Combined parametrization of GEn and $\gamma^{\ast} N \rightarrow \Delta (1232)$γ*N→Δ(1232) quadrupole form factors

Abstract

Models based on SU (6) symmetry breaking and large Nc limit provide relations between the pion cloud contributions to the γ * N → ∆(1232) quadrupole form factors, electric (GE) and Coulomb (GC ), and the neutron electric form factor GEn, suggesting that those form factors are dominated by the same physical processes. Those relations are improved in order to satisfy a fundamental constraint between the electric and Coulomb quadrupole form factors in the long wavelength limit, when the photon threemomentum vanishes (Siegert's theorem). Inspired by those relations, we study alternative parametrizations for the neutron electric form factor. The parameters of the new form are then determined by a combined fit to the GEn and the γ * N → ∆(1232) quadrupole form factor data. We obtain a very good description of the GE and GC data when we combine the pion cloud contributions with small valence quark contributions to the γ * N → ∆(1232) quadrupole form factors. The best description of the data is obtained when the second momentum of GEn is r 4 n ≃ −0.4 fm 4 . We conclude that the square radii associated with GE and GC , r 2 E and r 2 C , respectively, are large, revealing the long extension of the pion cloud. We conclude also that those square radii are related by r 2 E − r 2 C = 0.6 ± 0.2 fm 2 . The last result is mainly the consequence of the pion cloud effects and Siegert's theorem. PACS. 13.40.Gp Electromagnetic form factors -14.20.Gk Baryon resonances with S=0 -14.20.Dh Protons and neutrons

Quark model calculations of transition form factors at high photon virtualities

“…The original large-N c estimates of the pion cloud contributions [56,57,62], can be improved with a correction of the order 1/N 2 c in G E in order to be consistent with Siegert's theorem (4) [62][63][64]. Combining the valence quark contributions with an improved large-N c estimate of the pion cloud for G E [62], we obtain a good description of the quadrupole form factor data.…”

“…The model combines valence quark and pion cloud contributions. Data from [6,64]. Left: Estimate with pion cloud contribution from [62].…”

“…Figure 9. Neutron electric form factor G En determined by a global fit to the G En and G E and G C quadrupole form factors [64].…”

“…The pion cloud parametrizations for G E and G C are based on relations G E ∝ G En and G C ∝ G En , valid at low Q 2 [56,57,62,63], where G En is the neutron electric form factor. Inspired by the previous results, we use the relation between the γ * N → Δ(1232) G E and G C form factors and G En to obtain a more accurate parametrization of G En [64]. Our final parametrization for G En is presented in Fig.…”

Covariant model for Dalitz decays of decuplet baryons to octet baryons

“…How important are the physical poles associated with the meson resonances. How significant is the q 2 dependence of the form factors, and how are form factors modified near the pseudothreshold q 2 ¼ ðM B 0 − M B Þ 2 [15,29,[52][53][54][55].…”