arXiv subjects
Publications and source records attributed to A. Yu. Korchin.
A model is developed for electromagnetic form factors of the charged and neutral K-mesons. The formalism is based on ChPT Lagrangians with vector mesons. The form factors, calculated without fitting parameters, are in a good agreement with experiment for space-like and time-like photon momenta. Contribution of the two-kaon channels to the muon anomalous magnetic moment a_μis calculated.
Analytical expressions for the elastic and dynamic form factors (FFs) are derived in the shell model (SM) with a potential well of finite depth. The consideration takes into account the motion of the target-nucleus center of mass (CM). Explanation is suggested for a simultaneous shrinking of the density and momentum distributions of nucleons in nuclei. The convenient working formulae are given to handle the expectation values of relevant multiplicative operators in case of the 1s-1p shell nuclei.
A model is developed for electromagnetic form factor of the pion. One-loop corrections are included in the linear sigma-model. The rho-meson contribution is added in an extended VMD model. The form factor, calculated without fitting parameters, is in a good agreement with experiment for space-like and time-like photon momenta. Loop corrections to the two-pion hadronic contribution a^{(had, π)}_μto the muon anomalous magnetic moment are calculated. The optimal value of the sigma-meson mass appears to be close to the rho-meson mass.
Corrections to the Born approximation in photo-induced strangeness production off a proton are calculated in a semi-realistic microscopic model. The vertex corrections and internal contributions to the amplitude of the $γp \to K^+ Λ$ reaction are included on the one-loop level. Different gauge-invariant phenomenological prescriptions for the modification of the Born contribution via the introduction of form factors and contact terms are discussed. In particular, it is shown that the popular minimal-substitution method of Ohta corresponds to a special limit of the more realistic approach.
The chiral invariant QHD-III model of Serot and Walecka is applied in the calculation of some meson properties. The electromagnetic interaction is included by extending the symmetry of the model to the local U(1) \times SU(2)_{R} \times SU(2)_{L} group. The minimal and nonminimal contributions to the electromagnetic Lagrangian are obtained in a new representation of QHD-III. Strong decays of the axial-vector meson, a_{1} \to πρ, a_{1} \to πσ, and the electromagnetic decays ρ\to ππγ, a_{1} \to πγand ρ\to πγare calculated. The low-energy parameters for the π-πscattering are calculated in the tree-level approximation. The effect of the auxiliary Higgs bosons, introduced in QHD-III in order to generate masses of the vector and axial-vector mesons via the Higgs mechanism, is studied as well. This is done on the tree level for π-πscattering and on the level of one-loop diagrams for the a_{1} \to πγdecay. It is demonstrated that the model successfully describes some features of meson phenomenology in the non-strange sector.
Two-photon production in pp scattering is proposed as a means of studying virtual-pion emission. Such a process is complementary to real-pion emission in pp scattering. The virtual-pion signal is embedded in a background of double-photon bremsstrahlung. We have developed a model to describe this background process and show that in certain parts of phase space the virtual-pion signal gives significant contribution. In addition, through interference with the two-photon bremsstrahlung background, one can determine the relative phase of the virtual-pion process.
Coherent Compton scattering on light nuclei in the delta resonance region is studied in the impulse approximation and is shown to be a sensitive probe of the in-medium properties of the delta resonance. The elementary amplitude on a single nucleon is calculated from the unitary K-matrix approach developed previously. Modifications of the properties of the delta resonance due to the nuclear medium are accounted for through the self-energy operator of the delta, calculated from the one-pion loop. The dominant medium effects such as the Pauli blocking, mean-field modification of the nucleon and delta masses, and particle-hole excitations in the pion propagator are consistently included in nuclear matter.
Magnetic moment and radius of the nucleon are calculated in a nonlocal extension of the chiral linear σ-model. Properties of the nonlocal model under the vector and axial transformations are considered. The conserved electromagnetic and vector currents, and partially conserved axial vector current are obtained. In the calculation of the nucleon electromagnetic vertex the π- and σ-loop diagrams are included. Contribution from vector mesons is added in the vector meson dominance model with a gauge-invariant photon-meson coupling. The nonlocality parameter associated with the πN interaction is fixed from the experimental magnetic moment of the neutron. Other parameters (nonlocality parameter for the σN interaction and the mass of the σ-meson) are constrained by the magnetic moment of the proton. The calculated electric and magnetic mean-square radii of the proton and neutron are in satisfactory agreement with experiment.
Production of lepton pairs in gamma p -> e-e+ p reaction is studied at photon energies up to 1 GeV. We show that even if lepton charge is not measured there are extensive kinematical regimes where the nuclear process dominates over the Bethe-Heitler contribution. The decomposition is performed of the unpolarized cross section for the virtual Compton scattering in terms of response functions. These are expressed in terms of a polarization density matrix of the virtual photon and are shown to be sensitive to the properties of baryon resonances in the first and the second resonance regions. In the analysis a unitary K-matrix model is used, based on an effective Lagrangian including nucleon, mesons, and baryon resonances with masses up to 1.7 GeV. Results of the model are compared with data for real-photon Compton scattering and predictions are given for observables in photoproduction of lepton pairs.
The energy and four-momentum (Q^2) dependence of the photo-absorption cross section on the proton is calculated for helicity -1/2 and -3/2 states. An effective Lagrangian model is used, formulated in terms of meson and baryon degrees of freedom, which obeys crossing symmetry, unitarity, Lorentz and gauge invariance. The difference in the cross sections for the two helicity states, the Drell-Hearn-Gerasimov integral $I_{DHG}(Q^2)$, is evaluated at different $Q^2$. We find that at small momentum transfer the absolute value of $I_{DHG}(Q^2)$ first increases to reach a maximum at $Q^2 \approx 0.05 GeV^2$ before decreasing at higher $Q^2$.
Results of a relativistic model for pion- and photon-induced reactions on the proton are presented. The model is crossing symmetric and gauge invariant. The nucleon resonances P_11 (1440), P_11 (1710), D_13 (1520), S_11 (1535), S_11 (1650), P_33 (1232), P_33 (1600), S_31 (1620), and D_33 (1700) have been included explicitly in the calculation. Unitarity within the channel space πN + γN below the two-pion production threshold has been achieved by using the K-matrix approach. Strong and electromagnetic coupling parameters of the N* resonances have been determined from a fit to the pion-nucleon phase shifts, pion-photoproduction multipoles and Compton-scattering cross sections. The model is shown to describe simultaneously most of the available data. Results for the electric and magnetic polarizabilities of the nucleon are presented.
The process p+d \leftrightarrow 3He + γ* at intermediate energies is described using a covariant and gauge-invariant model, and a realistic pd3He vertex. Both photodisintegration of 3He and proton-deuteron capture with production of e+e- pairs are studied, and results for cross sections and response functions are presented. The effect of time-like formfactors on the dilepton cross sections is investigated as well.
We perform an expansion of the virtual Compton scattering amplitude for low energies and low momenta and show that this expansion covers the transition from the regime to be investigated in the scheduled photon electroproduction experiments to the real Compton scattering regime. We discuss the relation of the generalized polarizabilities of virtual Compton scattering to the polarizabilities of real Compton scattering.
We analyze virtual Compton scattering off the nucleon at low energies in a covariant, model-independent formalism. We define a set of invariant functions which, once the irregular nucleon pole terms have been subtracted in a gauge-invariant fashion, is free of poles and kinematical zeros. The covariant treatment naturally allows one to implement the constraints due to Lorentz and gauge invariance, crossing symmetry, and the discrete symmetries. In particular, when applied to the $ep\to e'p'γ$ reaction, charge-conjugation symmetry in combination with nucleon crossing generates four relations among the ten originally proposed generalized polarizabilities of the nucleon.
We show that lepton-pair production in Virtual Compton Scattering offers, through interference with the well-known Bethe-Heitler process, a sensitive probe to learn the longitudinal response of resonances and the electromagnetic nucleon form factors. This interference can be measured directly in terms of an asymmetry. The role of off-shell effects in the N-N-$γ$ vertices is investigated as well. An additional N-N-$γ-γ$ contact term in the amplitude, included to ensure gauge invariance of the model, cancels a substantial part of the off-shell effects.
We investigate the low-energy behavior of the four-point Green's function $Γ^{μν}$ describing virtual Compton scattering off the nucleon. Using Lorentz invariance, gauge invariance, and crossing symmetry, we derive the leading terms of an expansion of the operator in the four-momenta $q$ and $q'$ of the initial and final photon, respectively. The model-independent result is expressed in terms of the electromagnetic form factors of the free nucleon, i.e., on-shell information which one obtains from electron-nucleon scattering experiments. Model-dependent terms appear in the operator at $O(q_αq'_β)$, whereas the orders $O(q_αq_β)$ and $O(q'_αq'_β)$ are contained in the low-energy theorem for $Γ^{μν}$, i.e., no new parameters appear. We discuss the leading terms of the matrix element and comment on the use of on-shell equivalent electromagnetic vertices in the calculation of ``Born terms'' for virtual Compton scattering.
We present a relativistic calculation of pion scattering, pion photoproduction and Compton scattering on the nucleon in the energy region of the \Del-resonance (upto 450 MeV photon lab energy), in a unified framework which obeys the unitarity constraint. It is found that the recent data on the cross section for nucleon Compton scattering determine accurately the parameters of the electromagnetic nucleon--\Del\ coupling. The calculated pion-photoproduction partial-wave amplitudes agree well with the recent Arndt analysis.