42,504 research outputs found
The Effect of Combined Magnetic Geometries on Thermally Driven Winds I: Interaction of Dipolar and Quadrupolar Fields
Cool stars with outer convective envelopes are observed to have magnetic
fields with a variety of geometries, which on large scales are dominated by a
combination of the lowest order fields such as the dipole, quadrupole and
octupole modes. Magnetised stellar wind outflows are primarily responsible for
the loss of angular momentum from these objects during the main sequence.
Previous works have shown the reduced effectiveness of the stellar wind braking
mechanism with increasingly complex, but singular, magnetic field geometries.
In this paper, we quantify the impact of mixed dipolar and quadrupolar fields
on the spin-down torque using 50 MHD simulations with mixed field, along with
10 of each pure geometries. The simulated winds include a wide range of
magnetic field strength and reside in the slow-rotator regime. We find that the
stellar wind braking torque from our combined geometry cases are well described
by a broken power law behaviour, where the torque scaling with field strength
can be predicted by the dipole component alone or the quadrupolar scaling
utilising the total field strength. The simulation results can be scaled and
apply to all main-sequence cool stars. For Solar parameters, the lowest order
component of the field (dipole in this paper) is the most significant in
determining the angular momentum loss.Comment: 15 pages + 9 figures (main), 3 pages + 1 figure (appendix), accepted
for publication to Ap
Dynamical chiral symmetry breaking with Minkowski space integral representations
The fermion propagator is studied in the whole Minkowski space with the help
of the Schwinger-Dyson equations. Various integral representations are employed
to get solutions for the dynamical breaking of chiral symmetry in different
regimes of the coupling constant. In particular, in the case of massive boson,
we extend the singularity structure of the fermion propagator to the two real
pole Ansatz.Comment: 4 pages, 4 figures, published version in PR
Magneto-optical conductivity in graphene including electron-phonon coupling
We show how coupling to an Einstein phonon affects the absorption
peaks seen in the optical conductivity of graphene under a magnetic field .
The energies and widths of the various lines are shifted, and additional peaks
arise in the spectrum. Some of these peaks are Holstein sidebands, resulting
from the transfer of spectral weight in each Landau level (LL) into
phonon-assisted peaks in the spectral function. Other additional absorption
peaks result from transitions involving split LLs, which occur when a LL falls
sufficiently close to a peak in the self-energy. We establish the selection
rules for the additional transitions and characterize the additional absorption
peaks. For finite chemical potential, spectral weight is asymmetrically
distributed about the Dirac point; we discuss how this causes an asymmetry in
the transitions due to left- and right-handed circularly polarized light and
therefore oscillatory behavior in the imaginary part of the off-diagonal Hall
conductivity. We also find that the semiclassical cyclotron resonance region is
renormalized by an effective-mass factor but is not directly affected by the
additional transitions. Last, we discuss how the additional transitions can
manifest in broadened, rather than split, absorption peaks due to large
scattering rates seen in experiment.Comment: 24 pages, 21 figure
Phonon structures in the electronic density of states of graphene in magnetic field
Unlike in ordinary metals, in graphene, phonon structure can be seen in the
quasiparticle electronic density of states, because the latter varies on the
scale of the phonon energy. In a magnetic field, quantization into Landau
levels creates even more significant variations. We calculate the density of
states incorporating electron-phonon coupling in this case and find that the
coupling has pronounced new effects: shifting and broadening of Landau levels,
creation of new peaks, and splitting of any Landau levels falling near one of
the new peaks. Comparing our calculations with a recent experiment, we find
evidence for a phonon with energy similar to but somewhat greater than the
optical mode and a coupling corresponding to a mass enhancement
parameter .Comment: 6 pages, 4 figures, final version to be published in EP
Modelling strong interactions and longitudinally polarized vector boson scattering
We study scattering of the electroweak gauge bosons in 5D warped models.
Within two different models we determine the precise manner in which the Higgs
boson and the vector resonances ensure the unitarity of longitudinal vector
boson scattering. We identify three separate scales that determine the dynamics
of the scattering process in all cases. For a quite general background geometry
of 5D, these scales can be linked to a simple functional of the warp factor.
The models smoothly interpolate between a `composite' Higgs limit and a
Higgsless limit. By holographic arguments, these models provide an effective
description of vector boson scattering in 4D models with a strongly coupled
electroweak breaking sector.Comment: 30 pages, no figure
The Effect of Magnetic Variability on Stellar Angular Momentum Loss II: The Sun, 61 Cygni A, Eridani, Bootis A and Bootis A
The magnetic fields of low-mass stars are observed to be variable on decadal
timescales, ranging in behaviour from cyclic to stochastic. The changing
strength and geometry of the magnetic field should modify the efficiency of
angular momentum loss by stellar winds, but this has not been well quantified.
In Finley et al. (2018) we investigated the variability of the Sun, and
calculated the time-varying angular momentum loss rate in the solar wind. In
this work, we focus on four low-mass stars that have all had their surface
magnetic fields mapped for multiple epochs. Using mass loss rates determined
from astrospheric Lyman- absorption, in conjunction with scaling
relations from the MHD simulations of Finley & Matt (2018), we calculate the
torque applied to each star by their magnetised stellar winds. The variability
of the braking torque can be significant. For example, the largest torque for
Eri is twice its decadal averaged value. This variation is
comparable to that observed in the solar wind, when sparsely sampled. On
average, the torques in our sample range from 0.5-1.5 times their average
value. We compare these results to the torques of Matt et al. (2015), which use
observed stellar rotation rates to infer the long-time averaged torque on
stars. We find that our stellar wind torques are systematically lower than the
long-time average values, by a factor of ~3-30. Stellar wind variability
appears unable to resolve this discrepancy, implying that there remain some
problems with observed wind parameters, stellar wind models, or the long-term
evolution models, which have yet to be understood.Comment: 15 pages + 8 figures, accepted for publication to Ap
Towards a generalized theory of low-frequency sound source localization
Low-frequency sound source localization generates considerable amount of disagreement between audio/acoustics researchers, with some arguing that below a certain frequency humans cannot localize a source with others insisting that in certain cases localization is possible, even down to the lowest audible of frequencies. Nearly all previous work in this area depends on subjective evaluations to formulate theorems for low-frequency localization. This, of course, opens the argument of data reliability, a critical factor that may go some way to explain the reported ambiguities with regard to low-frequency localization. The resulting proposal stipulates that low-frequency source localization is highly dependent on room dimensions, source/listener location and absorptive properties. In some cases, a source can be accurately localized down to the lowest audible of frequencies, while in other situations it cannot. This is relevant as the standard procedure in live sound reinforcement, cinema sound and home-theater surround sound is to have a single mono channel for the low-frequency content, based on the assumption that human’s cannot determine direction in this band. This work takes the first steps towards showing that this may not be a universally valid simplification and that certain sound reproduction systems may actually benefit from directional low-frequency content
General criterion for oblivious remote state preparation
A necessary and sufficient condition is given for general exact remote state
preparation (RSP) protocols to be oblivious, that is, no information about the
target state can be retrieved from the classical message. A novel criterion in
terms of commutation relations is also derived for the existence of
deterministic exact protocols in which Alice's measurement eigenstates are
related to each other by fixed linear operators similar to Bob's unitaries. For
non-maximally entangled resources, it provides an easy way to search for RSP
protocols. As an example, we show how to reduce the case of partially entangled
resources to that of maximally entangled ones, and we construct RSP protocols
exploiting the structure of the irreducible representations of Abelian groups.Comment: 5 pages, RevTe
Illuminating the 1/x moment of parton distribution functions
The Weisberger relation, an exact statement of the parton model, elegantly
relates a high-energy physics observable, the 1/x moment of parton distribution
functions, to a nonperturbative low-energy observable: the dependence of the
nucleon mass on the value of the quark mass or its corresponding quark
condensate. We show that contemporary fits to nucleon structure functions fail
to determine this 1/x moment; however, deeply virtual Compton scattering can be
described in terms of a novel F_{1/x}(t) form factor which illuminates this
physics. An analysis of exclusive photon-induced processes in terms of the
parton-nucleon scattering amplitude with Regge behavior reveals a failure of
the high Q^2 factorization of exclusive processes at low t in terms of the
Generalized Parton-Distribution Functions which has been widely believed to
hold in the past. We emphasize the need for more data for the DVCS process at
large t in future or upgraded facilities.Comment: 11 pages, 3 figures, invited contribution to the 11th International
Conference on Meson-Nucleon Physics and the Structure of the Nucleon, sept.
10th-14th 2007, Juelich, German
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