The witchcraft trial in Moscow

https://stars.library.ucf.edu/prism/1519/thumbnail.jp

Non-perturbative intertwining between spin and charge correlations: A "smoking gun" single-boson-exchange result

We study the microscopic mechanism controlling the interplay between the local charge and local spin fluctuations in correlated electron systems, via a thorough investigation of the generalized on-site charge susceptibility of several fundamental many-electron models, such as the Hubbard atom, the Anderson impurity model and the Hubbard model. By decomposing the numerically determined generalized susceptibility in terms of physically transparent single-boson exchange processes, we unveil the microscopic mechanisms responsible for the breakdown of the self-consistent many-electron perturbation expansion. In particular, we unambiguously identify the origin of the significant suppression of its diagonal entries in (Matsubara) frequency space and the slight increase of the off-diagonal ones which cause the breakdown. The suppression effect on the diagonal elements directly originates from the electronic scattering on local magnetic moments, reflecting their increasingly longer lifetime as well as their enhanced effective coupling with the electrons. The slight and diffuse enhancement of the off-diagonal terms, instead, can be mostly ascribed to multiboson scattering processes. The strong intertwining between the spin and charge sector is partly weakened at the Kondo temperature due to a progressive reduction of the effective spin-fermion coupling of local magnetic fluctuations in the low frequency regime. Our analysis, thus, clarifies the precise way in which the physical information between different scattering channels of interacting electron problems is transferred and highlights the pivotal role played by such an intertwining in the physics of correlated electrons beyond the perturbative regime.Comment: 31 pages, 14 figures, submission to SciPos

Recent developments in effective field theory

We will give a short introduction to the one-nucleon sector of chiral perturbation theory and will address the issue of a consistent power counting and renormalization. We will discuss the infrared regularization and the extended on-mass-shell scheme. Both allow for the inclusion of further degrees of freedom beyond pions and nucleons and the application to higher-loop calculations. As applications we consider the chiral expansion of the nucleon mass to order O(q^6) and the inclusion of vector and axial-vector mesons in the calculation of nucleon form factors.Comment: 8 pages, 6 figures, invited talk given at International School of Nuclear Physics, 29th Course "Quarks in Hadrons and Nuclei", Erice, Sicily, 16 - 24 September 200

Light Cone QCD Sum Rules Analysis of the Axial N -> Delta Transition Form Factors

The axial N-> Delta(1232) transition form factors are calculated within the light cone QCD sum rules method. A comparison of our results with predictions of lattice theory and quark model calculations is pre- sented.Comment: 31 Pages, 9 Figure

Role of right posterior parietal cortex in maintaining attention to spatial locations over time

Recent models of human posterior parietal cortex (PPC) have variously emphasized its role in spatial perception, visuomotor control or directing attention. However, neuroimaging and lesion studies also suggest that the right PPC might play a special role in maintaining an alert state. Previously, assessments of right-hemisphere patients with hemispatial neglect have revealed significant overall deficits on vigilance tasks, but to date there has been no demonstration of a deterioration of performance over time--a vigilance decrement--considered by some to be a key index of a deficit in maintaining attention. Moreover, sustained attention deficits in neglect have not specifically been related to PPC lesions, and it remains unclear whether they interact with spatial impairments in this syndrome. Here we examined the ability of right-hemisphere patients with neglect to maintain attention, comparing them to stroke controls and healthy individuals. We found evidence of an overall deficit in sustaining attention associated with PPC lesions, even for a simple detection task with stimuli presented centrally. In a second experiment, we demonstrated a vigilance decrement in neglect patients specifically only when they were required to maintain attention to spatial locations, but not verbal material. Lesioned voxels in the right PPC spanning a region between the intraparietal sulcus and inferior parietal lobe were significantly associated with this deficit. Finally, we compared performance on a task that required attention to be maintained either to visual patterns or spatial locations, matched for task difficulty. Again, we found a vigilance decrement but only when attention had to be maintained on spatial information. We conclude that sustaining attention to spatial locations is a critical function of the human right PPC which needs to be incorporated into models of normal parietal function as well as those of the clinical syndrome of hemispatial neglect

A Chiral Effective Lagrangian for Nuclei

An effective hadronic lagrangian consistent with the symmetries of quantum chromodynamics and intended for applications to finite-density systems is constructed. The degrees of freedom are (valence) nucleons, pions, and the low-lying non-Goldstone bosons, which account for the intermediate-range nucleon-nucleon interactions and conveniently describe the nonvanishing expectation values of nucleon bilinears. Chiral symmetry is realized nonlinearly, with a light scalar meson included as a chiral singlet to describe the mid-range nucleon-nucleon attraction. The low-energy electromagnetic structure of the nucleon is described within the theory using vector-meson dominance, so that external form factors are not needed. The effective lagrangian is expanded in powers of the fields and their derivatives, with the terms organized using Georgi's ``naive dimensional analysis''. Results are presented for finite nuclei and nuclear matter at one-baryon-loop order, using the single-nucleon structure determined within the model. Parameters obtained from fits to nuclear properties show that naive dimensional analysis is a useful principle and that a truncation of the effective lagrangian at the first few powers of the fields and their derivatives is justified.Comment: 43 pages, REVTeX 3.0 with epsf.sty, plus 12 figure

Chiral perturbation theory - Success and challenge

Chiral perturbation theory is the effective field theory of the strong interactions at low energies. We will give a short introduction to chiral perturbation theory for mesons and will discuss, as an example, the electromagnetic polarizabilities of the pion. These have recently been extracted from an experiment on radiative $\pi^+$ photoproduction from the proton ($\gamma p\to \gamma \pi^+ n$) at the Mainz Microtron MAMI. Next we will turn to the one-baryon sector of chiral perturbation theory and will address the issue of a consistent power counting scheme. As examples of the heavy-baryon framework we will comment on the extraction of the axial radius from pion electroproduction and will discuss the generalized polarizabilities of the proton. Finally, we will discuss two recently proposed manifestly Lorentz-invariant renormalization schemes and illustrate their application in a calculation of the nucleon electromagnetic form factors.Comment: 12 pages, 13 figures, invited talk given at the Symposium 20 Years of Physics at the Mainz Microtron MAMI, 20 - 22 October 2005, Mainz, German

A new Perspective on the Scalar meson Puzzle, from Spontaneous Chiral Symmetry Breaking Beyond BCS

We introduce coupled channels of Bethe-Salpeter mesons both in the boundstate equation for mesons and in the mass gap equation for chiral symmetry. Consistency is insured by the Ward Identities for axial currents, which preserve the Goldstone boson nature of the pion and prevents a systematic shift of the hadron spectrum. We study the decay of a scalar meson coupled to a pair of pseudoscalars. We also show that coupled channels reduce the breaking of chiral symmetry, with the same Feynman diagrams that appear in the coupling of a scalar meson to a pair of pseudoscalar mesons. Exact calculations are performed in a particular confining quark model, where we find that the groundstate $I=0, ^3P_0 q \bar q$ meson is the f_0(980) with a partial decay width of 40MeV. We also find a 30% reduction of the chiral condensate due to coupled channels.Comment: 17 pages, Revtex, 8 eps figures, and several eps diagrams in equation

The unexpected resurgence of Weyl geometry in late 20-th century physics

Weyl's original scale geometry of 1918 ("purely infinitesimal geometry") was withdrawn by its author from physical theorizing in the early 1920s. It had a comeback in the last third of the 20th century in different contexts: scalar tensor theories of gravity, foundations of gravity, foundations of quantum mechanics, elementary particle physics, and cosmology. It seems that Weyl geometry continues to offer an open research potential for the foundations of physics even after the turn to the new millennium.Comment: Completely rewritten conference paper 'Beyond Einstein', Mainz Sep 2008. Preprint ELHC (Epistemology of the LHC) 2017-02, 92 pages, 1 figur

Electroexcitation of nucleon resonances

We review recent progress in the investigation of the electroexcitation of nucleon resonances, both in experiment and in theory. The most accurate results have been obtained for the electroexcitation amplitudes of the four lowest excited states, which have been measured in a range of Q2 up to 8 and 4.5 GeV2 for the Delta(1232)P33, N(1535)S11 and N(1440)P11, N(1520)D13}, respectively. These results have been confronted with calculations based on lattice QCD, large-Nc relations, perturbative QCD (pQCD), and QCD-inspired models. The amplitudes for the Delta(1232) indicate large pion-cloud contributions at low Q2 and don't show any sign of approaching the pQCD regime for Q2<7 GeV2. Measured for the first time, the electroexcitation amplitudes of the Roper resonance, N(1440)P11, provide strong evidence for this state as a predominantly radial excitation of a three-quark (3q) ground state, with additional non-3-quark contributions needed to describe the low Q2 behavior of the amplitudes. The longitudinal transition amplitude for the N(1535)S11 was determined and has become a challenge for quark models. Explanations may require large meson-cloud contributions or alternative representations of this state. The N(1520)D13 clearly shows the rapid changeover from helicity-3/2 dominance at the real photon point to helicity-1/2 dominance at Q2 > 0.5 GeV2, confirming a long-standing prediction of the constituent quark model. The interpretation of the moments of resonance transition form factors in terms of transition transverse charge distributions in infinite momentum frame is presented.Comment: 70 pages, 46 figures, will appear in Progress in Particle and Nuclear Physics, v.67, p.1, 201