A. Yu. Korchin

Structure analysis of the virtual Compton scattering amplitude at low energies

Published in: Phys. Rev. C 57 (1998) 941-952 Citing articles (CrossRef) citations recorded in [Science Citation Index] Abstract: We analyze virtual Compton scattering (VCS) off the nucleon at low energies in a covariant, model-independent formalism. We define a set of invariant functions which, for the structure-dependent part of the VCS amplitude, is free of poles and kinematical zeros. The covariant treatment naturally allows to implement the constraints due to Lorentz and gauge invariance, crossing symmetry, and the discrete symmetries. In particular, when applied to the photon electroproduction reaction, charge-conjugation symmetry in combination with nucleon crossing generates four relations among the ten originally proposed generalized polarizabilities of the nucleon. We perform a 1/M expansion of the VCS amplitude near the real-photon point and show how the generalized polarizabilities can be extracted from such an expansion. Finally, we discuss the real-photon limit including the spin-independent as well as the spin-dependent polarizabilities.

Pion and photon induced reactions on the nucleon in a unitary model

Abstract We present a relativistic calculation of pion scattering, pion photoproduction and Compton scattering on the nucleon in the energy region of the Δ-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-Δ coupling. The pion-photoproduction partial-wave amplitudes, calculated with parameters fitted to the pion-nucleon and Compton scattering, agree well with the recent Arndt analysis.

Dilepton production in nucleon-nucleon collisions and the low-energy theorem

Abstract We consider the process of dilepton production in nucleon-nucleon (NN) collisions NN → NN l + l − (virtual bremsstrahlung). The reaction amplitude in the rest frame of the virtual photon is presented in powers of the photon invariant mass M γ . It is shown that the leading term O( M γ −1 ) is expressed through the on-mass-shell NN T-matrix. If in addition the photon 3-momentum in the lab (or c.m.) frame is small enough the next term O( M γ 0 ) becomes model independent as well. The amplitude is presented in a covariant form which allows one to calculate late it in any reference frame and to compare it with the well-known low-energy theorem for the real photon NN bremsstrahlung. The decomposition of the exclusive cross section of the virtual NN bremsstrahlung in terms of response functions is carried out as well and the ratios of the response functions are calculated using the leading term in the amplitude.

Cross sections and electromagnetic response functions for radiative proton capture in

Abstract Exclusive differential cross sections of virtual-photon radiative proton capture in pd→ 3 He +e + e − at a proton energy of 190 MeV have been measured for the first time. The leptonic-angle dependence of the measured cross section is exploited in order to determine the electromagnetic response functions in the time-like region. The data are compared to a relativistic gauge-invariant impulse approximation and a Faddeev calculation. Differences between theoretical predictions and data are discussed in terms of enhanced magnetic radiation possibly originating from virtual Δ excitations.

Low-energy theorems for virtual nucleon-nucleon bremssrahlung; formalism and results

Abstract We present results for cross sections and response functions in virtual bremsstrahlung induced by nucleon-nucleon collisions NN → NN + e + e − , based on two different low-energy theorems. The first low-energy theorem is a generalization of Lows theorem for real-photon bremsstrahlung. The second low-energy theorem was derived by Liou et al. for real photons, and is generalized here to the case of virtual photons. The two low-energy theorems are compared by calculating the cross section and response functions in the kinematical region of possible future experiments. We find that for most observables the amplitude up to leading order only in the photon energy does not provide a reliable description, and that the next order term is very important. The differences between the two low-energy theorems turn out to be sizeable for pp , but quite small for pn virtual bremsstrahlung. We also pay attention to the correct implementation of the generalized Pauli principle and crossing symmetry which the reaction amplitude must obey.

Angular distributions and polarization of protons in thed(e, é p)n reaction

The effects of meson-exchange currents and final-state interactions in angular distributions and polarizations of protons for thed(e, é p)n reaction are investigated in a nonrelativistic approach. Special attention is paid to a certain consistency of the deuteron electromagnetic current with then-p interaction models employed. Kinematic regions for polarization observables accessible to measurements are determined. We discuss the possibility of an experiment to study the longitudinal-transverse interference of the deuteron current components. The calculatedd(e, é p)n cross section are compared with Saclay data.

Lepton-pair production in virtual Compton scattering off the proton

Abstract We show that lepton-pair production in virtual Compton scattering offers, through interference with the well-known Bethe-Heitler process, a sensitive probe to separate longitudinal and transverse response at positive photon invariant masses. This interference can be measured directly in terms of an asymmetry. The role of off-shell effects in the NNγ vertices is investigated as well. An additional NNγγ contact term in the amplitude, included to ensure gauge invariance of the model, cancels a substantial part of the off-shell effects, but introduces ambiguities which affect observables. The response of nucleon resonances is also investigated.

Quasifree peak parameters and sum rules for electron scattering on4He

The cross sections of the quasifree electron scattering from4He are calculated using the inclusivity of the (e,e′) reaction over final nuclear states. The quasifree peak position and shape are determined and related to the electronuclear sum rules. The developed approach yields a satisfactory description of the experimental data.

y-scaling in quasi-free electron scattering by nuclei

The origin of the so-calledy-dependences ofd2σ cross sections for the inclusive (e, e′) reaction on nuclei is studied using the properties of the time structure functions that determine the nuclear dynamic form factor. The validity of the approximations leading to two versions of they-scaling is verified in the framework of the single-particle (s.p.) model. A variant of the model-independent analysis (MIA) of the dependences is offered.

Decays of a neutral particle with zero spin and arbitrary CP parity into two off-mass-shell Z bosons

We investigate effects of CP symmetry violation in the decay of a scalar particle X (the Higgs boson) into two off-mass-shell Z bosons both decaying into a fermion–antifermion pair,