[Jlab-seminars] Physics Division/Theory Center Joint Seminar - Corrected Version

[Mary Fox]

Physics Divison/Theory Center Joint Seminar
Wed., Dec. 4, 2013
3:30 p.m. (coffee at 3:15 p.m.)
CEBAF Center, Room L102


Jorge Segovia
Argonne National Laboratory

*Recent Advances in the Calculation of Hadron Form Factors Using 
Dyson-Schwinger Equations of QCD*

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.

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 ?? 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 ?? 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 m? 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).

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.

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