Photo- and electro-excitation of the Δ-resonance at MAMI

Abstract

Abstract. Over the last decade accurate experiments at MAMI played an essential role to improve our understanding of the nucleon to ∆(1232) transition. Originally to a large extent motivated through intraquark hyperfine interactions anticipated in QCD-inspired quark models they showed that pionic degrees of freedom are essential. The meson cloud is mainly responsible for the observed quadrupole excitation strength and affects the magnetic dipole transition strength as well.

Delta and omega electromagnetic form factors in a Dyson-Schwinger/Bethe-Salpeter approach

“…The subleading s−, p− and d−wave amplitude components are significantly suppressed compared to this structure which corresponds to τ µρ 1 = δ µρ in Eq. (9). A similar observation holds for the nucleon amplitude and might indicate that orbital angular-momentum correlations in these baryons' amplitudes are dominated by pionic effects which are absent in our setup.…”

“…Information on ∆ 0 and ∆ − static electromagnetic properties is totally missing, and there are no experimental results for the evolution of the charge and magnetic properties with Q 2 = 0. On the other hand, further insight has been achieved through the measurement of the N γ∆ transition [3][4][5][6][7][8][9][10], * Electronic address: nicmorus@th.physik.uni-frankfurt.de where knowledge of the helicity amplitudes, the electric quadrupole and the Coulomb quadrupole form factors of the transition allows for an extraction of the ∆'s electric quadrupole moment [5].…”

Delta-baryon mass in a covariant Faddeev approach

“…Experimentally, the first excited state of the nucleon, the ∆ resonance, is observed in scattering pions, photons, or electrons off nucleon targets. Recent precise experiments at LEGS, BATES, ELSA, MAMI, and JLAB do not only report on its mass (M ∆ = 1.232 GeV) and width (Γ ∆ = 120 MeV) but also measure the electromagnetic N → ∆γ transition form factors [1,2,3,4,5,6,7]. The basic properties of the lightest resonance of the nucleon have been calculated within various approaches such as constituent-quark models [8,9,10,11,12,13,14,15,16,17], Skyrme models [18,19,20], chiral cloudy bag models [21,22], chiral effective field theory methods [23,24,25,26,27], and lattice-regularized QCD [28,29,30,31,32].…”

Ratio ofσL/σTforp(e,e'

“…(15) and in the response functions of Eqs. (8)(9)(10)(11), the CGLN amplitudes contain additional θ * K dependence (as well as Q 2 and W dependence). This suggests that Eq.…”