[Theory-seminars] Physics Divison/Theory Center Joint Seminar: Today at 3.30pm!

[Cesar Fernandez-Ramirez]

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 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).

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.


-- 
César Fernández-Ramírez

Center for Theoretical and Computational Physics
Thomas Jefferson National Accelerator Facility -- Jefferson Lab
12000 Jefferson Avenue, Newport News, VA 23606, USA

Phone: (+1) 757-269-7844
Fax: (+1) 757-269-7002
E-mail: cesar at jlab.org