10,666 research outputs found
The large CP phase in B(s)-anti-B(s) mixing from primary scalar unparticles
In this letter we consider the case of primary scalar unparticle
contributions to B(d,s) mixing. With particular emphasis on the impact of the
recent hint of new physics in the measurement of the B(s) mixing phase, phi(s),
we determine the allowed parameter space and impose bounds on the unparticle
couplings.Comment: 8 pages, 8 jpeg figures, using pdflatex. Typo corrected, reference
adde
Spin wave excitations: The main source of the temperature dependence of Interlayer exchange coupling in nanostructures
Quantum mechanical calculations based on an extended Heisenberg model are
compared with ferromagnetic resonance (FMR) experiments on prototype trilayer
systems Ni_7/Cu_n/Co_2/Cu(001) in order to determine and separate for the first
time quantitatively the sources of the temperature dependence of interlayer
exchange coupling. Magnon excitations are responsible for about 75% of the
reduction of the coupling strength from zero to room temperature. The remaining
25% are due to temperature effects in the effective quantum well and the
spacer/magnet interfaces.Comment: accepted for publication in PR
CP asymmetries at D0
Using two independent measurements of the semileptonic CP asymmetry in the
system, we constrain the CP violating phase of the system to be
. The data sample corresponds to an integrated
luminosity of 1.1 fb accumulated with D0 detector at the Fermilab
Tevatron collider. We also measure the direct CP violating asymmetry in the
decay to be (stat)(syst). The data corresponds to an integrated
luminosity of 1.6 fb.Comment: contributed paper to EPS07, Manchester, UK, manuscript number
EPSHEPP17
Chiral Condensate and Short-Time Evolution of QCD(1+1) on the Light-Cone
Chiral condensates in the trivial light-cone vacuum emerge if defined as
short-time limits of fermion propagators. In gauge theories, the necessary
inclusion of a gauge string in combination with the characteristic light-cone
infrared singularities contain the relevant non-perturbative ingredients
responsible for formation of the condensate, as demonstrated for the 't Hooft
model.Comment: 4 pages, Revtex
Limitations of rapid myelin water quantification using 3D bSSFP
Object: Imaging of the myelin water fraction (MWF) is conventionally performed using a multi-echo spin-echo sequence. This technique requires long acquisition times and therefore often suffers from a lack of volume coverage. In this work, the application of 3D balanced steady-state free precession (bSSFP) sequences to extract high-resolution myelin water maps is discussed. Materials and Methods: Based on a two-pool water exchange model, an approximate bSSFP signal equation is derived and applied to fit the flip angle dependence of the invivo bSSFP signal. Thereby, the MWF and signal amplitude are fitted, while a priori assumptions are made for the other parameters of the two-pool system. Results: The effects of magnetization transfer, finite RF pulses, B 0 and B 1 inhomogeneities, as well as variation of the constant fit parameters, are investigated. Acquisition and calculation of quantitative, high-resolution MWF maps from white matter of healthy volunteers based on bSSFP is feasible and averaged MWF fit results agree with literature. However, results from numerical simulations indicate a severe dependence of the derived MWF values on the constant two-pool parameters. Conclusion: The demonstrated dependence of the MWF on the two-pool parameters considerably impairs the applicability of the proposed metho
Requirements for contractility in disordered cytoskeletal bundles
Actomyosin contractility is essential for biological force generation, and is
well understood in highly organized structures such as striated muscle.
Additionally, actomyosin bundles devoid of this organization are known to
contract both in vivo and in vitro, which cannot be described by standard
muscle models. To narrow down the search for possible contraction mechanisms in
these systems, we investigate their microscopic symmetries. We show that
contractile behavior requires non-identical motors that generate large enough
forces to probe the nonlinear elastic behavior of F-actin. This suggests a role
for filament buckling in the contraction of these bundles, consistent with
recent experimental results on reconstituted actomyosin bundles.Comment: 10 pages, 6 figures; text shortene
Directed transport and localization in phase-modulated driven lattices
We explore the dynamics of non-interacting particles loaded into a
phase-modulated one-dimensional lattice formed by laterally oscillating square
barriers. Tuning the parameters of the driven unit cell of the lattice selected
parts of the classical phase space can be manipulated in a controllable manner.
We find superdiffusion in position space for all parameters regimes. A directed
current of an ensemble of particles can be created through locally breaking the
spatiotemporal symmetries of the time-driven potential. Magnitude and direction
of the current are tunable. Several mechanisms for transient localization and
trapping of particles in different wells of the driven unit cell are presented
and analyzed
Quantum Electrodynamics in the Light-Front Weyl Gauge
We examine QED(3+1) quantised in the `front form' with finite `volume'
regularisation, namely in Discretised Light-Cone Quantisation. Instead of the
light-cone or Coulomb gauges, we impose the light-front Weyl gauge . The
Dirac method is used to arrive at the quantum commutation relations for the
independent variables. We apply `quantum mechanical gauge fixing' to implement
Gau{\ss}' law, and derive the physical Hamiltonian in terms of unconstrained
variables. As in the instant form, this Hamiltonian is invariant under global
residual gauge transformations, namely displacements. On the light-cone the
symmetry manifests itself quite differently.Comment: LaTeX file, 30 pages (A4 size), no figures. Submitted to Physical
review D. January 18, 1996. Originally posted, erroneously, with missing
`Weyl' in title. Otherwise, paper is identica
A 3+1 Dimensional Light-Front Model with Spontaneous Breaking of Chiral Symmetry
We investigate a 3+1 dimensional toy model that exhibits spontaneous
breakdown of chiral symmetry, both in a light-front (LF) Hamiltonian and in a
Euclidean Schwinger-Dyson (SD) formulation. We show that both formulations are
completely equivalent --- provided the renormalization is properly done. For
the model considered, this means that if one uses the same transverse momentum
cutoff on the SD and LF formulations then the vertex mass in the LF calculation
must be taken to be the same as the current quark mass in the SD calculation.
The kinetic mass term in the LF calculation is renormalized non-trivially,
which is eventually responsible for the mass generation of the physical fermion
of the model.Comment: 6 pages, REVTE
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