545 research outputs found
Stabilization of the Electroweak Scale in 3-3-1 Models
One way of avoiding the destabilization of the electroweak scale through a
strong coupled regime naturally occurs in models with a Landau-like pole at the
TeV scale. Hence, the quadratic divergence contributions to the scalar masses
are not considered as a problem anymore since a new nonperturbative dynamic
emerges at the TeV scale. This scale should be an intrinsic feature of the
models and there is no need to invoke any other sort of protection for the
electroweak scale. In some models based on the gauge symmetry, a nonperturbative dynamics arise and it stabilizes
the electroweak scale.Comment: 10 pages. Version with some improvements and corrections in the tex
Vanishing Dimensions and Planar Events at the LHC
We propose that the effective dimensionality of the space we live in depends
on the length scale we are probing. As the length scale increases, new
dimensions open up. At short scales the space is lower dimensional; at the
intermediate scales the space is three-dimensional; and at large scales, the
space is effectively higher dimensional. This setup allows for some fundamental
problems in cosmology, gravity, and particle physics to be attacked from a new
perspective. The proposed framework, among the other things, offers a new
approach to the cosmological constant problem and results in striking collider
phenomenology and may explain elongated jets observed in cosmic-ray data.Comment: v1: 5 pages revtex, 1 eps figure; v2: includes extensive discussion
on violation of Lorentz invariance, featured in a Nature editorial [Nature
466 (2010) 426] http://www.nature.com/news/2010/100720/full/466426a.html; v3:
discussion expanded, matching journal versio
Noncommutative Geometry and the standard model with neutrino mixing
We show that allowing the metric dimension of a space to be independent of
its KO-dimension and turning the finite noncommutative geometry F-- whose
product with classical 4-dimensional space-time gives the standard model
coupled with gravity--into a space of KO-dimension 6 by changing the grading on
the antiparticle sector into its opposite, allows to solve three problems of
the previous noncommutative geometry interpretation of the standard model of
particle physics:
The finite geometry F is no longer put in "by hand" but a conceptual
understanding of its structure and a classification of its metrics is given.
The fermion doubling problem in the fermionic part of the action is resolved.
The spectral action of our joint work with Chamseddine now automatically
generates the full standard model coupled with gravity with neutrino mixing and
see-saw mechanism for neutrino masses. The predictions of the Weinberg angle
and the Higgs scattering parameter at unification scale are the same as in our
joint work but we also find a mass relation (to be imposed at unification
scale).Comment: Typos removed, to appear in JHE
Pole- versus MS-mass definitions in the electroweak theory
Two different two-loop relations between the pole- and the MS-mass of the top
quark have been derived in the literature which were based on different
treatments of the tadpole diagrams. In addition, the limit M_W^2/m_t^2 \to 0
was employed in one of the calculations. It is shown that, after appropriate
transformations, the results of the two calculations are in perfect agreement.
Furthermore we demonstrate that the inclusion of the non-vanishing mass of the
W-boson leads to small modifications only.Comment: 6 pages, 6 figures; references added, figure 1 change
Invisible Events with Radiative Photons at LEP
A study of the radiative neutrino counting reaction at LEP1 and LEP2 energies is presented. An approximate expression
for the spectrum of the observed photon is derived within the framework of the
-dependent structure function approach. This is compared with an exact
expression and found in agreement within the foreseen experimental accuracy.
This model describing single-photon radiation can be applied to the more
general case of initial-state single-photon emission accompanying invisible
final-state events. Higher-order QED corrections due to undetected
initial-state radiation are also included. The implementation in a Monte Carlo
event generator is briefly described.Comment: 10 pages, LaTeX, 7 figures available via anonymous ftp at:
ftp://cobra1.pv.infn.it/pub/phot/, files fig#n.ps with #=1,...,
The Equivalence Theorem and Effective Lagrangians
We point out that the equivalence theorem, which relates the amplitude for a
process with external longitudinally polarized vector bosons to the amplitude
in which the longitudinal vector bosons are replaced by the corresponding
pseudo-Goldstone bosons, is not valid for effective Lagrangians. However, a
more general formulation of this theorem also holds for effective interactions.
The generalized theorem can be utilized to determine the high-energy behaviour
of scattering processes just by power counting and to simplify the calculation
of the corresponding amplitudes. We apply this method to the phenomenologically
most interesting terms describing effective interactions of the electroweak
vector and Higgs bosons in order to examine their effects on vector-boson
scattering and on vector-boson-pair production in annihilation. The
use of the equivalence theorem in the literature is examined.Comment: 20 pages LaTeX, BI-TP 94/1
Reduced neural connectivity but increased task-related activity during working memory in de novo Parkinson patients
Objective: Patients with Parkinson's disease (PD) often suffer from impairments in executive functions, such as working memory deficits. It is widely held that dopamine depletion in the striatum contributes to these impairments through decreased activity and connectivity between task-related brain networks. We investigated this hypothesis by studying task-related network activity and connectivity within a sample of de novo patients with PD, versus healthy controls, during a visuospatial working memory task. Methods: Sixteen de novo PD patients and 35 matched healthy controls performed a visuospatial n-back task while we measured their behavioral performance and neural activity using functional magnetic resonance imaging. We constructed regions-of-interest in the bilateral inferior parietal cortex (IPC), bilateral dorsolateral prefrontal cortex (DLPFC), and bilateral caudate nucleus to investigate group differences in task-related activity. We studied network connectivity by assessing the functional connectivity of the bilateral DLPFC and by assessing effective connectivity within the frontoparietal and the frontostriatal networks. Results: PD patients, compared with controls, showed trend-significantly decreased task accuracy, significantly increased task-related activity in the left DLPFC and a trend-significant increase in activity of the right DLPFC, left caudate nucleus, and left IPC. Furthermore, we found reduced functional connectivity of the DLPFC with other task-related regions, such as the inferior and superior frontal gyri, in the PD group, and group differences in effective connectivity within the frontoparietal network. Interpretation: These findings suggest that the increase in working memory-related brain activity in PD patients is compensatory to maintain behavioral performance in the presence of network deficits. Hum Brain Mapp 36:1554-1566, 2015. (c) 2015 Wiley Periodicals, Inc
From arbitrariness to ambiguities in the evaluation of perturbative physical amplitudes and their symmetry relations
A very general calculational strategy is applied to the evaluation of the
divergent physical amplitudes which are typical of perturbative calculations.
With this approach in the final results all the intrinsic arbitrariness of the
calculations due to the divergent character is still present. We show that by
using the symmetry properties as a guide to search for the (compulsory) choices
in such a way as to avoid ambiguities, a deep and clear understanding of the
role of regularization methods emerges. Requiring then an universal point of
view for the problem, as allowed by our approach, very interesting conclusions
can be stated about the possible justifications of most intriguing aspect of
the perturbative calculations in quantum field theory: the triangle anomalies.Comment: 16 pages, no figure
Consequences of a Possible Di-Gamma Resonace at TRISTAN
If high mass di-gamma events observed at LEP are due to the production of a
di-gamma resonance via its leptonic coupling, its consequences can be observed
at TRISTAN. We find that a predicted decay branching rate is too small to
account for the observed events if the resonance spin is zero, due to a strong
cancellation in the decay amplitudes. Such a cancellation is absent if the
resonance has a spin two. We study the consequences of a tensor production in
the processes , and at TRISTAN
energies. Complete helicity amplitudes with tensor boson exchange contributions
are given, and the signal can clearly be identified from various distributions.
TRISTAN experiments are also sensitive to the virtual tensor boson exchange
effects, which reduce to the contact interaction terms in the high mass limit.Comment: 23 pages in revtex, 7 figures (not included) available upon request,
KEK-TH-35
Consistency in Perturbative Calculations and Radiatively Induced Lorentz and CPT Violations
The origin of the radiatively induced Lorentz and CPT violations, in
perturbative evaluations, of an extended version of QED, is investigated. Using
a very general calculational method, concerning the manipulations and
calculations involving divergent amplitudes, we clearly identify the possible
sources of contributions for the violating terms. We show that consistency in
the perturbative calculations, in a broader sense, leaves no room for the
existence of radiatively induced contributions which is in accordance with what
was previously conjectured and recently advocated by some authors supported on
general arguments.Comment: 8 pages, Revte
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