# The 12th International Workshop on the Physics of Excited Nucleons

10-14 June 2019
Bonn, Campus Poppelsdorf
Europe/Zurich timezone

## Undressing the nucleon with photons

12 Jun 2019, 15:00
30m
HS 5 ()

### HS 5

Baryon spectrum through meson photoproduction

### Speaker

Helmut Haberzettl (The George Washington University)

### Description

Allowing arbitrary dressing mechanisms for both pions and nucleons, it is shown
that combining the respective fully dressed propagators with fully dressed
electromagnetic currents in a consistent manner completely removes all
dressing effects from half-on-shell combinations of the form
(propagator)$\times$(current). The key to this exact result is the realization
that the coupling procedures for the electromagnetic field are necessarily
different when applied to the respective scalar dressing functions of Dirac and
scalar particles. This means, for example, that the usual gauge-invariant
Ball-Chiu current ansatz for spin-1/2 particles suffers from an incomplete
coupling procedure. Immediate and direct practical consequences for the
descriptions of pion photoproduction off the nucleon and real Compton
scattering processes on the pion and on the nucleon are discussed. For the
latter processes, in particular, it is found that all dressing effects
cancel exactly, except for contributions where hadrons loop around
two photon insertion points.

### Primary author

Helmut Haberzettl (The George Washington University)