7,730 research outputs found
U_A(1) Problems and Gluon Topology - Anomalous Symmetry in QCD
Many of the distinctive and subtle features of the dynamics in the
channel in QCD can be related to gluon topology, more precisely to the
topological susceptibility ,
where Q = {\a_s\over8\pi} {\rm tr} G_{\m\n} \tilde G^{\m\n} is the gluon
topological charge density. The link is the axial (ABJ) anomaly. In
this lecture, we describe the anomalous chiral Ward identities in a
functional formalism and show how two apparently unrelated ` problems'
-- the mass of the and the violation of the Ellis-Jaffe sum rule in
polarised deep-inelastic scattering -- can be explained in terms of the gluon
topological susceptibility. They are related through a extension of
the Goldberger-Treiman formula, which is derived here for QCD with both
massless and massive quarks.Comment: Lecture at 1998 Zuoz Summer School, `Hidden Symmetries and Higgs
Phenomena'. 22 pages, plain TeX, 2 ps or eps figure
and the topological susceptibility
The radiative decays \eta'(\eta)\rta\gamma\gamma are discussed. The
modifications of the conventional PCAC formulae due to the gluonic contribution
to the flavour singlet axial anomaly are given. The decay constants satisfy a
modified Dashen formula which generalises the Witten--Veneziano formula for the
mass of the . It is shown how the topological susceptibility in QCD with
massive, dynamical quarks may be extracted from measurements of
\eta'(\eta)\rta\gamma\gamma.Comment: 4 pages, LaTeX, uses espcrc2.sty. To appear in Proceedings, QCD99
Montpellie
The Proton-Spin Crisis: another ABJ anomaly?
Contents: 1. Introduction; 2. The First Moment Sum Rule for ; 3. The
Parton Model and the `Proton Spin'; 4. The CPV Method and Topological Charge
Screening; 5. Experiment; 6. Semi-Inclusive Polarised DIS.Comment: Lecture at the International School of Subnuclear Physics, `From the
Planck Length to the Hubble Radius', Erice, 1998. 22 pages, plain TeX, 2ps or
eps figure
When to make the sensory social: Registering in copresent openings
This article provides the first detailed empirical analysis of naturally-occurring videorecorded openings during which participants make the sensory social through the action of registering – calling joint attention to a selected, publicly perceivable referent so others shift their sensory attention to it. Examining sequence-initial actions that register referents for which a participant is regarded as responsible, this study elucidates a systematic preference organization which observably guides when and how people initiate registering sequences sensitive to both referent ownership and referent value. Analysis shows how choosing to register an owned referent puts involved participants’ face, affiliation, and social relationship on the line
When to Make the Sensory Social: Registering in Face-to-Face Openings
This article analyzes naturally occurring video-recorded openings during which participants make the sensory social through the action of registering—calling joint attention to a selected, publicly perceiv- able referent so others shift their sensory attention to it. It examines sequence-initial actions that register referents for which a participant is regarded as responsible. Findings demonstrate a systematic preference organization which observably guides when and how people initiate registering sequences sensitive to ownership of, and displayed stance toward, the target referent. Analysis shows how registering an owned referent achieves intersubjectivity and puts involved participants’ face, affiliation, and social relationship on the line. A video abstract is available at https://youtu.be/rNL70vawG3
Being a Good Parent in Parent-Teacher Conferences
This research advances our understanding of what constitutes a good parent in the course of actual social interaction. Examining video-recorded naturally occurring parent-teacher conferences, this article shows that, while teachers deliver student-praising utterances, parents may display that they are gaining knowledge; but when teachers’ actions adumbrate student-criticizing utterances, parents systematically display prior knowledge. This article elucidates the details of how teachers and parents tacitly collaborate to enable parents to express student-troubles first, demonstrating that parents display competence -- appropriate involvement with children’s schooling -- by asserting their prior knowledge of, and/or claiming/describing their efforts to remedy, student-troubles. People (have to) display competence generically in interaction. By explicating how parents display competence, this article offers insights for several areas of communication research
The Refractive Index of Curved Spacetime: the Fate of Causality in QED
It has been known for a long time that vacuum polarization in QED leads to a
superluminal low-frequency phase velocity for light propagating in curved
spacetime. Assuming the validity of the Kramers-Kronig dispersion relation,
this would imply a superluminal wavefront velocity and the violation of
causality. Here, we calculate for the first time the full frequency dependence
of the refractive index using world-line sigma model techniques together with
the Penrose plane wave limit of spacetime in the neighbourhood of a null
geodesic. We find that the high-frequency limit of the phase velocity (i.e. the
wavefront velocity) is always equal to c and causality is assured. However, the
Kramers-Kronig dispersion relation is violated due to a non-analyticity of the
refractive index in the upper-half complex plane, whose origin may be traced to
the generic focusing property of null geodesic congruences and the existence of
conjugate points. This puts into question the issue of micro-causality, i.e.
the vanishing of commutators of field operators at spacelike separated points,
in local quantum field theory in curved spacetime.Comment: 43 pages, 19 figures, JHEP3, conclusions respecting microcausality
modifie
`Superluminal' Photon Propagation in QED in Curved Spacetime is Dispersive and Causal
It is now well-known that vacuum polarisation in QED can lead to superluminal
low-frequency phase velocities for photons propagating in curved spacetimes. In
a series of papers, we have shown that this quantum phenomenon is dispersive
and have calculated the full frequency dependence of the refractive index,
explaining in detail how causality is preserved and various familiar results
from quantum field theory such as the Kramers-Kronig dispersion relation and
the optical theorem are realised in curved spacetime. These results have been
criticised in a recent paper by Akhoury and Dolgov arXiv:1003.6110 [hep-th],
who assert that photon propagation is neither dispersive nor necessarily
causal. In this note, we point out a series of errors in their work which have
led to this false conclusion.Comment: 11 page
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