26 research outputs found
Vector Meson Mixing and Charge Symmetry Violation
We discuss the consistency of the traditional vector meson dominance (VMD)
model for photons coupling to matter, with the vanishing of vector meson-meson
and meson-photon mixing self-energies at q^2=0. This vanishing of vector mixing
has been demonstrated in the context of rho-omega mixing for a large class of
effective theories. As a further constraint on such models, we here apply them
to a study of photon-meson mixing and VMD. As an example we compare the
predicted momentum dependence of one such model with a momentum-dependent
version of VMD discussed by Sakurai in the 1960's. We find that it produces a
result which is consistent with the traditional VMD phenomenology. We conclude
that comparison with VMD phenomenology can provide a useful constraint on such
models.Comment: 7 pages, uses epsfig.sty. Publication details added to title pag
Extracting the rho-omega mixing amplitude from the pion form-factor
We re-examine and extend a recent analysis which showed that the rho-omega
mixing amplitude cannot be unambiguously extracted from the pion
electromagnetic form-factor in a model independent way. In particular, we focus
on the argument that the extraction is sensitive to the presence of any
intrinsic omega_I -> 2pi coupling. Our extended analysis confirms the original
conclusion, with only minor, quantitative differences. The extracted mixing
amplitude is shown to be sensitive to both the intrinsic coupling omega_I ->
2pi and to the value assumed for the mass of the neutral rho meson.Comment: 11 pages, publication details added to title pag
Analysis of rho-omega interference in the pion form-factor
The formalism underlying the analysis of e+e- -> pi+pi- in the rho-omega
interference region is carefully revisited. We show that the standard neglect
of the pure I=0 omega, omega_I, `direct'' coupling to pi-pi is not valid, and
extract those combinations of the direct coupling and \rho-omega mixing allowed
by experiment. The latter is shown to be only very weakly constrained by
experiment, and we conclude that data from the e+e- -> pi+pi- interference
region cannot be used to fix the value of rho-omega mixing in a
model-independent way unless the errors on the experimental phase can be
significantly reduced. Certain other modifications of the usual formalism
necessitated by the unavoidable momentum-dependence of rho-omega mixing are
also discussed.Comment: 15 pages of REVTEX, uses epsfig.sty with 1 figure. Entire manuscript
available as a ps file at http://www.physics.adelaide.edu.au/theory/home.html
or ftp://adelphi.adelaide.edu.au/pub/theory/ADP-95-50.T197.ps Minor changes
to text and some equations for publication in Physics Letters
Rho-omega Mixing and the Pion Electromagnetic Form-Factor
The suggestion of momentum dependence in the amplitude for rho-omega mixing
has generated concern over related implications for vector meson dominance and
the photon-rho coupling. We discuss two established representations of vector
meson dominance and show that one of these is completely consistent with such a
coupling. We then apply it to a calculation of the pion electromagnetic
form-factor. Our analysis leads to a new value for the on-shell rho-omega
mixing amplitude of (-3800 +/- 370) MeV^2.Comment: 11 pages with epsfig.sty. Publication details added to title pag
Precision and neuronal dynamics in the human posterior parietal cortex during evidence accumulation
Primate studies show slow ramping activity in posterior parietal cortex (PPC) neurons during perceptual decision-making. These findings have inspired a rich theoretical literature to account for this activity. These accounts are largely unrelated to Bayesian theories of perception and predictive coding, a related formulation of perceptual inference in the cortical hierarchy. Here, we tested a key prediction of such hierarchical inference, namely that the estimated precision (reliability) of information ascending the cortical hierarchy plays a key role in determining both the speed of decision-making and the rate of increase of PPC activity. Using dynamic causal modelling of magnetoencephalographic (MEG) evoked responses, recorded during a simple perceptual decision-making task, we recover ramping-activity from an anatomically and functionally plausible network of regions, including early visual cortex, the middle temporal area (MT) and PPC. Precision, as reflected by the gain on pyramidal cell activity, was strongly correlated with both the speed of decision making and the slope of PPC ramping activity. Our findings indicate that the dynamics of neuronal activity in the human PPC during perceptual decision-making recapitulate those observed in the macaque, and in so doing we link observations from primate electrophysiology and human choice behaviour. Moreover, the synaptic gain control modulating these dynamics is consistent with predictive coding formulations of evidence accumulation
Effects of Symmetry Breaking on the Strong and Electroweak Interactions of the Vector Nonet
Starting from a chiral invariant and quark line rule conserving Lagrangian of
pseudoscalar and vector nonets we introduce first and second order symmetry
breaking as well as quark line rule violating terms and fit the parameters, at
tree level, to many strong and electroweak processes. A number of predictions
are made. The electroweak interactions are included in a manifestly gauge
invariant manner. The resulting symmetry breaking pattern is discussed in
detail. Specifically, for the ``strong'' interactions, we study all the vector
meson masses and V -> \phi \phi decays, including isotopic spin violations. In
the electroweak sector we study the { rho^0 , omega , phi } -> e^+e^- decays, {
pi^+ , K^+ , K^0 } ``charge radii'', K_{l3} ``slope factor'' and the overall
e^+e^- -> pi^+ pi^- process. It is hoped that the resulting model may be useful
as a reasonable description of low energy physics in the range up to about 1
GeV.Comment: 43 pages (LaTeX), 5 PostScript figures are included as
uuencoded-compressed-tar file at the en
Constraints on the momentum dependence of rho-omega mixing
Within a broad class of models we show that the amplitude for rho^0-omega
mixing must vanish at the transition from timelike to spacelike four momentum.
Hence in such models the mixing is either zero everywhere or is necessarily
momentum-dependent. This lends support to the conclusions of other studies of
rho-omega mixing and calls into question standard assumptions about the role of
rho-omega mixing in the theoretical understanding of charge-symmetry breaking
in nuclear systems.Comment: 8 pages. Publication details added to title pag
Isospin-Violating Meson-Nucleon Vertices as an Alternate Mechanism of Charge-Symmetry Breaking
We compute isospin-violating meson-nucleon coupling constants and their
consequent charge-symmetry-breaking nucleon-nucleon potentials. The couplings
result from evaluating matrix elements of quark currents between nucleon states
in a nonrelativistic constituent quark model; the isospin violations arise from
the difference in the up and down constituent quark masses. We find, in
particular, that isospin violation in the omega-meson--nucleon vertex dominates
the class IV CSB potential obtained from these considerations. We evaluate the
resulting spin-singlet--triplet mixing angles, the quantities germane to the
difference of neutron and proton analyzing powers measured in elastic
scattering, and find them commensurate to those computed
originally using the on-shell value of the - mixing amplitude.
The use of the on-shell - mixing amplitude at has been
called into question; rather, the amplitude is zero in a wide class of models.
Our model possesses no contribution from - mixing at , and
we find that omega-meson exchange suffices to explain the measured
analyzing power difference~at~183 MeV.Comment: 20 pages, revtex, 3 uuencoded PostScript figure
Meson Spectroscopy in AdS/CFT with Flavour
We compute the meson spectrum of an N=2 super Yang-Mills theory with
fundamental matter from its dual string theory on AdS_5 x S_5 with a D7-brane
probe. For scalar and vector mesons with arbitrary R-charge the spectrum is
computed in closed form by solving the equations for D7-brane fluctuations; for
matter with non-zero mass m_q it is discrete, exhibits a mass gap of order m_q
/ sqrt(g_s N) and furnishes representations of SO(5) even though the manifest
global symmetry of the theory is only SO(4). The spectrum of mesons with large
spin J is obtained from semiclassical, rotating open strings attached to the
D7-brane. It displays Regge-like behaviour for J << sqrt(g_s N), whereas for J
>> sqrt(g_s N) it corresponds to that of two non-relativistic quarks bound by a
Coulomb potential. Meson interactions, baryons and `giant gauge bosons' are
briefly discussed.Comment: LaTeX, 39 pages, 4 figures, uses epsf. v2: typos corrected.
references adde
Reducing the environmental impact of surgery on a global scale: systematic review and co-prioritization with healthcare workers in 132 countries
Background
Healthcare cannot achieve net-zero carbon without addressing operating theatres. The aim of this study was to prioritize feasible interventions to reduce the environmental impact of operating theatres.
Methods
This study adopted a four-phase Delphi consensus co-prioritization methodology. In phase 1, a systematic review of published interventions and global consultation of perioperative healthcare professionals were used to longlist interventions. In phase 2, iterative thematic analysis consolidated comparable interventions into a shortlist. In phase 3, the shortlist was co-prioritized based on patient and clinician views on acceptability, feasibility, and safety. In phase 4, ranked lists of interventions were presented by their relevance to high-income countries and low–middle-income countries.
Results
In phase 1, 43 interventions were identified, which had low uptake in practice according to 3042 professionals globally. In phase 2, a shortlist of 15 intervention domains was generated. In phase 3, interventions were deemed acceptable for more than 90 per cent of patients except for reducing general anaesthesia (84 per cent) and re-sterilization of ‘single-use’ consumables (86 per cent). In phase 4, the top three shortlisted interventions for high-income countries were: introducing recycling; reducing use of anaesthetic gases; and appropriate clinical waste processing. In phase 4, the top three shortlisted interventions for low–middle-income countries were: introducing reusable surgical devices; reducing use of consumables; and reducing the use of general anaesthesia.
Conclusion
This is a step toward environmentally sustainable operating environments with actionable interventions applicable to both high– and low–middle–income countries