D. K. Fedorov

Electroproduction of the Roper resonance on the proton: the role of the three-quark core and the molecular

The Roper resonance is considered as a mixed state of a three-quark core configuration and a hadron molecular component N+sigma. Based on this ansatz we study electroproduction of the Roper resonance. The strong and electromagnetic couplings induced by the quark core are calculated in the 3P0 model. The contribution of the vector meson cloud to the electromagnetic transition is given in the framework of the VMD model. Results are compared with the recent JLab electroproduction data.

N\sigma

Abstract The interference term between s - and t -pole contributions to the p ( e , e ′ π + ) n cross section is evaluated on the basis of the constituent quark model. It is shown that the contribution of baryon s -poles can be modeled by a nonlocal extension of the Kroll–Ruderman contact term. This contribution is in a destructive interference with the pion t -pole that is essential to improve the description of recent JLab data at the invariant mass W = 1.95 GeV . Some predictions are made for a new JLab measurement at higher values W = 2.1 – 2.3 GeV and Q 2 centered at 1.6 and 2.45 GeV 2 / c 2 .

component

The electroproduction of charged rho mesons on a nucleon under conditions of quasielastic kinematics [Q2 > 2 (GeV/c)2, W > 3 GeV, and ωρ ≃ q0 in the laboratory frame] is considered. The main diagrams involved in this process are taken into account, and it is shown that the case of rho-meson knockout by a longitudinal photon is dominated by the t-channel pole diagram featuring a rho meson, so that direct rho-meson electroknockout makes a leading contribution to the process being considered. Questions concerning the gauge invariance of the electroproduction process are considered in detail, along with contact terms that inevitably arise in employing form factors in ρNN vertices and which are necessary for restoring gauge invariance. It is shown that the structure of these contact terms is not determined exclusively by the requirement that the electroproduction amplitude be gauge-invariant and may be such that the corresponding correction to the dominant diagram due to these terms proves to be suppressed in the quasielastic region (with increasing Q2), with the result that the t-pole process featuring a rho meson remains dominant; at the same time, the contribution of this correction is significant at low Q2 or in the case of photoproduction, modifying substantially the electroproduction cross section.

Quark model description of quasi-elastic pion knockout from the proton at JLAB

It is shown that, under kinematical conditions of quasielastic knockout, the differential cross section for the exclusive process p(e, e′π+)n induced by longitudinal virtual photons γ*L changes its physical nature as the recoil momentum |k| of the spectator nucleon grows from values in the range |k| ∼ 0.1–0.4 GeV/c, which are typical in studying the pion cloud of the nucleon, to values around |k| ∼ 1 GeV/c. Calculations within the 3P0 quark microscopic model reveal that the amplitudes for the deexcitation of vector and pseudovector mesons via the processes ρ+ + γ*L → π+ and b1+ + γ*L → π+ become dominant here. Concurrently, the effective momentum distributions develop a substantial angular anisotropy of about 50% with respect to the angle between the momentum of the photon γ*L and the recoil momentum of the spectator nucleon, the longitudinal differential cross sections undergoing respective changes. In this region, the energy of knock-on pions is expected to be about 5 GeV under conditions of quasilastic kinematics.

Quasielastic rho-meson knockout from nucleons by high-energy electrons

The electroproduction of charged ρ mesons on a nucleon under conditions of quasielastic kinematics (Q2 = 2−3 (GeV/c)2, W = 2 GeV) is considered. Dominant diagrams are taken into account. It is shown that the transverse cross section for ρ-meson knockout, in just the same was as the respective longitudinal cross section, is dominated by the t-pole diagram involving a virtual ρ meson. This means that the leading contribution to the process comes from direct ρ-meson knockout.

Leading role pseudovector and vector mesons in quasielastic pion electroproduction on a nucleon by longitudinal photons at comparatively high virtual momenta in the range 0.4 ≤ |k| ≤ 1 GeV/c

It is shown that the amplitude for the direct knockout of ρ0 mesons plays an important role at energies W above the resonance region, W ≳ 2 GeV, and rather high values of Q2 ≳ 1.5−2 GeV2/c2 and that it corresponds to the t pole in the channel of the virtual decay p → p + π0 accompanied by quark-spin flip upon momentum transfer to the meson, π0 + γ*T → ρ0. The contributions of several scalar mesons (p → p + f0 channel), the contribution of the tensor meson f2, and effects of the interference between different contributions were taken into account in addition to the contribution of the π0 meson. The vectorand tensor-meson-dominance models were used to estimate the respective vertex constants. Within the proposed mechanism, the differential cross sections dσL/dt and dσT/dt were calculated for several W and Q2 values at which the respective total cross sections were measured by the CLAS Collaboration. Agreement with data on the transverse part of the total cross section, σT, was attained, whereby the assumption of a dominant role of magnetic spin transitions in the meson skin of the nucleon under conditions of quasielastic-knockout kinematics is confirmed. At the same time, the contribution of spin-flip transitions is suppressed in the longitudinal part of the cross section, σL, and this is confirmed by the results of our calculations. The behavior of the differential cross section dσT/dt is predicted with an eye to future experiments.

Knockout of charged ρ mesons from a nucleon by high-energy electrons in the quasielastic-kinematics region with the participation of transverse virtual photons

It is shown that, under kinematical conditions of quasielastic knockout, the differential cross section for the exclusive process p(e, e′π+)n induced by longitudinal virtual photons γ*L changes its physical nature as the recoil momentum |k| of the spectator nucleon grows from values in the range |k| ∼ 0.1–0.4 GeV/c, which are typical in studying the pion cloud of the nucleon, to values around |k| ∼ 1 GeV/c. Calculations within the 3P0 quark microscopic model reveal that the amplitudes for the deexcitation of vector and pseudovector mesons via the processes ρ+ + γ*L → π+ and b1+ + γ*L → π+ become dominant here. Concurrently, the effective momentum distributions develop a substantial angular anisotropy of about 50% with respect to the angle between the momentum of the photon γ*L and the recoil momentum of the spectator nucleon, the longitudinal differential cross sections undergoing respective changes. In this region, the energy of knock-on pions is expected to be about 5 GeV under conditions of quasilastic kinematics.

Electroproduction of ρ0 mesons on protons in quasielastic kinematics at intermediate energies and spin-flip mechanism of direct meson knockout

AbstractThe possibility of studying q4

Potential of quasielastic meson knockout from a nucleon by high-energy electrons and pions for studying the structure of exotic excited states of a final-state baryon

\bar q