1,602,278 research outputs found
Limits on Lorentz violation from charged-pion decay
Charged-pion decay offers many opportunities to study Lorentz violation.
Using an effective field theory approach, we study Lorentz violation in the
lepton, W-boson, and quark sectors and derive the differential pion-decay rate,
including muon polarization. Using coordinate redefinitions we are able to
relate the first-generation quark sector, in which no bounds were previously
reported, to the lepton and W-boson sector. This facilitates a tractable
calculation, enabling us to place bounds on the level of on
first-generation quark parameters. Our expression for the pion-decay rate can
be used to constrain Lorentz violation in future experiments.Comment: 12 pages, 1 figure, Accepted for publication in Phys. Rev.
Craters as sand traps: Dynamics, history, and morphology of modern sand transport in an active Martian dune field
Aeolian transport of sand is abundant on modern-day Mars, as revealed by remote sensing measurements of the motion of dunes, and of the meter-scale ripples that mantle them. We study a large-scale natural sand trap within the Meroe Patera dune field: a 1.8-km diameter crater which features a dune-free “shadow” in its lee. We compare the volume of sand trapped within this crater to the sand volume that would be expected to cover the area of the crater and its dune-free shadow behind it if the crater were not present. We find that the crater holds less sand than this “missing” volume would predict, implying that sand escapes from the crater over time. Modern day imagery shows an apparent lack of sand escaping from the Meroe crater, however, suggesting that changes in the wind regime at the site may have allowed sand to escape in the past. The persistence of an altered dune morphology all the way to the far downwind edge of the dune field suggests consistent wind conditions over the time of the crater-dune field interaction
Effective Operator Treatment of the Lipkin Model
We analyze the Lipkin Model using effective operator techniques. We present
both analytical and numerical results for effective Hamiltonians. The accuracy
of the cluster approximation is investigated.Comment: To appear in Phys.Rev.
Axion-Dilaton Domain Walls and Fake Supergravity
Dynamical systems methods are used to investigate domain-wall solutions of a
two-parameter family of models in which gravity is coupled to an axion, and to
a dilaton with an exponential potential of either sign. A complete global
analysis is presented for (i) constant axion and (ii) flat walls, including a
study of bifurcations and a new exact domain-wall solution with non-constant
axion. We reconsider `fake supergravity' issues in light of these results. We
show, by example, how domain walls determine multi-valued superpotentials that
branch at stationary points that are not stationary points of the potential,
and we apply this result to potentials with anti-de Sitter vacua. We also show
by example that `adapted' truncation to a single-scalar model is sometimes
inconsistent, and we propose a `generalized' fake supergravity formalism that
applies in some such cases.Comment: 43pp, 19 figures; minor corrections and extensions; one additional
figur
Leonardo's rule, self-similarity and wind-induced stresses in trees
Examining botanical trees, Leonardo da Vinci noted that the total
cross-section of branches is conserved across branching nodes. In this Letter,
it is proposed that this rule is a consequence of the tree skeleton having a
self-similar structure and the branch diameters being adjusted to resist
wind-induced loads
Dynamical Symmetries Reflected in Realistic Interactions
Realistic nucleon-nucleon (NN) interactions, derived within the framework of
meson theory or more recently in terms of chiral effective field theory, yield
new possibilities for achieving a unified microscopic description of atomic
nuclei. Based on spectral distribution methods, a comparison of these
interactions to a most general Sp(4) dynamically symmetric interaction, which
previously we found to reproduce well that part of the interaction that is
responsible for shaping pairing-governed isobaric analog 0+ states, can
determine the extent to which this significantly simpler model Hamiltonian can
be used to obtain an approximate, yet very good description of low-lying
nuclear structure. And furthermore, one can apply this model in situations that
would otherwise be prohibitive because of the size of the model space. In
addition, we introduce a Sp(4) symmetry breaking term by including the
quadrupole-quadrupole interaction in the analysis and examining the capacity of
this extended model interaction to imitate realistic interactions. This
provides a further step towards gaining a better understanding of the
underlying foundation of realistic interactions and their ability to reproduce
striking features of nuclei such as strong pairing correlations or collective
rotational motion.Comment: 10 pages, 4 figures, Proceedings of the XXV International Workshop on
Nuclear Theory, June 26-July 1, 2006, Rila Mountains, Bulgari
Spectral microscopic mechanisms and quantum phase transitions in a 1D correlated problem
In this paper we study the dominant microscopic processes that generate
nearly the whole one-electron removal and addition spectral weight of the
one-dimensional Hubbard model for all values of the on-site repulsion . We
find that for the doped Mott-Hubbard insulator there is a competition between
the microscopic processes that generate the one-electron upper-Hubbard band
spectral-weight distributions of the Mott-Hubbard insulating phase and
finite-doping-concentration metallic phase, respectively. The spectral-weight
distributions generated by the non-perturbative processes studied here are
shown elsewhere to agree quantitatively for the whole momentum and energy
bandwidth with the peak dispersions observed by angle-resolved photoelectron
spectroscopy in quasi-one-dimensional compounds.Comment: 18 pages, 2 figure
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