Hadron structure in the description of electromagnetic reactions

Koch, J.H.; Pascalutsa, Vladimir; Scherer, Stefan

doi:10.1103/physrevc.65.045202

Cited by 31 publications

(37 citation statements)

References 52 publications

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“…[7] which is based on the same set of Born diagrams, but concerning the electromagnetic form factors we employ a prescription proposed in Refs. [13,14] where an arbitrary form factor FðQ 2 Þ can be accommodated by the following replacement of the currents:…”

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confidence: 99%

Deeply inelastic pions in the exclusive reactionp(e,e′π+)nabove the resonance region

2008

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A model for the pðe; e 0 þ Þn reaction which combines an improved treatment of gauge invariant mesonexchange currents and hard deep-inelastic scattering (DIS) of virtual photons off nucleons is proposed. It is shown that DIS dominates and explains the transverse response at moderate and high photon virtualities Q 2 , whereas the longitudinal response is dominated by hadronic degrees of freedom and the pion electromagnetic form factor. This leads to a combined description of the longitudinal and transverse components of the cross section in a wide range of photon virtuality Q 2 and momentum transfer to the target t and solves the longstanding problem of the observed large transverse cross sections. The latter are shown to be sensitive to the intrinsic transverse momentum distribution of partons.At Jefferson Laboratory (JLAB) the exclusive reaction pðe; e 0 þ Þn has been investigated for a range of photon virtualities up to Q 2 ' 5 GeV 2 at an invariant mass of the þ n system around the onset of deep-inelastic regime, W ' 2 GeV [1-3]. A separation of the cross section into the transverse T and longitudinal L components has been performed. The longitudinal cross section L is well understood in terms of the pion quasielastic knockout mechanism [4] because of the pion pole at low Àt. This makes it possible to study the charge form factor of the pion at momentum transfer much bigger than in the scattering of pions from atomic electrons [5]. On the other hand, the T is predicted to be suppressed by $1=Q 2 with respect to L for sufficiently high Q 2 ) Ã 2 QCD [6]. However, the data from the À CT experiment [3] show that T is large at JLAB energies. At Q 2 ¼ 3:91 GeV 2 T is by about a factor of 2 larger than L and at Q 2 ¼ 2:15 GeV 2 it has the same size as L in agreement with previous JLAB measurements [1]. Theoretically, the model of Ref. [7], which is generally considered to be a guideline for the experimental analysis and extraction of the pion form factor, underestimates T at Q 2 ¼ 2:15 GeV 2 and at Q 2 ¼ 3:91 GeV 2 by about 1 order of magnitude [3]. Previous measurements at values of Q 2 ¼ 1:6ð2:45Þ GeV 2 [1] show a similar problem in the understanding of T . Even at smaller JLAB [2] and much higher Cornell [8] values of Q 2 , there is a disagreement between model calculations based on the hadron-exchange scenario and experimental data; see Ref.[9] for a possible interpretation and references therein.In this paper we first generalize the treatment of Ref.[7] for the longitudinal contribution. We then propose a resolution of the T problem. The idea followed here is to complement the soft hadronlike interaction types shown in Fig. 1 which dominate in photoproduction and low Q 2 electroproduction by direct hard interaction of virtual photons with partons followed by the hadronization process into þ n channel, to form the þ -electroproduction framework. As we shall show, then the large T in the reaction pðe; e 0 þ Þn can be readily explained and both L and T can be described from low up to high values of Q 2 .The exclus...

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Deeply inelastic pions in the exclusive reactionp(e,e′π+)nabove the resonance region

2008

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“…From the point of view of EFT it is not consistent to consider a minimal coupling only (see, e.g., Ref. [12]). Using symmetry arguments only, c 0 and κ 0 are free parameters of the most general effective Lagrangian [13].…”

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Quantum Electrodynamics for Vector Mesons

et al. 2005

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Quantum electrodynamics for ρ mesons is considered. It is shown that, at tree level, the value of the gyromagnetic ratio of the ρ + is fixed to 2 in a self-consistent effective quantum field theory. Further, the mixing parameter of the photon and the neutral vector meson is equal to the ratio of electromagnetic and strong couplings, leading to the mass difference M ρ 0 − M ρ ± ∼ 1 MeV at tree order.PACS numbers: 11.10. Gh, 12.39.Fe The question of the intrinsic magnetic moment of (elementary) particles of arbitrary spin s has been discussed controversially in the literature and is still of great interest. On the one hand, low-energy theorems and the optical theorem require that the gyromagnetic ratio g ≃ 2 for a particle with arbitrary spin s different from zero (at least for particles which do not participate in strong interactions) [1]. On the other hand, general arguments have been given that the minimal coupling leads to 1/s for this quantity [2]. Finally, the investigations of Ref. [3] regarding the theory of charged vector mesons interacting with the electromagnetic field suggested that the gyromagnetic ratio depends on a free parameter, thus allowing it to take any value. Below, we will address this question from the point of view of effective field theory (EFT).In Ref.[4] we have shown how the universal coupling of the ρ meson and the Kawarabayashi-Suzuki-RiadzuddinFayyazuddin (KSRF) relation [5,6] follow from the requirement that chiral perturbation theory of pions, nucleons, and ρ mesons is a consistent EFT. Although EFTs are non-renormalizable in the traditional sense, the general principles of EFT [7] require that all ultraviolet divergences can be absorbed into the redefinition of fields and parameters of the most general Lagrangian [8]. Imposing the renormalizability in this sense one finds that not all parameters of the most general Lagrangian are free but satisfy consistency conditions [9]. In this Letter we will use similar arguments for the effective Lagrangian including, in addition, the interaction with photons to show that the gyromagnetic ratio is fixed to g = 2 at tree level. Furthermore, the mixing parameter of the photon and the neutral vector meson is also fixed and leads to M ρ 0 − M ρ ± ∼ 1 MeV at tree order.We start with the chirally invariant effective Lagrangian including vector mesons in the form given by Weinberg [10], containing all interaction terms which respect Lorentz invariance, the discrete symmetries, and chiral symmetry. The electromagnetic interaction is introduced by adding all terms with photon fields which are allowed by U(1) gauge invariance,Here, B µ is a U(1) gauge vector field, V a µ (a = 1, 2, 3) denote the Cartesian components of an isospin triplet of vector fields, and Ψ is an isospin doublet of nucleon fields with mass m 0 [11]. Furthermore,All fields and coupling constants in Eq. (1) are bare quantities. From the point of view of EFT it is not consistent to consider a minimal coupling only (see, e.g., Ref. [12]). Using symmetry arguments only, c 0 and κ 0...

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“…Following the arguments given in, e.g., [11,12], we find that an arbitrary form factor F (Q 2 ) can be accommodated by the following replacement the current:…”

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γ*NΔform factors from a relativistic dynamical model of pion electroproduction

et al. 2004

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We obtain the electromagnetic form factors of the γN ∆ transition by analyzing recent pionelectroproduction data using a fully relativistic dynamical model. Special care is taken to satisfy Ward-Takahashi identities for the Born term in the presence of form factors thereby allowing the use of realistic electromagnetic form factors of the nucleon and pion. We parametrize the Q 2 dependence of the bare γN ∆ form factors by a three-parameter form which is consistent with the asymptotic behavior inferred from QCD. The parameters of the bare γN ∆ form factors are the only free parameters of the model and are fitted to the differential cross-section and multipoleanalysis data up to Q 2 = 4 (GeV/c) 2 in the ∆(1232)-resonance region. This analysis emphasizes the significance of the pion-cloud effects in the extraction of the resonance parameters.

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Hadron structure in the description of electromagnetic reactions

Cited by 31 publications

References 52 publications

Deeply inelastic pions in the exclusive reactionp(e,e′π+)nabove the resonance region

Deeply inelastic pions in the exclusive reactionp(e,e′π+)nabove the resonance region

Quantum Electrodynamics for Vector Mesons

γ*NΔform factors from a relativistic dynamical model of pion electroproduction

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