90 research outputs found
The polarized EMC effect
Conference details: Quark confinement and the hadron spectrum VII : 7th Conference on Quark Confinement and the Hadron Spectrum, QCHS7, Ponta Delgada, Açores, Portugal, 2-7 September 2006 / José EmÃlio F. T. Ribeiro (ed.): pp. 248-250We calculate both the spin independent and spin dependent nuclear structure functions in an effective quark theory. The nucleon is described as a composite quark-diquark state, and the nucleus is treated in the mean field approximation. We predict a sizable polarized EMC effect, which could be confirmed in future experiments.W. Bentz, I. C. Cloet, and A. W. Thoma
Covariance, Dynamics and Symmetries, and Hadron Form Factors
We summarise applications of Dyson-Schwinger equations to the theory and
phenomenology of hadrons. Some exact results for pseudoscalar mesons are
highlighted with details relating to the U_A(1) problem. We describe inferences
from the gap equation relating to the radius of convergence for expansions of
observables in the current-quark mass. We recapitulate upon studies of nucleon
electromagnetic form factors, providing a comparison of the ln-weighted ratios
of Pauli and Dirac form factors for the neutron and proton.Comment: 9 pages, 2 figures. Contribution to proceedings of Workshop on
Exclusive Reactions at High Momentum Transfer, May 21-24, 2007, Jefferson
Lab, Newport News, V
Spin-dependent structure functions in nuclear matter and the polarized EMC effect
An excellent description of both spin-independent and spin-dependent quark
distributions and structure functions has been obtained with a modified
Nambu-Jona-Lasinio model, which is free of unphysical thresholds for nucleon
decay into quarks - hence incorporating an important aspect of confinement. We
utilize this model to investigate nuclear medium modifications to structure
functions and find that we are readily able to reproduce both nuclear matter
saturation and the experimental F^A_2N / F_2N ratio, that is, the EMC effect.
Applying this framework to determine g^A_1p, we find that the ratio g^A_1p /
g_1p differs significantly from 1, with the quenching caused by the nuclear
medium being about twice that of the spin-independent case. This represents an
exciting result, which if confirmed experimentally, will reveal much about the
quark structure of nuclear matter.Comment: 4 pages, 4 figure
Baryon Resonance Phenomenology
The Japan Hadron Facility will provide an unprecedented opportunity for the
study of baryon resonance properties. This talk will focus on the chiral
nonanalytic behaviour of magnetic moments exclusive to baryons with open decay
channels. To illustrate the novel features associated with an open decay
channel, we consider the ``Access'' quark model, where an analytic continuation
of chiral perturbation theory is employed to connect results obtained using the
constituent quark model in the limit of SU(3)-flavour symmetry to empirical
determinations.Comment: 10 pages, 4 figures, ws-procs9x6.cls(included), Proceedings from the
Joint CSSM/JHF Workshop, Adelaide, March 14-21, 200
Exposing the dressed quark's mass
This snapshot of recent progress in hadron physics made in connection with
QCD's Dyson-Schwinger equations includes: a perspective on confinement and
dynamical chiral symmetry breaking (DCSB); a pre'cis on the physics of
in-hadron condensates; results on the hadron spectrum, including
dressed-quark-core masses for the nucleon and Delta, their first radial
excitations, and the parity-partners of these states; an illustration of the
impact of DCSB on the electromagnetic pion form factor, thereby exemplifying
how data can be used to chart the momentum-dependence of the dressed-quark mass
function; and a prediction that F_1^{p,d}/F_1^{p,u} passes through zero at
Q^2\approx 5m_N^2 owing to the presence of nonpointlike scalar and axial-vector
diquark correlations in the nucleon.Comment: 10 pages, 4 figures, 2 tables. Contribution to the Proceedings of the
4th Workshop on Exclusive Reactions at High Momentum Transfer, Thomas
Jefferson National Accelerator Facility Newport News, Virginia, 18-21 May
201
Pion electromagnetic form factor at spacelike momenta
A novel method is employed to compute the pion electromagnetic form factor,
F_\pi(Q^2), on the entire domain of spacelike momentum transfer using the
Dyson-Schwinger equation (DSE) framework in quantum chromodynamics (QCD). The
DSE architecture unifies this prediction with that of the pion's valence-quark
parton distribution amplitude (PDA). Using this PDA, the leading-order,
leading-twist perturbative QCD result for Q^2 F_\pi(Q^2) underestimates the
full computation by just 15% on Q^2>~8GeV^2, in stark contrast with the result
obtained using the asymptotic PDA. The analysis shows that hard contributions
to the pion form factor dominate for Q^2>~8GeV^2 but, even so, the magnitude of
Q^2 F_\pi(Q^2) reflects the scale of dynamical chiral symmetry breaking, a
pivotal emergent phenomenon in the Standard Model.Comment: 5 pages, 2 figures. To appear in Phys. Rev. Let
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