arXiv subjects
Publications and source records attributed to M. Benmerrouche.
We study the reaction $π^\pm p \to π^\pm π^+ n$ and $π^- p \to π^0 π^0 n$ in heavy-baryon chiral perturbation theory of chiral order three from threshold up to pion laboratory kinetic energies of 400 MeV. We find that the contributions from amplitudes of chiral order three are large and play an essential role in reproducing the experimental data.
We study the nuclear (or A) dependence of the coherent eta photoproduction reaction in a relativistic impulse approximation approach. We use a standard relativistic parameterization of the elementary amplitude, based on a set of four Lorentz- and gauge-invariant amplitudes, to calculate the coherent production cross section from He-4, C-12, and Ca-40. In contrast to nonrelativistic treatments, our approach maintains the full relativistic structure of the process. The nuclear structure affects the process through the ground-state tensor density. This density is sensitive to relativistic effects and depends on A in a different manner than the vector density used in nonrelativistic approaches. This peculiar dependence results in He-4 having a cross section significantly smaller than that of C-12---in contrast to existent nonrelativistic calculations. Distortion effects are incorporated through an eta-nucleus optical potential that is computed in a simple ``t rho'' approximation.
Global fits to the shape of the first QCD Laplace sum rule exhibiting sensitivity to pion-resonance [$Π(1300)$] parameters are performed, leading to predictions for the pion-resonance mass and decay constant. Two scenarios are considered which differ only in their treatment of the dimension-six quark condensate $< O_6>$. The first scenario assumes an effective scale for $< O_6>$ from other sum-rule applications which is assumed to be independent of the physical value of the quark mass, while the second scenario requires self-consistency between the value of $< O_6>$ and the current algebra constraint $2m< \bar q q>=-f_π^2m_π^2$. Predictions of the pion-resonance mass $M_π$ and decay constant $F_π$ are obtained in these two scenarios. A byproduct of this analysis is a prediction of the renormalization-group invariant quark mass $(\hat m_u+\hat m_d)/2$.
A global fit to the shape of the first QCD Laplace sum rule exhibiting sensitivity to pion-resonance [$Π(1300)$] parameters is performed, leading to predictions for the pion-resonance mass and decay constant. This fit leads to predictions of $M_π = 1.15 \pm 0.15 GeV$ for the pion-resonance mass, and $F_π^2=(4.5\pm 2.1).(f_π^2m_π^4/M_π^4)$ for the pion-resonance decay constant.
We study coherent eta-photoproduction from nuclei in a relativistic impulse approximation approach. For the elementary production amplitude we use a standard relativistic parameterization based on a set of four Lorentz- and gauge-invariant amplitudes. The photonuclear amplitude is evaluated without recourse to a nonrelativistic reduction; the full relativistic structure of the amplitude is maintained. On general arguments we show that the coherent process is sensitive to only one of the elementary amplitudes. Moreover, we show that the nuclear structure information is fully contained in the ground-state tensor density. The tensor density is evaluated in a mean-field approximation to the Walecka model and it is shown to be sensitive to relativistic effects. Distortion effects are incorporated through an eta-nucleus optical potential that is computed in a simple t-rho approximation.
We analyze the available data on eta photo- and electroproduction, around W~1535 MeV, in the framework of the effective Lagrangian approach, and extract, in a nearly model-independent fashion, the electrostrong amplitude for the gamma+N--> N*(1535)--> N+eta processes. Quark model approaches are shown to be quite inadequate to explain this property at all Q^2. In particular, at high Q^2, the extracted amplitude falls much slower than the predictions of the quark model, as a function of Q^2, a situation similar to the electroexcitation and decay of Delta(1232). A QCD explanation of these observations is urgently needed.
We analyze the existing data on electroproduction of eta mesons in the region of W~1.5GeV, and extract an electrostrong form factor for the N*(1535) electroexcitation and decay into the eta-N channel, which is found to be relatively insensitive to the uncertainties of the effective Lagrangian approach. this extracted quantity is of interest in the QCD description of relevant baryons.
Making use of QCD sum rules a lower bound is found which relates the electromagnetic polarizability $α_{_E}$ and mean-square radius $\langle r_π^2\rangle$ of charged pions through the intrinsic polarizability $\tildeα_{_E}= α_{_E}-α\langle r_π^2\rangle/(3M_π)$. We find that if present constraints on the QCD continuum (duality) threshold are accepted, this lower bound on the intrinsic polarizability $\tildeα_{_E}$ is incompatible with some previous determinations of $α_{_E}$ and $\langle r_π^2\rangle$.
From the new Mainz data of eta photoproduction off protons\cite{ref1} and deuterons\cite{ref2}, we can extract, via the effective Lagrangian approach (ELA)\cite{ref3}, the electrostrong parameters $ξ_{p}$ and $ξ_{n}$ for the N$^*$(1535) excitation. These parameters are, in units of $10^{-4} MeV^{-1}$, $(2.20\pm 0.15)$ and $(-1.86\pm 0.20)$ respectively. They yield the ratio of the helicity amplitudes for the excitation of N$^*$(1535) by photons off neutrons and protons to be $A^{n}_{1/2}/A^{p}_{1/2}=-0.84\pm 0.15$, a quantity of fundamental importance in the baryon structure. Best estimates in the quark model are close to this ratio.
Using the recent precise measurements of eta photopro- duction in proton and deuteron targets, we extract the ratio of the helicity amplitudes A^n_1/2 / A^p_1/2, for the excitation of N^*(1535), in the effective Lagrangian approach, It is fairly model-independent, free from the final-state interaction effects, and negative as predicted by the quark models. We stress the importance of polarization observables in further elucidation of the N^*(1520) photoexcitation amplitudes.
We extract a parameter $ξ$, a characteristic product of the strong and electromagnetic amplitudes for the N^*(1535), to be (2.20+-0.15) in unit of 10^(-4)MeV^(-1) from the new data on photoproduction of etas off proton.
We discuss the eta and eta prime photo- and electroproduction as a way to probe the nucleon resonance structure.
A new technique based on Hölder's integral inequality is applied to QCD sum-rules to provide fundamental constraints on the sum-rule parameters. These constraints must be satisfied if the sum-rules are to consistently describe integrated physical cross-sections, but these constraints do not require any experimental data and therefore can be applied to any hadronic spectral function. As an illustration of this technique the Laplace sum-rules of the light-quark correlation function for the vector and the axial-vector currents are examined in detail. We find examples of inconsistency between the inequalities and sum-rule parameters used in some previous analyses of the vector and axial-vector channels.
We examine eta prime photoproduction, as a novel tool to study baryon resonances around 2 GeV, of particular interest to the quark shell model, which predicts a number of them. We find important roles of the form factors at the strong vertices, and show that the N^*(2080) can be probed efficiently by this reaction.
We investigate eta photoproduction in the $N^{*}(1535)$ resonance region within the effective Lagrangian approach (ELA), wherein leading contributions to the amplitude at the tree level are taken into account. These include the nucleon Born terms and the leading $t$-channel vector meson exchanges as the non-resonant pieces. In addition, we consider five resonance contributions in the $s$- and $u$- channel; besides the dominant $N^{*}(1535)$, these are: $N^{*}(1440),N^{*}(1520),N^{*}(1650)$ and $N^{*}(1710)$. The amplitudes for the $π^\circ$ and the $η$ photoproduction near threshold have significant differences, even as they share common contributions, such as those of the nucleon Born terms. Among these differences, the contribution to the $η$ photoproduction of the $s$-channel excitation of the $N^{*}(1535)$ is the most significant. We find the off-shell properties of the spin-3/2 resonances to be important in determining the background contributions. Fitting our effective amplitude to the available data base allows us to extract the quantity $\sqrt{χΓ_η} A_{1/2}/Γ_T$, characteristic of the photoexcitation of the $N^{*}(1535)$ resonance and its decay into the $η$-nucleon channel, of interest to precise tests of hadron models. At the photon point, we determine it to be $(2.2\pm 0.2)\times 10^{-1} GeV^{-1}$ from the old data base, and $(2.2\pm 0.1) \times 10^{-1} GeV^{-1}$ from a combination of old data base and new Bates data. We obtain the helicity amplitude for $N^{*}(1535)\rightarrow γp$ to be $A_{1/2}=(97\pm 7)\times 10^{-3} GeV^{-1/2}$ from the old data base, and $A_{1/2}=(97\pm 6)\times 10^{-3} GeV^{-1/2}$ from the combination of the old data base and new Bates data, compared with the results of the analysis of pion photoproduction yielding $74\pm 11$, in the same units.