New low-Q(2) measurements of the gamma*N -> Delta(1232) Coulomb quadrupole form factor, pion cloud parametrizations and Siegert's theorem

Author(s)

Ramalho, G.

• Document type: Journal article
• Source: EUROPEAN PHYSICAL JOURNAL A; v. 54, n. 5, p. 6-pg., 2018-05-14.

Abstract

The novel measurements of the gamma*N -> Delta(1232) Coulomb quadrupole form factor in the range Q(2) = 0.04-0.13 GeV2 changed the trend of the previous data. With the new data the electric and Coulomb form factors are both in remarkable agreement with estimates of the pion cloud contributions to the quadrupole form factors at low Q(2). The pion cloud contributions to the electric and Coulomb form factors can be parametrized by the relations G(E) proportional to G(En)/(1+ Q(2)/2M(Delta)(M-Delta-M)) and G(C) proportional to G(En), where G(En) is the neutron electric form factor, and M, M-Delta are the nucleon and Delta masses, respectively. Those parametrizations are in full agreement with Siegert's theorem, which states that G(E) = M-Delta-M/2M(Delta) G(C) at the pseudothreshold, when Q(2) = -(M-Delta - M)(2), and improve previous parametrizations. Also a small valence quark component estimated by a covariant quark model contributes to this agreement. The combination of the new data with the new parametrization for G(E) concludes an intense period of studying the gamma*N -> Delta(1232) quadrupole form factors at low Q(2), with the agreement between theory and data. (AU)

• FAPESP's process: 17/02684-5 - Study of the electromagnetic and axial structure of the baryons in the vacuum and in nuclear medium
• Grantee: Gilberto Tomás Ferreira Ramalho
• Support Opportunities: Research Grants - Young Investigators Grants