9,810 research outputs found
A custodial symmetry for Zbb
We show that a subgroup of the custodial symmetry O(3) that protects delta
rho from radiative corrections can also protect the Zbb coupling. This allows
one to build models of electroweak symmetry breaking, such as Higgsless, Little
Higgs or 5D composite Higgs models, that are safe from corrections to Z-> bb.
We show that when this symmetry protects Zbb it cannot simultaneously protect
Ztt and Wtb. Therefore one can expect to measure sizable deviations from the SM
predictions of these couplings at future collider experiments. We also show
under what circumstances Zb_R b_R can receive corrections in the right
direction to explain the anomaly in the LEP/SLD forward-backward asymmetry
A^b_{FB}.Comment: 11 pages. v2: minor correction
Chiral Perturbation Theory in Few-Nucleon Systems
The low-energy effective theory of nuclear physics based on chiral symmetry
is reviewed. Topics discussed include the nucleon-nucleon force, few-body
potentials, isospin violation, pion-deuteron scattering, proton-neutron
radiative capture, pion photoproduction on the deuteron, and pion production in
proton-proton collisions.Comment: 15 pages, 2 figures, Latex, aipproc.sty and epsfig, invited talk at
the 6th Conference on the Intersections of Particle and Nuclear Physics, Big
Sky, May 199
Curvature induced toroidal bound states
Curvature induced bound state (E < 0) eigenvalues and eigenfunctions for a
particle constrained to move on the surface of a torus are calculated. A limit
on the number of bound states a torus with minor radius a and major radius R
can support is obtained. A condition for mapping constrained particle wave
functions on the torus into free particle wave functions is established.Comment: 6 pages, no figures, Late
A Higher-Order Calculation of Scattering in Cut-Off Effective Field Theory
We report a next-to-leading-order (NLO) chiral perturbation theory
calculation of the neutron-proton scattering cross section in the
channel using a cut-off regularization. The inclusion of two-pion exchanges in
the irreducible diagrams -- or potential -- figuring at NLO is found to be
important in enlarging the domain of validity of the effective field theory. We
are able to reproduce the {\it empirical} scattering phase shift up to p=300
MeV -- which is comparable to the cutoff scale involved -- with an agreement
which is superior to results of other effective field theory approaches. We
also discuss the role of the cutoff as a renormalization prescription and the
importance of the explicit pion degree of freedom in scattering process.Comment: Substantial changes made in texts and Fig.2. To appear in Phys. Lett.
A framework for power analysis using a structural equation modelling procedure
BACKGROUND: This paper demonstrates how structural equation modelling (SEM) can be used as a tool to aid in carrying out power analyses. For many complex multivariate designs that are increasingly being employed, power analyses can be difficult to carry out, because the software available lacks sufficient flexibility. Satorra and Saris developed a method for estimating the power of the likelihood ratio test for structural equation models. Whilst the Satorra and Saris approach is familiar to researchers who use the structural equation modelling approach, it is less well known amongst other researchers. The SEM approach can be equivalent to other multivariate statistical tests, and therefore the Satorra and Saris approach to power analysis can be used. METHODS: The covariance matrix, along with a vector of means, relating to the alternative hypothesis is generated. This represents the hypothesised population effects. A model (representing the null hypothesis) is then tested in a structural equation model, using the population parameters as input. An analysis based on the chi-square of this model can provide estimates of the sample size required for different levels of power to reject the null hypothesis. CONCLUSIONS: The SEM based power analysis approach may prove useful for researchers designing research in the health and medical spheres
Effective Field Theory of Nuclear Forces
The application of the effective field theory (EFT) method to nuclear systems
is reviewed. The roles of degrees of freedom, QCD symmetries, power counting,
renormalization, and potentials are discussed. EFTs are constructed for various
energy regimes of relevance in nuclear physics, and are used in systematic
expansions to derive nuclear forces in terms of a number of parameters that
embody information about QCD dynamics. Two-, three-, and many-nucleon systems,
including external probes, are considered.Comment: 83 pages, 20 figures, commissioned for Prog. Part. Nucl. Phy
Geometric Phase, Curvature, and Extrapotentials in Constrained Quantum Systems
We derive an effective Hamiltonian for a quantum system constrained to a
submanifold (the constraint manifold) of configuration space (the ambient
space) by an infinite restoring force. We pay special attention to how this
Hamiltonian depends on quantities which are external to the constraint
manifold, such as the external curvature of the constraint manifold, the
(Riemannian) curvature of the ambient space, and the constraining potential. In
particular, we find the remarkable fact that the twisting of the constraining
potential appears as a gauge potential in the constrained Hamiltonian. This
gauge potential is an example of geometric phase, closely related to that
originally discussed by Berry. The constrained Hamiltonian also contains an
effective potential depending on the external curvature of the constraint
manifold, the curvature of the ambient space, and the twisting of the
constraining potential. The general nature of our analysis allows applications
to a wide variety of problems, such as rigid molecules, the evolution of
molecular systems along reaction paths, and quantum strip waveguides.Comment: 27 pages with 1 figure, submitted to Phys. Rev.
Effects of dilution and disorder on magnetism in diluted spin systems
The influence of configurational disorder on the magnetic properties of
diluted Heisenberg spin systems is studied with regard to the ferromagnetic
stability of diluted magnetic semiconductors. The equation of motion of the
magnon Green's function is decoupled by Tyablikov approximation. With supercell
approach, the concentrations of magnetic ions are determined by the size of the
supercell in which there is only one magnetic ion per supercell in our method.
In order to distinguish the influence of dilution and disorder, there are two
kinds of supercells being used: the \textit{diluted and ordered} case and the
\textit{diluted and disordered} case. The configurational averaging of magnon
Green function due to disorder is treated in the augmented space formalism. The
random exchange integrals between two supercells are treated as a matrix. The
obtained magnon spectral densities are used to calculate the temperature
dependence of magnetization and Curie temperature. The results are shown as
following: (i) dilution leads to increasing the averaged distance of two
magnetic ions, further decreases the effective exchange integrals and is main
reason to reduce Curie temperature; (ii) spatial position disorder of magnetic
ions results in the dispersions of the exchange integrals between two
supercells and slightly changes ferromagnetic transition temperature; (iii) the
exponential damping of distance dependence obviously reduces Curie temperature
and should be set carefully in any phenomenological model.Comment: 8 pages, 3 figures, accepted by physica status solidi (b); added
references and the corresponding remark
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An ASKAP Search for a Radio Counterpart to the First High-significance Neutron Star-Black Hole Merger LIGO/Virgo S190814bv
We present results from a search for a radio transient associated with the LIGO/Virgo source S190814bv, a likely neutron star-black hole (NSBH) merger, with the Australian Square Kilometre Array Pathfinder. We imaged a 30 deg2 field at ΔT = 2, 9, and 33 days post-merger at a frequency of 944 MHz, comparing them to reference images from the Rapid ASKAP Continuum Survey observed 110 days prior to the event. Each epoch of our observations covers 89% of the LIGO/Virgo localization region. We conducted an untargeted search for radio transients in this field, resulting in 21 candidates. For one of these, AT2019osy, we performed multiwavelength follow-up and ultimately ruled out the association with S190814bv. All other candidates are likely unrelated variables, but we cannot conclusively rule them out. We discuss our results in the context of model predictions for radio emission from NSBH mergers and place constrains on the circum-merger density and inclination angle of the merger. This survey is simultaneously the first large-scale radio follow-up of an NSBH merger, and the most sensitive widefield radio transients search to-date
Eurasian Arctic greening reveals teleconnections and the potential for novel ecosystems
Arctic warming has been linked to observed increases in tundra shrub cover and growth in recent decades on the basis of significant relationships between deciduous shrub growth/biomass and temperature. These vegetation trends have been linked to Arctic sea ice decline and thus to the sea ice/albedo feedback known as Arctic amplification. However, the interactions between climate, sea ice and tundra vegetation remain poorly understood. Here we reveal a 50- year growth response over a >100,000 km2 area to a rise in summer temperature for alder (Alnus) and willow (Salix), the most abundant shrub genera respectively at and north of the continental treeline. We demonstrate that whereas plant productivity is related to sea ice in late spring, the growing season peak responds to persistent synoptic-scale air masses over West Siberia associated with Fennoscandian weather systems through the Rossby wave train. Substrate is important for biomass accumulation, yet a strong correlation between growth and temperature encompasses all observed soil types. Vegetation is especially responsive to temperature in early summer. These results have significant implications for modelling present and future Low Arctic vegetation responses to climate change, and emphasize the potential for structurally novel ecosystems to emerge fromwithin the tundra zone.Vertaisarviointia edeltävä käsikirjoitu
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