Q2-evolution of nucleon-to-resonance transition form factors in a QCD-inspired vector-meson-dominance model

Abstract

We adopt the vector-meson-dominance approach to investigate Q 2 -evolution of N R-transition form factors (N denotes nucleon and R an excited resonance) in the first and second resonance regions. The developed model is based upon conventional γN R-interaction Lagrangians, introducing three form factors for spin-3/2 resonances and two form factors for spin-1/2 nucleon excitations. Lagrangian form factors are expressed as dispersionlike expansions with four or five poles corresponding to the lowest excitations of the mesons ρ(770) and ω(782). Correct high-Q 2 form factor behavior predicted by perturbative QCD is due to phenomenological logarithmic renormalization of electromagnetic coupling constants and linear superconvergence relations between the parameters of the meson spectrum. The model is found to be in good agreement with all the experimental data on Q 2 -dependence of the transitions N ∆(1232), N N (1440), N N (1520), N N (1535). We present fit results and model predictions for high-energy experiments proposed by JLab. Besides, we make special emphasis on the transition to perturbative domain of N ∆(1232) form factors.

Low-Q2 scaling behavior of the form-factor ratios for the NΔ(1232)-transition

“…Thus, it is interesting to compare the quasi-scaling in the model developed here and in the popular ones discussed in the literature [26,32]. We find that the hypothesis of the quasi-scaling of the FFs ratios in the non-gauge-invariant model [26,33] is in much poorer agreement with the data. In the same time, the gauge invariant FFs of Ref.…”

“…It is interesting that the quasi-scaling of the another popular (non-point-and non-gauge-invariant) choice of the FFs [27,33] is in much poorer agreement with the data, χ 2 /DOF = 2.58 (The curves are shown in Fig. 2, the bottom panel.)…”

“…The Section 4 reviews the vector-meson-dominance model of Ref. [33]. In Section 5 we contrast our model with the experimental data on the Q 2 -dependence of the N∆(1232)-transition and test the hypothesis of the quasi-scaling behavior of the transition FFs.…”

“…Within the vector-meson-dominance (VMD) model [33], the FFs F f (Q 2 ) are given by dispersionlike expansions…”

Electromagnetic transition form factors of the Roper resonance in a phenomenological field theory

“…Theoretical studies of the transverse and scalar (longitudinal) helicity amplitudes have been performed in various frameworks such as nonrelativistic constituent quark models (including relativistic corrections) [25][26][27], relativistic quark models [28][29][30][31], chiral quark models [32][33][34], different hybrid models [35,36], approaches including vector-meson-dominance features [37,38], and lattice QCD [39,40]. Even though the empirical data for the transverse and scalar (longitudinal) helicity amplitudes can be described fairly well for larger values of the squared momentum transfer in the framework of relativistic quark models as well as in lattice QCD, neither of the two approaches predicts the behavior in the low-Q 2 region correctly [9,12,29,33,37,39,40].…”