5,651 research outputs found
Singlet-Assisted Electroweak Baryogenesis in Effective Field Theory and SO(6)/SO(5) Composite Higgs Models
It is known that the Baryon-Asymmetry in the Universe (BAU) can be generated in Electroweak Baryogenesis when minimally extending the Standard Model (SM) with a scalar singlet. A particularly well-researched class of models features a Z2 symmetry of the singlet that is spontaneously broken around the electroweak scale. However, such a scenario generically leads to phenomenologically problematic domain walls. Here, a thermal history in which the Z2 symmetry is not restored at high temperatures is envisioned, as accomplished by introducing D ≤ 6 operators in a general effective field theory (EFT). Introducing non-renormalizable operators is the logical consequence of understanding the theory as the low-energy tail of a more complete theory, such as in Composite Higgs (CH) scenarios, which can additionally address the hierarchy problem and the flavor hierarchy puzzle. Here I present a comprehensive analysis of the Yukawa terms and the scalar potential generated up to D = 6 terms in various SO(6)/SO(5) CH models. To this end, a Spurion Analysis with SM fermions embedded in the (1), 6, 15 or 20′ representations of SO(6) is performed. The most promising model is successfully matched to the general EFT, showing that it can generate the correct BAU
Z<sub>2</sub> Non-Restoration and Composite Higgs: Singlet-Assisted Baryogenesis w/o Topological Defects
Simple scalar-singlet extensions of the Standard Model with a (spontaneouslybroken) symmetry allow for a strong first order electroweak phasetransition, as sought in order to realize electroweak baryogenesis. Howeverthey generically also lead to the emergence of phenomenologically problematicdomain walls. Here we present a framework with a real scalar singlet thatfeatures a different thermal history that avoids this problem by neverrestoring the symmetry in the early universe. This is accomplished byconsidering operators that emerge on general grounds, understanding themodel as the low energy tail of a more complete theory, like for example incomposite Higgs scenarios. Sticking to the latter framework, we present aconcrete composite realization of the idea. To this end, weadditionally provide a complete classification of the structure of the Higgspotential (and the Yukawa couplings) in models with fermions inthe or of .<br
Energetics and electronic structure of phenyl-disubstituted polyacetylene: A first-principles study
Phenyl-disubstituted polyacetylene (PDPA) is an organic semiconductor which
has been studied during the last years for its efficient photo-luminescence. In
contrast, the molecular geometry, providing the basis for the electronic and
optical properties, has been hardly investigated. In this paper, we apply a
density-functional-theory based molecular-dynamics approach to reveal the
molecular structure of PDPA in detail. We find that oligomers of this material
are limited in length, being stable only up to eight repeat units, while the
polymer is energetically unfavorable. These facts, which are in excellent
agreement with experimental findings, are explained through a detailed analysis
of the bond lengths. A consequence of the latter is the appearance of
pronounced torsion angles of the phenyl rings with respect to the plane of the
polyene backbone, ranging from up to . We point out
that such large torsion angles do not destroy the conjugation of the
electrons from the backbone to the side phenyl rings, as is evident from the
electronic charge density.Comment: 9 pages, 7 figures, accepted for publication in Phys. Rev.
Spin fluctuations and superconductivity in noncentrosymmetric heavy fermion systems CeRhSi and CeIrSi
We study the normal and the superconducting properties in noncentrosymmetric
heavy fermion superconductors CeRhSi and CeIrSi. For the normal state,
we show that experimentally observed linear temperature dependence of the
resistivity is understood through the antiferromagnetic spin fluctuations near
the quantum critical point (QCP) in three dimensions. For the superconducting
state, we derive a general formula to calculate the upper critical field
, with which we can treat the Pauli and the orbital depairing effect on
an equal footing. The strong coupling effect for general electronic structures
is also taken into account. We show that the experimentally observed features
in , the huge value up to 30(T), the downward
curvatures, and the strong pressure dependence, are naturally understood as an
interplay of the Rashba spin-orbit interaction due to the lack of inversion
symmetry and the spin fluctuations near the QCP. The large anisotropy between
and is explained in terms of
the spin-orbit interaction. Furthermore, a possible realization of the
Fulde-Ferrell- Larkin-Ovchinnikov state for is studied. We
also examine effects of spin-flip scattering processes in the pairing
interaction and those of the applied magnetic field on the spin fluctuations.
We find that the above mentioned results are robust against these effects. The
consistency of our results strongly supports the scenario that the
superconductivity in CeRhSi and CeIrSi is mediated by the spin
fluctuations near the QCP.Comment: 21pages, 13figures, to be published in Phys. Rev.
Theory of nonlinear optical properties of phenyl-substituted polyacetylenes
In this paper we present a theoretical study of the third-order nonlinear
optical properties of poly(diphenyl)polyacetylene (PDPA) pertaining to the
third-harmonic-generation (THG) process. We study the aforesaid process in
PDPA's using both the independent electron Hueckel model, as well as
correlated-electron Pariser-Parr-Pople (P-P-P) model. The P-P-P model based
calculations were performed using various configuration interaction (CI)
methods such as the the multi-reference-singles-doubles CI (MRSDCI), and the
quadruples-CI (QCI) methods, and the both longitudinal and the transverse
components of third-order susceptibilities were computed. The Hueckel model
calculations were performed on oligo-PDPA's containing up to fifty repeat
units, while correlated calculations were performed for oligomers containing up
to ten unit cells. At all levels of theory, the material exhibits highly
anisotropic nonlinear optical response, in keeping with its structural
anisotropy. We argue that the aforesaid anisotropy can be divided over two
natural energy scales: (a) the low-energy response is predominantly
longitudinal and is qualitatively similar to that of polyenes, while (b) the
high-energy response is mainly transverse, and is qualitatively similar to that
of trans-stilbene.Comment: 13 pages, 7 figures (included), to appear in Physical Review B (April
15, 2004
Consequence of Hawking radiation from 2d dilaton black holes
We investigate the CGHS model through numerical calculation. The behavior of
the mass function, which we introduced in our previous work as a ``local
mass'', is examined. We found that the mass function takes negative values,
which means that the amount of Hawking radiation becomes greater than the
initial mass of the black hole as in the case of the RST model.Comment: 17pages, 5 figures (three of them are attached, the other 2 figures
are available on request. Some mistakes including typographic errors have
been correcte
A study on correlation effects in two dimensional topological insulators
We investigate correlation effects in two dimensional topological insulators
(TI). In the first part, we discuss finite size effects for interacting systems
of different sizes in a ribbon geometry. For large systems, there are two pairs
of well separated massless modes on both edges. For these systems, we analyze
the finite size effects using a standard bosonization approach. For small
systems, where the edge states are massive Dirac fermions, we use the
inhomogeneous dynamical mean field theory (DMFT) combined with iterative
perturbation theory as an impurity solver to study interaction effects. We show
that the finite size gap in the edge states is renormalized for weak
interactions, which is consistent with a Fermi-liquid picture for small size
TIs. In the second part, we investigate phase transitions in finite size TIs at
zero temperature focusing on the effects of possible inter-edge Umklapp
scattering for the edge states within the inhomogeneous DMFT using the
numerical renormalization group. We show that correlation effects are
effectively stronger near the edge sites because the coordination number is
smaller than in the bulk. Therefore, the localization of the edge states around
the edge sites, which is a fundamental property in TIs, is weakened for strong
coupling strengths. However, we find no signs for "edge Mott insulating states"
and the system stays in the topological insulating state, which is
adiabatically connected to the non-interacting state, for all interaction
strengths smaller than the critical value. Increasing the interaction further,
a nearly homogeneous Mott insulating state is stabilized.Comment: 20 page
Research on pre-slaughter stress and meat quality: A review of challenges faced under practical conditions
Transportation and handling of animals are important components in meat production systems. Animals destined for slaughter are stressed by factors such as loading, transportation, restraint, handling, and novelty of the slaughter environment, adverse weather conditions, hunger, thirst and fatigue. When an animal is stressed in the pre-slaughter environment, there is a rapid release of enzymes, cortisols and catecholamines which may lead to depletion of glycogen, high meat ultimate pH (pHu) and dark cuts. Pre-slaughter stress also affects the physiology of the animal resulting in an increase in creatine kinase (CK) activity, glucose, lactate and other blood metabolites. Pre-slaughter stress and meat quality is a wide topic and some research have been done albeit was conducted under experimental and controlled conditions. In real life situations many variables are not controlled. Data collection in practical conditions is complex because it is difficult to accurately take some of the measurements; for example the onset of the release of the enzymes and hormones. Moreover, there are difficulties in taking measurements from slaughterhouses and butcheries. Challenges that researchers, marketers and consumers face under practical conditions either on-farm, research stations, transfer centres, slaughter plants, abattoirs or butcheries; with regard to pre-slaughter stress and meat quality may affect the quality of the final products developed under such conditions. This review therefore aims to give a detailed outline on pre-slaughter stress, meat science research and challenges under practical conditions
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