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A. I. L'vov

Publications and source records attributed to A. I. L'vov.

At least 37 records · Page 2Linked to original sources
arXiv2000

Analysis of data on low-energy gamma-p scattering and determination of proton polarizabilities

An analysis of all experimental data of 50's - 90's on elastic gamma-p scattering at photon energies below 150 MeV (10 experimental works) is performed in order to determine the electric (alpha_p) and magnetic (beta_p) polarizabilities of the proton. A fit of these data with the two free parameters, alpha_p and beta_p, embedded into a theoretical cross section obtained on the basis of finite-energy s-channel dispersion relations gives the following world-average values of the proton polarizabilities (in units of 10^{-4} fm^3): alpha_p = 11.7 +- 0.8 (stat+sys) +- 0.7 (model), beta_p = 2.3 +- 0.9 (stat+sys) +- 0.7 (model), where the first error takes into account statistical and systematic errors of the experimental cross sections and the second error does model uncertainties in the theoretical cross section.

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arXiv2000

First experimental results on creation and decay of eta-mesic nuclei

First experimental results on photoproduction of eta-mesic nuclei are analyzed. In an experiment performed at the 1 GeV electron synchrotron of the Lebedev Physical Institute, correlated (pi+,n) pairs arising from the reaction γ+ C12 -> N + (A-1)_eta -> N + (pi+,n) + (A-2) and flying transversely to the photon beam have been observed. When the photon energy exceeds the eta-meson production threshold, a distribution of the (pi+,n) pairs over their total energy is found to have a peak in the subthreshold region of the internal-conversion process eta p -> pi+ n which signals about formation of eta-mesic nuclei.

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arXiv2000

Deuteron Compton scattering and electromagnetic polarizabilities of the nucleon

Differential cross section of deuteron Compton scattering has been calculated using the Bonn NN-potential, a consistent set of meson-exchange currents and seagulls, and lowest- and higher-order electromagnetic polarizabilities of the nucleon. Estimates of the polarizabilities of the neutron are obtained from recent experimental data.

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arXiv2000

The GDH sum rule for the Delta isobar: A possible anomaly?

The GDH sum rule is discussed for the Delta(1232) resonance. It is shown that apart from ordinary excitations to higher-energy states, the sum rule contains a large negative contribution due to de-excitation into the nucleon state. Therefore, a fulfillment of the sum rule assumes a strong coupling of Delta^+ and Delta^0 to resonances of spin 5/2 and higher. Calculations performed in quark models suggest that D15(1675) may be such a resonance. However, its strength is found to be not sufficient for bringing the GDH sum rule to a theoretically expected positive magnitude.

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arXiv2000

Creation and decay of eta-mesic nuclei

First experimental results on photoproduction of eta-mesic nuclei are analyzed. In an experiment performed at the 1 GeV electron synchrotron of the Lebedev Physical Institute, correlated pi+n pairs arising from the reaction gamma + 12C -> N + eta(A-1) -> N + pi+ + n + (A-2) and flying transversely to the photon beam have been observed. When the photon energy exceeds the eta-meson production threshold, a distribution of the pi+n pairs over their total energy is found to have a peak in the subthreshold region of the internal-conversion process eta p -> pi+ n which signals about formation of eta-mesic nuclei.

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arXiv2000

Deuteron Compton scattering below pion photoproduction threshold

Deuteron Compton scattering below pion photoproduction threshold is considered in the framework of the nonrelativistic diagrammatic approach with the Bonn OBE potential. A complete gauge-invariant set of diagrams is taken into account which includes resonance diagrams without and with NN-rescattering and diagrams with one- and two-body seagulls. The seagull operators are analyzed in detail, and their relations with free- and bound-nucleon polarizabilities is discussed. It is found that both dipole and higher-order polarizabilities of the nucleon are needed for a quantitative description of recent experimental data. An estimate of the isospin-averaged dipole electromagnetic polarizabilities of the nucleon and the polarizabilities of the neutron is obtained from the data.

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arXiv1999

Search for eta-mesic nuclei in photoproduction processes

We present preliminary results of an experiment performed at the 1-GeV electron synchrotron of the Lebedev Physical Institute. Using the bremsstrahlung photon beam with the end-point energy of 650-850 MeV and the carbon target, correlated pi+/n pairs with opening angle 180 deg and energies E_pi=300 MeV, E_n=100 MeV have been observed. They arise from the process gamma + 12C -> N + eta + (A-1) -> N + pi+ + n + (A-2) and provide evidence for the existence of eta-11B and eta-11C eta-mesic nuclei.

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arXiv1999

Compton Scattering by Nuclei

The concept of Compton scattering by even-even nuclei from giant-resonance to nucleon-resonance energies and the status of experimental and theoretical researches in this field are outlined. Nuclear Compton scattering in the giant-resonance energy-region provides information on the dynamical properties of the in-medium mass of the nucleon. The electromagnetic polarizabilities of the nucleon in the nuclear medium can be extracted from nuclear Compton scattering data obtained in the quasi-deuteron energy-region. Recent results are presented for two-body effects due to the mesonic seagull amplitude and due to the excitation of nucleon internal degrees of freedom accompanied by meson exchanges. Due to these studies the in-medium electromagnetic polarizabilities are by now well understood, whereas the understanding of nuclear Compton scattering in the Delta-resonance range is only at the beginning. Phenomenological methods how to include retardation effects in the scattering amplitude are discussed and compared with model predictions.

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arXiv1999

Generalized polarizabilities of the pion in chiral perturbation theory

We present a calculation of the virtual Compton scattering amplitude for $γ^\ast+π\to γ+π$ in the framework of chiral perturbation theory at ${\cal O}(p^4)$. We explicitly derive expressions for generalized electromagnetic polarizabilities and discuss alternative definitions of these quantities.

hep-ph↗
arXiv1998

Search for eta-mesic nuclei in photo-mesonic processes

Preliminary results of an experiment performed at the Lebedev Physical Institute with the bremsstrahlung photon beams of the end-point energies 650 and 850 MeV are presented. Correlated pi+n pairs with the opening angle theta = 180 deg and the energies E_π= 300 MeV, E_n = 100 MeV from the process gamma + 12C -> N + eta(A-1) -> N + pi+ n + (A-2) were observed. They provide an evidence for an existence of the eta-11B and eta-11C eta-mesic nuclei.

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arXiv1998

Spin polarizabilities of the nucleon

Spin polarizabilities of the nucleon are discussed in the framework of fixed-t and backward-angle dispersion relations, chiral perturbation theory, and the non-relativistic quark model. Calculations with the dispersion relations generally confirm findings from HBChPT and disagree with a recent experimental result from LEGS for the backward spin polarizability.

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arXiv1998

Production and decay of eta-mesic nuclei

Using the Green function method, binding effects on produced eta-mesons in the two-stage reaction γ+ A \to N + η+ (A-1) \to N + (πN) + (A-2) are studied. The energy spectrum of the correlated pi-N pairs which arise from decays of etas inside the nucleus is strongly affected by an attractive eta-nucleus optical potential. Its resonant behavior gives a clear signal of formating intermediate eta-mesic nuclei.

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arXiv1998

Deuteron Compton scattering below pion photoproduction threshold

Deuteron Compton scattering below pion photoproduction threshold is considered in the framework of the non-relativistic diagrammatic approach with the Bonn OBE potential. The complete gauge-invariant set of diagrams is taken into account which includes resonance diagrams without and with rescattering and diagrams with one- and two-body seagulls. The obtained results are compared with predictions of other models and with experimental data. A possibility of determining isospin-averaged electromagnetic polarizabilities of the nucleon is discussed.

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arXiv1998

Electric polarizability of nuclei and a longitudinal sum rule

Recently, a longitudinal sum rule for the electric polarizability of nuclei was used to revise a relativistic correction in a dipole sum rule for the polarizability (nucl-th/9802011). This revision is shown to be wrong because of neglecting an asymptotic contribution in the underlying dispersion relation. The status and correct use of the longitudinal sum rule is clarified.

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arXiv1998

Sum rule for the backward spin polarizability of the nucleon from a backward dispersion relation

A new sum rule for $γ_π$, the backward spin polarizability of the nucleon, is derived from a backward-angle dispersion relation. Taking into account single- and multi-pion photoproduction in the s-channel up to the energy 1.5 GeV and resonances in the t-channel with mass below 1.5 GeV, it is found for the proton and neutron that $[γ_π]_p$ = -39.5 +/- 2.4 and $[γ_π]_n$ = 52.5 +/- 2.4, respectively, in units of 10^{-4} fm^4.

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arXiv1998

Low-Energy Compton Scattering of Polarized Photons on Polarized Nucleons

The general structure of the cross section of $γN$ scattering with polarized photon and/or nucleon in initial and/or final state is systematically described and exposed through invariant amplitudes. A low-energy expansion of the cross section up to and including terms of order $ω^4$ is given which involves ten structure parameters of the nucleon (dipole, quadrupole, dispersion, and spin polarizabilities). Their physical meaning is discussed in detail. Using fixed-t dispersion relations, predictions for these parameters are obtained and compared with results of chiral perturbation theory. It is emphasized that Compton scattering experiments at large angles can fix the most uncertain of these structure parameters. Predictions for the cross section and double-polarization asymmetries are given and the convergence of the expansion is investigated. The feasibility of the experimental determination of some of the struture parameters is discussed.

hep-ph↗
arXiv1997

Polarization Phenomena in Small-Angle Photoproduction of e+e- Pairs and the Gerasimov-Drell-Hearn Sum Rule

Photoproduction of $e^+e^-$ pairs at small angles is investigated as a tool to determine the functions $f_1$ and $f_2$ entering the real-photon forward Compton scattering amplitude. The method is based on an interference of the Bethe-Heitler and the virtual Compton scattering mechanisms, generating an azimuthal asymmetry in the $e^+$ versus $e^-$ yield. The general case of a circularly polarized beam and a longitudinally polarized target allows one to determine both the real and imaginary parts of $f_1$ as well as $f_2$. The imaginary part of $f_2$ requires target polarization only. We calculate cross sections and asymmetries of the reaction $p(γ,e^+e^-)p$, estimate corrections and backgrounds, and propose suitable kinematical regions to perform the experiment. Our investigation shows that photoproduction of $e^+e^-$-pairs off the proton and light nuclei may serve as a rather sensitive test of the validity of the Gerasimov-Drell-Hearn sum rule.

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