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Physics Divison/Theory Center Joint Seminar<br>
Wed., Dec. 4, 2013<br>
3:30 p.m. (coffee at 3:15 p.m.)<br>
CEBAF Center, Room L102<br>
<br>
<br>
Jorge Segovia<br>
Argonne National Laboratory<br>
<br>
<b>Recent Advances in the Calculation of Hadron Form Factors Using
Dyson-Schwinger Equations of QCD</b><br>
<br>
Elastic and transition form factors of nucleon excited states
provide vital information about their structure and composition.
They are a measurable and physical manifestation of the nature of
the hadrons constituents and the dynamics that binds them together.
In this respect, two emergent phenomena of Quantum Chromodynamics
(QCD), confinement and dynamical chiral symmetry breaking, appear to
play an important role; and Dyson-Schwinger equations (DSEs) have
been established as a nonperturbative quantum field theoretical
approach for the study of continuum strong QCD which is able to
connect such emergent phenomena with the behaviour of form factors.<br>
<br>
In this presentation, I provide an example of the contemporary
application of DSEs to the study of elastic and transition form
factors of N ∗ -states analyzing the electromagnetic γ∗ p → ∆+
transition. This reaction has stimulated a great deal of theoretical
analysis, and speculation about: the shape deformation of involved
hadrons; the relevance of perturbative QCD to processes involving
moderate momentum transfers; and the role that experiments on
resonance electroproduction can play in exposing nonperturbative
characteristics of QCD. The small-Q2 behaviour of the ∆ elastic form
factors is a necessary element in computing the γ∗ N → ∆ transition
form factors. I calculate the core contributions to the ∆+
electromagnetic form factors and compare to lattice data, both at
different pion masses. The ∆ elastic form factors appear to be very
sensitive to mπ and consequently to m∆ . Hence, given that the
parameters which define extant simulations of lattice-regularised
QCD produce ∆-resonance masses that are very large, the form factors
obtained therewith are a poor guide to properties of the ∆(1232).<br>
<br>
Finally, the measurement of form factors at high-Q2 virtualities is
actually challenging the theoretical computation techniques. This
presentation is intended to close with the description of a recently
introduced method to extract parton distribution amplitudes (PDAs)
from the light-front projections of the hadron bound-state
amplitudes (BSAs). These PDAs are necessary objects in the
computation of hard exclusive processes.<br>
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