133 research outputs found
A combined study of the pion's static properties and form factors
We study consistently the pion's static observables and the elastic and
\gamma*\gamma -> \pi^0 transition form factors within a light-front model.
Consistency requires that all calculations are performed within a given model
with the same and single adjusted length or mass-scale parameter of the
associated pion bound-state wave function. Our results agree well with all
extent data including recent Belle data on the \gamma*\gamma -> \pi^0 form
factor at large q^2, yet the BaBar data on this transition form factor resists
a sensible comparison. We relax the initial constraint on the bound-state wave
function and show the BaBar data can partially be accommodated. This, however,
comes at the cost of a hard elastic form factor not in agreement with
experiment. Moreover, the pion charge radius is about 40% smaller than its
experimentally determined value. It is argued that a decreasing charge radius
produces an ever harder form factor with a bound-state amplitude difficultly
reconcilable with soft QCD. We also discuss why vector dominance type models
for the photon-quark vertex, based on analyticity and crossing symmetry, are
unlikely to reproduce the litigious transition form factor data.Comment: 14 pages, 7 figures, 2 Tables; minor changes; 2 new references added
(Ref. 54 and 57); version to be published in Few Body Physic
Light-Front Model of Transition Form-Factors in Heavy Meson Decay
Electroweak transition form factors of heavy meson decays are important
ingredients in the extraction of the Cabibbo-Kobayashi-Maskawa (CKM) matrix
elements from experimental data. In this work, within a light-front framework,
we calculate electroweak transition form factor for the semileptonic decay of
mesons into a pion or a kaon. The model results underestimate in both cases
the new data of CLEO for the larger momentum transfers accessible in the
experiment. We discuss possible reasons for that in order to improve the model.Comment: Paper with 5 pages and 2 eps figures. To appear to Nuclear Physics B.
Talk at Light Cone 2009: Relativistic Hadronic and Particle Physics (LC
2009), Sao Jose dos Campos, S.P, Brazil, 8-13 Jul 2009
On the quark-gluon vertex and quark-ghost kernel: combining lattice simulations with Dyson-Schwinger equations
We investigate the dressed quark-gluon vertex combining two established nonperturbative approaches to QCD: the Dyson-Schwinger equation (DSE) for the quark propagator and lattice-regularized simulations for the quark, gluon and ghost propagators. The vertex is modeled using a generalized Ball-Chiu ansatz parameterized by a single form
actor X̃_0 which effectively represents the quark-ghost scattering kernel. The solution space
of the DSE inversion for X̃_0 is highly degenerate, which can be dealt with by a numerical regularization scheme. We consider two possibilities: (i) linear regularization and (ii) the Maximum Entropy Method. These two numerical approaches yield compatible X̃_0 functions for the range of momenta where lattice data is available and feature a strong enhancement of the generalized Ball-Chiu vertex for momenta below 1 GeV. Our ansatz for the quark-gluon vertex is then used to solve the quark Dyson-Schwinger equation which yields a mass function in good agreement with lattice simulations and thus provides adequate dynamical chiral symmetry breaking
Pion structure in the nuclear medium
Using the light-front pion wave function based on a Bethe-Salpeter amplitude
model, we study the properties of the pion in symmetric nuclear matter. The
pion model we adopt is well constrained by previous studies to explain the pion
properties in vacuum. In order to consistently incorporate the constituent up
and down quarks of the pion immersed in symmetric nuclear matter, we use the
quark-meson coupling model, which has been widely applied to various hadronic
and nuclear phenomena in a nuclear medium with success. We predict the
in-medium modifications of the pion lectromagnetic form factor, charge radius
and weak decay constant in symmetric nuclear matter.Comment: Use revtex4, 7 figures. Title change. Revised text and figures.
Accepted, Physical Review C (August, 2014
State sampling dependence of the Hopfield network inference
The fully connected Hopfield network is inferred based on observed
magnetizations and pairwise correlations. We present the system in the glassy
phase with low temperature and high memory load. We find that the inference
error is very sensitive to the form of state sampling. When a single state is
sampled to compute magnetizations and correlations, the inference error is
almost indistinguishable irrespective of the sampled state. However, the error
can be greatly reduced if the data is collected with state transitions. Our
result holds for different disorder samples and accounts for the previously
observed large fluctuations of inference error at low temperatures.Comment: 4 pages, 1 figure, further discussions added and relevant references
adde
Looking into the matter of light-quark hadrons
In tackling QCD, a constructive feedback between theory and extant and
forthcoming experiments is necessary in order to place constraints on the
infrared behaviour of QCD's \beta-function, a key nonperturbative quantity in
hadron physics. The Dyson-Schwinger equations provide a tool with which to work
toward this goal. They connect confinement with dynamical chiral symmetry
breaking, both with the observable properties of hadrons, and hence provide a
means of elucidating the material content of real-world QCD. This contribution
illustrates these points via comments on: in-hadron condensates; dressed-quark
anomalous chromo- and electro-magnetic moments; the spectra of mesons and
baryons, and the critical role played by hadron-hadron interactions in
producing these spectra.Comment: 11 pages, 7 figures. Contribution to the Proceedings of "Applications
of light-cone coordinates to highly relativistic systems - LIGHTCONE 2011,"
23-27 May, 2011, Dallas. The Proceedings will be published in Few Body
System
Survey of nucleon electromagnetic form factors
A dressed-quark core contribution to nucleon electromagnetic form factors is
calculated. It is defined by the solution of a Poincare' covariant Faddeev
equation in which dressed-quarks provide the elementary degree of freedom and
correlations between them are expressed via diquarks. The nucleon-photon vertex
involves a single parameter; i.e., a diquark charge radius. It is argued to be
commensurate with the pion's charge radius. A comprehensive analysis and
explanation of the form factors is built upon this foundation. A particular
feature of the study is a separation of form factor contributions into those
from different diagram types and correlation sectors, and subsequently a
flavour separation for each of these. Amongst the extensive body of results
that one could highlight are: r_1^{n,u}>r_1^{n,d}, owing to the presence of
axial-vector quark-quark correlations; and for both the neutron and proton the
ratio of Sachs electric and magnetic form factors possesses a zero.Comment: 43 pages, 17 figures, 12 tables, 5 appendice
Academic freedom: in justification of a universal ideal
This paper examines the justification for, and benefits of, academic freedom to academics, students, universities and the world at large. The paper surveys the development of the concept of academic freedom within Europe, more especially the impact of the reforms at the University of Berlin instigated by Wilhelm von Humboldt. Following from this, the paper examines the reasons why the various facets of academic freedom are important and why the principle should continue to be supported
Couplings of light I=0 scalar mesons to simple operators in the complex plane
The flavour and glue structure of the light scalar mesons in QCD are probed
by studying the couplings of the I=0 mesons and to the
operators , and to two photons. The Roy dispersive
representation for the amplitude is used to determine the
pole positions as well as the residues in the complex plane. On the real axis,
is constrained to solve the Roy equation together with elastic
unitarity up to the K\Kbar threshold leading to an improved description of
the . The problem of using a two-particle threshold as a matching
point is discussed. A simple relation is established between the coupling of a
scalar meson to an operator and the value of the related pion form-factor
computed at the resonance pole. Pion scalar form-factors as well as two-photon
partial-wave amplitudes are expressed as coupled-channel Omn\`es dispersive
representations. Subtraction constants are constrained by chiral symmetry and
experimental data. Comparison of our results for the couplings with
earlier determinations of the analogous couplings of the lightest I=1 and
scalar mesons are compatible with an assignment of the ,
, , into a nonet. Concerning the gluonic operator
we find a significant coupling to both the and the
.Comment: 31 pages, 5 figure
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