Octet baryon electromagnetic form factor double ratios (GE*/GM*)/(GE

Abstract

The ratios of the baryon electric (GE) to magnetic (GM ) form factors, GE/GM , can provide us with important information on the structure of baryons in vacuum as demonstrated in recent studies for the proton and neutron systems. It has been argued that the corresponding in-medium ratios, G * E /G * M , can also provide useful information on the electromagnetic structure of baryons in a nuclear medium. Although the ratios may not be measured directly in experiments except for the proton at present, the double ratios (G * E /G * M )/(GE/GM ) can be directly related to the polarization-transfer measurements of bound baryons. In the present work we estimate, for the first time, the double ratios (G * E /G * M )/(GE/GM ) for all the members of octet baryons in symmetric nuclear matter in the range Q 2 = 0-3 GeV 2 , where Q 2 = −q 2 with q being the four-momentum transfer.

Electromagnetic form factors of baryons in nuclear medium

“…The electromagnetic structure of the quark is parametrized using a vector meson dominance (VMD) form, which includes contributions of vector meson poles depending on the (3) channel (isovector, isoscalar and strange quark) [3,13]. The radial wave functions are parametrized in terms of two momentum range scales according to the number of strange quarks ( (3) symmetry breaking) [2,13]. To determine the parameters of the radial wave functions, we extend the model to the lattice QCD regime, in a region where meson cloud effects are suppressed (large pion mass region).…”

“…The parameters are then adjusted to the lattice QCD data (details can be found in Refs. [2,3,13,14]). In addition to the valence quarks, we consider also contributions associated to pion cloud dressing of the baryon cores determined by the cloudy bag model [15], using the (3) pion-baryon interaction [16], with a 2 -dependence calibrated by the physical data for the nucleon and by the hyperon magnetic moments [3,13].…”

“…Since the discovery of the EMC effect [1] it has been known that the hadrons change their properties in a nuclear medium [2,3]. The natural interpretation is that a strong mean field modifies the properties of the degrees of freedom of QCD, the quarks and gluons.…”

“…Measurements of the proton electromagnetic form factor double ratio were performed already at MAMI [5] and Jefferson Lab [6] using the 4 He( ì , ì ) 3 H reaction (proton in medium). The data show signs of medium modifications, but the small magnitude of the effect may be a consequence of the averaged low density of the 4 He system compared with the normal nuclear density 0 = 0.15 fm −3 [2,3]. The extension of the method to the neutron is planned for the future ( 4 He( ì , ì ) 3 He reaction) [7].…”

Quark model calculations of transition form factors at high photon virtualities

“…(1), we can express the transition current in terms of the quark electromagnetic form factors f i± (i = 1, 2) and the radial wave functions ψ N and ψ R for resonances with no strange quarks and determine the transition form factors [7,14,23,24]. Taking advantage of the quark form factor structure, based on vector meson dominance, the model has been extended to the lattice QCD regime (heavy pions and reduced meson cloud) [8,12,21], to the nuclear medium [25,26] and to the timelike regime (Q 2 < 0) [15,17,27,28].…”