arXiv subjects

L. Tiator

Publications and source records attributed to L. Tiator.

100 records · Page 6Linked to original sources

arXiv1996

New Insights into the Coupling of $η$ and $f_1$ Mesons to the Nucleon

We show that the contact couplings of neutral pseudoscalar and axial mesons to the nucleon are proportional to $Δs$, the fraction of nucleon spin carried by the strange quark sea, and thus are strongly suppressed relative to the couplings of charged mesons to the nucleon. On the other side, recent high accuracy data on $η$ photoproduction at threshold reveal the need for non--negligibile $ηNN$ vertices, while fitting $\bar N N$ phase shifts by means of effective meson exchange potentials requires a substantial presence of $f_1$ mesons there. We here advocate the idea to attribute the couplings of neutral pseudoscalar and axial vector mesons to effective triangular diagrams containig non--strange mesons and demonstrate its usefulness in describing available data.

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arXiv1996

The position and the residues of the delta resonance pole in pion photoproduction

We have analyzed the $M_{1+}^{(3/2)}$ and $E_{1+}^{(3/2)}$ multipole amplitudes of pion photoproduction in the framework of fixed-$t$ dispersion relations. Applying the speed plot technique to our results for these multipoles, we have determined the position and the residues of the $Δ$ (1232) resonance pole. The pole is found at total $c.m.$ energy $W = (1211 - 50i)$ MeV on the second Riemann sheet, and the ratio of the electric and magnetic residues is $R_Δ = - 0.035 - 0.046 i$, resulting in an E2/M1 ratio for the "dressed" delta resonance of $- 3.5 \%$.

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arXiv1996

Quasifree Eta Photoproduction from Nuclei

Quasifree $η$ photoproduction from nuclei is studied in the Distorted Wave Impulse Approximation (DWIA). The elementary eta production operator contains Born terms, vector meson and nucleon resonance contributions and provides an excellent description of the recent low energy Mainz measurements on the nucleon. The resonance sector includes the $S_{11}(1535)$, $P_{11}(1440)$ and $D_{13}(1520)$ states whose couplings are fixed by independent electromagnetic and hadronic data. Different models for the $ηN$ t-matrix are used to construct a simple $ηA$ optical potential based on a $t ρ$-approximation. We find that the exclusive $A(γ,ηN)B$ process can be used to study medium modifications of the $N^*$ resonances, particularly if the photon asymmetry can be measured. The inclusive $A(γ, η)X$ reaction is compared to new data obtained on $^{12}C$, $^{40}Ca$, and is found to provide a clear distinction between different models for the $ηN$ t-matrix.

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arXiv1996

Elastic Pion Scattering on the Deuteron in a Multiple Scattering Model

Pion elastic scattering on deuterium is studied in the KMT multiple scattering approach developed in momentum space. Using a Paris wave function and the same methods and approximations as commonly used in pion scattering on heavier nuclei excellent agreement with differential cross section data is obtained for a wide range of pion energies. Only for $T_π>250$ MeV and very backward angles, discrepancies appear that are reminiscent of disagreements in pion scattering on $^3$He, $^3$H, and $^4$He. At low energies the second order corrections have been included. Polarization observables are studied in detail. While tensor analyzing powers are well reproduced, vector analyzing powers exhibit dramatic discrepancies.

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arXiv1996

Coherent $π^0$ and $η$ Photoproduction on the Deuteron

Coherent pion photoproduction, $d(γ,π^0)d$, is calculated in a coupled channels approach in momentum space using an elementary production operator that includes Born terms, vector meson exchange and $M1/E2$ $Δ$-resonance excitation. The final state interaction is treated in the KMT multiple scattering approach that describes $πd$ elastic scattering well. Differential cross sections are found to agree well with existing ($γ, π^0$) data. The polarization observables $Σ, iT_{11}$ and the tensor analyzing powers are predicted for forthcoming experiments. A number of transparent relationships between polarization observables and the elementary production operator is derived. Finally, calculations are performed for coherent eta photoproduction, $d (γ, η)d$, in the threshold region which agree well with a very recent Mainz experiment.

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arXiv1996

On the Coupling of the $η$ Meson to the Nucleon

The pseudoscalar and pseudovector $ηN$ coupling constants are calculated from an effective vertex associated with the $a_0(980)πN$ triangle diagram. The predicted values are in agreement with the ones concluded from fitting $η$ photoproduction amplitudes. In this context we stress the importance of the properties of the scalar meson octet for $η$ meson physics.

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arXiv1995

Photo- and Electroproduction of Eta Mesons

Eta photo- and electroproduction off the nucleon is investigated in an effective lagrangian approach that contains Born terms and both vector meson and nucleon resonance contributions. In particular, we review and develop the formalism for coincidence experiments with polarization degrees of freedom. The different response functions appearing in single and double polarization experiments have been studied. We will present calculations for structure functions and kinematical conditions that are most sensitive to details of the lagrangian, in particular with regard to contributions of nucleon resonances beyond the dominant $S_{11}$(1535) resonance.

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arXiv1994

Two-body Mechanisms in Pion Photoproduction on the Trinucleon

A breakdown of the Impulse Approximation is studied in pion photoproduction on $^3$He at high momentum transfers. The usual DWIA formalism with Faddeev wave functions which works well for small momentum transfers deviates from experimental measurements by up to two orders of magnitude for $Q^2>6 fm^{-2}$. It is found that the explicit inclusion of two-body mechanisms, where the photon is absorbed on one nucleon and the pion is emitted from another nucleon can restore agreement with the data.

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arXiv1994

The $η$NN Coupling in Eta Photoproduction

Low-energy eta photoproduction on the nucleon is studied in an effective Lagrangian approach that contains Born terms, vector meson and nucleon resonance contributions. The resonance sector includes the $S_{11}(1535)$, $P_{11}(1440)$ and $D_{13}(1520)$ states whose couplings are fixed by independent electromagnetic and hadronic data. The available $(γ,η)$ data are employed to discuss the difference between pseudoscalar and pseudovector Born terms and to determine the magnitude of the $ηNN$ coupling constant. We present multipoles that are most sensitive to the various model ingredients and demonstrate how these multipoles may be accessed in polarization observables. Cross section calculations are presented for eta photoproduction on light nuclei.

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arXiv1993

Pion interaction with the trinucleon up to the eta production threshold

Pion elastic, charge exchange scattering and induced eta production on the trinucleon systems are investigated in a coupled-channels approach in momentum space with Fadeev wave functions. The channel $πN \rightarrow ηN$ is included using an isobar model with S-, P-, and D-wave resonances. While the coherent reactions like $^3$He($π,π)^3$He can be reasonably well reproduced up to $T_π$=500 MeV, large discrepancies appear for the incoherent processes, $^3$He($π^-,π^0)^3$H and $^3$He($π^-,η)^3$H at backward angles and energies above $Δ$-resonance. In the forward direction the $(π,η)$ calculations underestimate the experimental measurements very close to threshold but agreement with the data improves with increasing pion energy. Predictions are made for the asymmetries of the various reactions on polarized $^3$He.

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