745 research outputs found
A dynamical collective calculation of supernova neutrino signals
We present the first calculations with three flavors of collective and shock
wave effects for neutrino propagation in core-collapse supernovae using
hydroynamical density profiles and the S matrix formalism. We explore the
interplay between the neutrino-neutrino interaction and the effects of multiple
resonances upon the time signal of positrons in supernova observatories. A
specific signature is found for the inverted hierarchy and a large third
neutrino mixing angle and we predict, in this case, a dearth of lower energy
positrons in Cherenkov detectors midway through the neutrino signal and the
simultaneous revelation of valuable information about the original fluxes. We
show that this feature is also observable with current generation neutrino
detectors at the level of several sigmas.Comment: 4 pages, 5 figure
Stimulated Neutrino Transformation with Sinusoidal Density Profiles
Large amplitude oscillations between the states of a quantum system can be
stimulated by sinusoidal external potentials with frequencies that are similar
to the energy level splitting of the states or a fraction thereof. Situations
when the applied frequency is equal to an integer fraction of the energy level
splittings are known as parametric resonances. We investigate this effect for
neutrinos both analytically and numerically for the case of arbitrary numbers
of neutrino flavors. We look for environments where the effect may be observed
and find that supernova are the one realistic possibility due to the necessity
of both large densities and large amplitude fluctuations. The comparison of
numerical and analytic results of neutrino propagation through a model
supernova reveals it is possible to predict the locations and strengths of the
stimulated transitions that occur.Comment: 14 pages, 6 figure
Holonomic constraints : an analytical result
Systems subjected to holonomic constraints follow quite complicated dynamics
that could not be described easily with Hamiltonian or Lagrangian dynamics. The
influence of holonomic constraints in equations of motions is taken into
account by using Lagrange multipliers. Finding the value of the Lagrange
multipliers allows to compute the forces induced by the constraints and
therefore, to integrate the equations of motions of the system. Computing
analytically the Lagrange multipliers for a constrained system may be a
difficult task that is depending on the complexity of systems. For complex
systems, it is most of the time impossible to achieve. In computer simulations,
some algorithms using iterative procedures estimate numerically Lagrange
multipliers or constraint forces by correcting the unconstrained trajectory. In
this work, we provide an analytical computation of the Lagrange multipliers for
a set of linear holonomic constraints with an arbitrary number of bonds of
constant length. In the appendix of the paper, one would find explicit formulas
for Lagrange multipliers for systems having 1, 2, 3, 4 and 5 bonds of constant
length, linearly connected.Comment: 13 pages, no figures. To appear in J. Phys. A : Math. The
BBN For Pedestrians
The simplest, `standard' model of Big Bang Nucleosynthesis (SBBN) assumes
three light neutrinos (N_nu = 3) and no significant electron neutrino
asymmetry, leaving only one adjustable parameter: the baryon to photon ratio
eta. The primordial abundance of any one nuclide can, therefore, be used to
measure the baryon abundance and the value derived from the observationally
inferred primordial abundance of deuterium closely matches that from current,
non-BBN data, primarily from the WMAP survey. However, using this same estimate
there is a tension between the SBBN-predicted 4He and 7Li abundances and their
current, observationally inferred primordial abundances, suggesting that N_nu
may differ from the standard model value of three and/or that there may be a
non-zero neutral lepton asymmetry (or, that systematic errors in the abundance
determinations have been underestimated or overlooked). The differences are not
large and the allowed ranges of the BBN parameters permitted by the data are
quite small. Within these ranges, the BBN-predicted abundances of D, 3He, 4He,
and 7Li are very smooth, monotonic functions of eta, N_nu, and the lepton
asymmetry. It is possible to describe the dependencies of these abundances (or
powers of them) upon the three parameters by simple, linear fits which, over
their ranges of applicability, are accurate to a few percent or better. The
fits presented here have not been maximized for their accuracy but, for their
simplicity. To identify the ranges of applicability and relative accuracies,
they are compared to detailed BBN calculations; their utility is illustrated
with several examples. Given the tension within BBN, these fits should prove
useful in facilitating studies of the viability of proposals for non-standard
physics and cosmology, prior to undertaking detailed BBN calculations.Comment: Submitted to a Focus Issue on Neutrino Physics in New Journal of
Physics (www.njp.org
Deep-water sediment wave formation: Linear stability analysis of coupled flow/bed interaction
International audienceA linear stability analysis is carried out for the interaction of an erodible sediment bed with a sediment-laden, stratified flow above the bed, such as a turbidity or bottom current. The fluid motion is described by the full, two-dimensional Navier-Stokes equations in the Boussinesq approximation, while erosion is modelled as a diffusive flux of particles from the bed into the fluid. The stability analysis shows the existence of both Tollmien-Schlichting and internal wave modes in the stratified boundary layer. For the internal wave mode, the stratified boundary layer acts as a wave duct, whose height can be determined analytically from the Brunt-Val frequency criterion. Consistent with this criterion, distinct unstable perturbation wavenumber regimes exist for the internal wave mode, which are associated with different numbers of pressure extrema in the wall-normal direction. For representative turbidity current parameters, the analysis predicts unstable wavelengths that are consistent with field observations. As a key condition for instability to occur, the base flow velocity boundary layer needs to be thinner than the corresponding concentration boundary layer. For most of the unstable wavenumber ranges, the phase relations between the sediment bed deformation and the associated wall shear stress and concentration perturbations are such that the sediment waves migrate in the upstream direction, which again is consistent with field observations. © 2011 Cambridge University Press
Trench-parallel flow and seismic anisotropy in the Mariana and Andean subduction systems
Shear- wave splitting measurements above the mantle wedge of the Mariana(1) and southern Andean(2,3) subduction zones show trench-parallel seismically fast directions close to the trench and abrupt rotations to trench- perpendicular anisotropy in the back arc. These patterns of seismic anisotropy may be caused by three-dimensional flow associated with along- strike variations in slab geometry(1-5). The Mariana and Andean subduction systems are associated with the largest along- strike variations of slab geometry observed on Earth(6,7) and are ideal for testing the link between slab geometry and solid- state creep processes in the mantle. Here we show, with fully three- dimensional non- newtonian subduction zone models, that the strong curvature of the Mariana slab and the transition to shallow slab dip in the Southern Andes give rise to strong trench- parallel stretching in the warm- arc and warm- back-arc mantle and to abrupt rotations in stretching directions that are accompanied by strong trench- parallel stretching. These models show that the patterns of shear- wave splitting observed in the Mariana and southern Andean systems may be caused by significant three- dimensional flow induced by along- strike variations in slab geometry.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62601/1/nature06429.pd
Neutrinos And Big Bang Nucleosynthesis
The early universe provides a unique laboratory for probing the frontiers of
particle physics in general and neutrino physics in particular. The primordial
abundances of the relic nuclei produced during the first few minutes of the
evolution of the Universe depend on the electron neutrinos through the
charged-current weak interactions among neutrons and protons (and electrons and
positrons and neutrinos), and on all flavors of neutrinos through their
contributions to the total energy density which regulates the universal
expansion rate. The latter contribution also plays a role in determining the
spectrum of the temperature fluctuations imprinted on the Cosmic Background
Radiation (CBR) some 400 thousand years later. Using deuterium as a baryometer
and helium-4 as a chronometer, the predictions of BBN and the CBR are compared
to observations. The successes of, as well as challenges to the standard models
of particle physics and cosmology are identified. While systematic
uncertainties may be the source of some of the current tensions, it could be
that the data are pointing the way to new physics. In particular, BBN and the
CBR are used to address the questions of whether or not the relic neutrinos
were fully populated in the early universe and, to limit the magnitude of any
lepton asymmetry which may be concealed in the neutrinos.Comment: Accepted for publication in the Proceedings of Nobel Symposium 129,
"Neutrino Physics"; to appear in Physics Scripta, eds., L Bergstrom, O.
Botner, P. Carlson, P. O. Hulth, and T. Ohlsso
Enhancement of the superconducting transition temperature in Nb/Permalloy bilayers by controlling the domain state of the ferromagnet
In (S/F) hybrids the suppression of superconductivity by the exchange field
h_ex of the ferromagnet can be partially lifted when different directions of
h_ex are sampled simultaneously by the Cooper pair. In F/S/F trilayer
geometries where the magnetization directions of the two F-layers can be
controlled separately, this leads to the so-called spin switch. Here we show
that domain walls in a single F-layer yield a similar effect. We study the
transport properties of Ni_0.8Fe_0.2/Nb bilayers structured in strips of
different sizes. For large samples a clear enhancement of superconductivity
takes place in the resistive transition, in the very narrow field range (order
of 0.5 mT) where the magnetization of the Py layer switches and many domains
are present. This effect is absent in microstructured samples. Comparison of
domain wall width \delta_w to the temperature dependent superconductor
coherence length \xi_S(T) shows that \delta_w ~ \xi_S(T), which means that the
Cooper pairs sample a large range of different magnetization directions.Comment: 4 pages, 5 figure
Exchange bias in Co/CoO core-shell nanowires: Role of the antiferromagnetic superparamagnetic fluctuations
The magnetic properties of Co (=15 nm, =130nm) nanowires are reported.
In oxidized wires, we measure large exchange bias fields of the order of 0.1 T
below T ~ 100 K. The onset of the exchange bias, between the ferromagnetic core
and the anti-ferromagnetic CoO shell, is accompanied by a coercivity drop of
0.2 T which leads to a minimum in coercivity at K. Magnetization
relaxation measurements show a temperature dependence of the magnetic viscosity
S which is consistent with a volume distribution of the CoO grains at the
surface. We propose that the superparamagnetic fluctuations of the
anti-ferromagnetic CoO shell play a key role in the flipping of the nanowire
magnetization and explain the coercivity drop. This is supported by
micromagnetic simulations. This behavior is specific to the geometry of a 1D
system which possesses a large shape anisotropy and was not previously observed
in 0D (spheres) or 2D (thin films) systems which have a high degree of symmetry
and low coercivities. This study underlines the importance of the AFM
super-paramagnetic fluctuations in the exchange bias mechanism.Comment: 10 pages, 10 figures, submitted to Phys. Rev.
Invalidation of the Kelvin Force in Ferrofluids
Direct and unambiguous experimental evidence for the magnetic force density
being of the form in a certain geometry - rather than being the
Kelvin force - is provided for the first time. (M is the
magnetization, H the field, and B the flux density.)Comment: 4 pages, 4 figure
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