L. L. Sviridova

The exclusive reaction of pion quasi-elastic knockout from nucleon p(e, e′ π)B to various states of the final baryon-spectator B and the quark models for the pion cloud of nucleon

The pion momentum distributions (MDs) in four channels of virtual decay p→B+π, B = N, Δ, N1/2-(1535), N1/2+(1440) are calculated in two models, the microscopic model of 3P0 scalar q−q fluctuation with the pion as a composite q−q-system and the chiral semi-microscopic model of πq interaction with the pion as a structureless Goldstone boson. The results of the above models are similar for the baryon states B = N, Δ, N1/2-(1535) but are rather different for the Roper resonance N1/2+(1440) which corresponds to excitation of two oscillator quanta in the nucleon. The experimental investigation of pion MDs by means of the reaction of quasi-elastic knockout of pion by an electron of a few GeV energy p(e, e′ π)B may be very suitable for Jefferson Laboratory, Virginia (JLab).

Quark Microscopic Picture for Vector and Pseudoscalar Mesons in the Nucleon and Their Quasielastic Knockout by High-Energy Electrons

A technique for projecting a multiquark wave function in the microscopic model of a 3P0 scalar fluctuation onto the virtual-decay channels N → N + ρ and N → N + π is formulated (at a more general level for the latter than previously). The amplitude for the electromagnetic transition ρ + γT* → π in electron-induced quasielastic rho-meson knockout followed by rho-meson conversion to a pion is considered. Theoretical results obtained in this way are contrasted against available experimental data, and reasonable agreement is found for cross-section values. This confirms a universal character of the 3P0 model. The precision of relevant experiments is as yet insufficient for comparing the momentum distribution of the rho meson from the channel N → N + ρ with its theoretical counterpart.

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

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.

What new information about the microscopic quark structure of meson-baryon degrees of freedom in nucleons can be deduced from data on quasielastic pion knockout?

The emergence of B + π virtual subsystems, which manifest themselves in quasielastic-knockout processes of the N(e, e′π)B type, where B is the final-state baryon in various excited states, is considered within the microscopic quark model of a 3P0 localized scalar fluctuation of the QCD vacuum. The relevant technique for taking into account the rearrangement of quark degrees of freedom is demonstrated. The model provides a good description of known momentum distributions for B=N channels. The momentum distributions are predicted for N → π + B channels, where B=Δ, N*(1535), or N**(1440). It would be of interest to study these channels in exclusive coincidence experiments at accelerators producing electron beams of energy ranging between 5 and 10 GeV.

Scale of corrections to the t-channel pole approximation in quasielastic pion knockout from a nucleon by high-energy electrons

An investigation of a dominant role of the simplest t-channel pole diagrams in pion electroproduction on nucleons for quasielastic-knockout kinematics at an electron energy of a few GeV is completed-namely, the competition between the t-channel pion and rho-meson pole amplitudes, on one hand, and the s-channel pole amplitude (tree diagram), on the other hand, is considered. When the virtual-photon mass is sufficiently large [Q2>2(GeV/c)2], the last amplitude does not make significant contributions to relevant cross sections, either the longitudinal (dσL/dt) or the transverse (dσT/dt) one. At Q2=0.7(GeV/c)2, the term associated with the interference between the t-channel pion-pole amplitude and the s-channel pole amplitude is still noticeable in the longitudinal cross section. The vertex functions gρNN(t) as obtained from the cross section for the quasielastic knockout of rho mesons and from the cross section for pion photoproduction are compared. Their disagreement must give impetus to a further development of the gauge-invariant theory of pion photoproduction.

Probing the mesonic structure of the nucleon by means of exclusive quasielastic pion knockout in (e, e′π) reactions at high energies

By including the Z diagram in an analysis performed in the laboratory frame (instantaneous form of dynamics), the notion of quasielastic pion knockout by protons and electrons [(p, 2p) and (e, e′p) reactions treated in terms of the relevant pole diagrams] is generalized to the relativistic case where a meson is quasielastically knocked out of a nucleon by an electron having an energy of a few GeV. The concept of the wave function is introduced for the pion (and for other mesons), and its relation to the vertex constant GπNN and the vertex function gπNN(k2) is indicated. The spectroscopic factor SNBℳ is defined as the normalization of the wave function for the meson ℳ. It is shown by two methods that, under the kinematical conditions of quasielastic knockout that include the condition Eπ≫mπ (Eπ is the energy of the knock-on pion) and the condition that the square Q2 of the virtual pion mass is large, the competing tree diagram is suppressed in relation to the pole diagram (this is not so in the case of pion photoproduction). From data of a p(e, e′π+)n experiment involving longitudinal virtual photons γL*, the momentum distribution |Ψpnπ(k)|2 of pions in the nucleon is extracted for the first time over the entire range of significant momenta k, and this result is used to determine the cutoff constant Λπ and the value of Spnπ≈0.2. The momentum distribution of positive rho mesons in the soft section of the spectrum is determined from experimental data on the process p(e, e′π+)n proceeding through the mechanism ρ++γT* → π+ involving transverse photons. A way to determine the momentum distribution of omega mesons through data on the process p(e, e′π0)p is indicated. Two forms of dynamics—instantaneous form and that of light-front dynamics (the latter does not involve the Z diagram)—are compared for the example where the calculations are performed for the spectroscopic factor SNBℳ.

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

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.

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

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.

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

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