13,806 research outputs found
Higgs Triplets, Decoupling, and Precision Measurements
Electroweak precision data has been extensively used to constrain models
containing physics beyond that of the Standard Model. When the model contains
Higgs scalars in representations other than SU(2) singlets or doublets, and
hence rho not equal to one at tree level, a correct renormalization scheme
requires more inputs than the three needed for the Standard Model. We discuss
the connection between the renormalization of models with Higgs triplets and
the decoupling properties of the models as the mass scale for the scalar
triplet field becomes much larger than the electroweak scale. The requirements
of perturbativity of the couplings and agreement with electroweak data place
strong restrictions on models with Higgs triplets. Our results have important
implications for Little Higgs type models and other models with rho not equal
to one at tree level.Comment: 23 page
FISCAL FORESIGHT AND INFORMATION FLOWS
Fiscal foresight---the phenomenon that legislative and implementation lags ensure that private agents receive clear signals about the tax rates they face in the future---is intrinsic to the tax policy process. This paper develops an analytical framework to study the econometric implications of fiscal foresight. Simple theoretical examples show that foresight produces equilibrium time series with nonfundamental representations, which misalign the agents' and the econometrician's information sets. Economically meaningful shocks to taxes, therefore, cannot generally be extracted from statistical innovations in conventional ways. Econometric analyses that fail to align agents' and the econometrician's information sets can produce distorted inferences about the effects of tax policies. The paper documents the sensitivity of econometric inferences of tax effects to details about how tax information flows into the economy. We show that alternative assumptions about the information flows that give rise to fiscal foresight can reconcile the diverse empirical findings in the literature on anticipated tax changes.
Electron tomography at 2.4 {\AA} resolution
Transmission electron microscopy (TEM) is a powerful imaging tool that has
found broad application in materials science, nanoscience and biology(1-3).
With the introduction of aberration-corrected electron lenses, both the spatial
resolution and image quality in TEM have been significantly improved(4,5) and
resolution below 0.5 {\AA} has been demonstrated(6). To reveal the 3D structure
of thin samples, electron tomography is the method of choice(7-11), with
resolutions of ~1 nm^3 currently achievable(10,11). Recently, discrete
tomography has been used to generate a 3D atomic reconstruction of a silver
nanoparticle 2-3 nm in diameter(12), but this statistical method assumes prior
knowledge of the particle's lattice structure and requires that the atoms fit
rigidly on that lattice. Here we report the experimental demonstration of a
general electron tomography method that achieves atomic scale resolution
without initial assumptions about the sample structure. By combining a novel
projection alignment and tomographic reconstruction method with scanning
transmission electron microscopy, we have determined the 3D structure of a ~10
nm gold nanoparticle at 2.4 {\AA} resolution. While we cannot definitively
locate all of the atoms inside the nanoparticle, individual atoms are observed
in some regions of the particle and several grains are identified at three
dimensions. The 3D surface morphology and internal lattice structure revealed
are consistent with a distorted icosahedral multiply-twinned particle. We
anticipate that this general method can be applied not only to determine the 3D
structure of nanomaterials at atomic scale resolution(13-15), but also to
improve the spatial resolution and image quality in other tomography
fields(7,9,16-20).Comment: 27 pages, 17 figure
Quark energy loss and shadowing in nuclear Drell-Yan process
The energy loss effect in nuclear matter is another nuclear effect apart from
the nuclear effects on the parton distribution as in deep inelastic scattering
process. The quark energy loss can be measured best by the nuclear dependence
of the high energy nuclear Drell-Yan process. By means of three kinds of quark
energy loss parameterizations given in literature and the nuclear parton
distribution extracted only with lepton-nucleus deep inelastic scattering
experimental data, measured Drell-Yan production cross sections are analyzed
for 800GeV proton incident on a variety of nuclear targets from FNAL E866. It
is shown that our results with considering the energy loss effect are much
different from these of the FNAL E866 who analysis the experimental data with
the nuclear parton distribution functions obtained by using the deep inelastic
lA collisions and pA nuclear Drell-Yan data . Considering the existence of
energy loss effect in Drell-Yan lepton pairs production,we suggest that the
extraction of nuclear parton distribution functions should not include
Drell-Yan experimental data.Comment: 12 page
Breakdown of the lattice polaron picture in La0.7Ca0.3MnO3 single crystals
When heated through the magnetic transition at Tc, La0.7Ca0.3MnO3 changes
from a band metal to a polaronic insulator. The Hall constant R_H, through its
activated behavior and sign anomaly, provides key evidence for polaronic
behavior. We use R_H and the Hall mobility to demonstrate the breakdown of the
polaron phase. Above 1.4Tc, the polaron picture holds in detail, while below,
the activation energies of both R_H and the mobility deviate strongly from
their polaronic values. These changes reflect the presence of metallic,
ferromagnetic fluctuations, in the volume of which the Hall effect develops
additional contributions tied to quantal phases.Comment: 11 pages, 3 figures, final version to appear in Phys. Rev. B Rapi
Berry's phase contribution to the anomalous Hall effect of gadolinium
When conduction electrons are forced to follow the local spin texture, the
resulting Berry phase can induce an anomalous Hall effect (AHE). In gadolinium,
as in double-exchange magnets, the exchange interaction is mediated by the
conduction electrons and the AHE may therefore resemble that of chromium
dioxide and other metallic double-exchange ferromagnets. The Hall resistivity,
magnetoresistance, and magnetization of single crystal gadolinium were measured
in fields up to 30 T. Measurements between 2 K and 400 K are consistent with
previously reported data. A scaling analysis for the Hall resistivity as a
function of the magnetization suggests the presence of a Berry's-phase
contribution to the anomalous Hall effect.Comment: 6 pages, 7 figures, submitted to Phys. Rev.
Reentrant spin glass behavior in a layered manganite La1.2Sr1.8Mn2O7 single crystals
We report here a detailed study of AC/DC magnetization and
longitudinal/transverse transport properties of
LaSrMnO single crystals below = 121 K. We
find that the resistivity upturn below 40 K is related to the reentrant spin
glass phase at the same temperature, accompanied by additional anomalous Hall
effects. The carrier concentration from the ordinary Hall effects remains
constant during the transition and is close to the nominal doping level (0.4
holes/Mn). The spin glass behavior comes from the competition between
ferromagnetic double exchange and antiferromagnetic superexchange interactions,
which leads to phase separation, i.e. a mixture of ferromagnetic and
antiferromagnetic clusters, representing the canted antiferromagnetic state.Comment: 5 pages, 5 figures, submitted to Phys. Rev.
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