson qubits,
and braiding errors in proposed Majorana-based topological quantum computers.
Quasiparticle dynamics have been studied in detail in metallic superconductors
but remain relatively unexplored in semiconductor-superconductor structures,
which are now being intensely pursued in the context of topological
superconductivity. To this end, we introduce a new physical system comprised of
a gate-confined semiconductor nanowire with an epitaxially grown superconductor
layer, yielding an isolated, proximitized nanowire segment. We identify
Andreev-like bound states in the semiconductor via bias spectroscopy, determine
the characteristic temperatures and magnetic fields for quasiparticle
excitations, and extract a parity lifetime (poisoning time) of the bound state
in the semiconductor exceeding 10 ms.Comment: text and supplementary information combine
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