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G. Orlandini

Publications and source records attributed to G. Orlandini.

At least 55 records · Page 3Linked to original sources
arXiv2004

The importance of few-nucleon physics at low energy

This manuscript originated from the discussion at the workshop on the "Future of Few-body Low Energy Experimental Physics" (FFLEEP), which was held at the University of Trento on December 4-7, 2002 and has been written in its present form on March 19, 2003. It illustrates a selection of theoretical advancements in the nuclear few-body problem, including two- and many-nucleon interactions, the three-nucleon bound and scattering system, the four-body problem, the A-body (A$>$4) problem, and fields of related interest, such as reactions of astrophysical interest and few-neutron systems. Particular attention is called to the contradictory situation one experiences in this field: while theory is currently advancing and has the potential to inspire new experiments, the experimental activity is nevertheless rapidly phasing out. If such a trend will continue, advancements in this area will become critically difficult.

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arXiv2004

Recent Developments in the Lorentz Integral Transform (LIT) Method

Recent results on electromagnetic reactions into the continuum of systems with A from 3 to 7 are presented. They have been obtained using the Lorentz Integral Transform (LIT) method. The method is shortly reviewed, emphasizing how all the results, though obtained with the sole ingredient of the N-N potential, contain the full complicated dynamics of the A-body system, both in the initial and in the final states.

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arXiv2004

Ab initio calculation of Li7 photodisintegration

The Li7 total photoabsorption cross section is calculated microscopically. As nucleon-nucleon interaction the semi-realistic central AV4' potential with S- and P-wave forces is taken. The interaction of the final 7-nucleon system is fully taken into account via the Lorentz Integral Transform (LIT) method. For the calculation of the LIT we use expansions in hyperspherical harmonics (HH) in conjunction with the HH effective interaction (EIHH) approach. The convergence of the LIT expansion is discussed in detail. The calculated cross section agrees quite well with the available experimental data, which cover an energy range from threshold up to 100 MeV.

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arXiv2003

Electromagnetic break-up of nuclei with A = 3 - 7

This talk contains a short review of some of the progresses made in the last three years in the calculations of electromagnetic cross sections of light nuclei up to A=7. Since many of them have been possible thanks to the use of the Lorentz Integral Transform (LIT) method, both for inclusive and exclusive reactions, I will first make a few remarks on the method, stressing its essential points and then show results for different nuclei. One of the interesting outcomes is e.g. the appearing of typical collective motion features from ab initio six-body calculations. When a comparison with available experimental data is attempted, it is rather disappointing to realize that low-energy data are old, incomplete and not accurate enough to disantangle interesting effects, showing the need of a major experimental program in this direction, together with more theoretical efforts to implement modern realistic forces in continuum calculations of $A\geq 4$ systems.

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arXiv2003

Application of the Lorentz-Transform Technique to Meson Photoproduction

We show that the Lorentz integral transform (LIT) technique which has been successfully applied to photoreactions in light nuclei can also be applied to photoreactions involving particle production. A simple model where results are easily calculable in the traditional fashion is used to test the technique. Specifically we compute inclusive $π^+$ photoproduction from deuterium for photon energies less than 200 MeV using a Yamaguchi model for the NN interaction. It is demonstrated that although the response functions for inclusive meson production do not have favourable asymptotic behavior one can nontheless extract them by inversion of the transform. The implication is that one can treat realistic problems of photo-meson production including all final state interactions by means of the LIT technique.

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arXiv2003

Gaussian Sum-Rule Analysis of Scalar Gluonium and Quark Mesons

Gaussian sum-rules, which are related to a two-parameter Gaussian-weighted integral of a hadronic spectral function, are able to examine the possibility that more than one resonance makes a significant contribution to the spectral function. The Gaussian sum-rules, including instanton effects, for scalar gluonic and non-strange scalar quark currents clearly indicate a distribution of the resonance strength in their respective spectral functions. Furthermore, analysis of a two narrow resonance model leads to excellent agreement between theory and phenomenology in both channels. The scalar quark and gluonic sum-rules are remarkably consistent in their prediction of masses of approximately 1.0 GeV and 1.4 GeV within this model. Such a similarity would be expected from hadronic states which are mixtures of gluonium and quark mesons.

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arXiv2002

Gaussian Sum-Rules, Scalar Gluonium, and Instantons

Gaussian sum-rules relate a QCD prediction to a two-parameter Gaussian-weighted integral of a hadronic spectral function, providing a clear conceptual connection to quark-hadron duality. In contrast to Laplace sum-rules, the Gaussian sum-rules exhibit enhanced sensitivity to excited states of the hadronic spectral function. The formulation of Gaussian sum-rules and associated analysis techniques for extracting hadronic properties from the sum-rules are reviewed and applied to scalar gluonium. With the inclusion of instanton effects, the Gaussian sum-rule analysis results in a consistent scenario where the gluonic resonance strength is spread over a broad energy range below 1.6 GeV, and indicates the presence of gluonium content in more than one hadronic state.

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arXiv2002

Benchmark Calculation of the Three-Nucleon Photodisintegration

A benchmark is set on the three-nucleon photodisintegration calculating the total cross section with modern realistic two- and three-nucleon forces (AV18, UrbIX) using both the Faddeev equations and the Lorentz Integral Transform method. This test shows that the precision of three-body calculations involving continuum states is considerably higher than experimental uncertainties. Effects due to retardations, higher multipoles, meson exchange currents and Coulomb force are studied.

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arXiv2001

Benchmark Test Calculation of a Four-Nucleon Bound State

In the past, several efficient methods have been developed to solve the Schroedinger equation for four-nucleon bound states accurately. These are the Faddeev-Yakubovsky, the coupled-rearrangement-channel Gaussian-basis variational, the stochastic variational, the hyperspherical variational, the Green's function Monte Carlo, the no-core shell model and the effective interaction hyperspherical harmonic methods. In this article we compare the energy eigenvalue results and some wave function properties using the realistic AV8' NN interaction. The results of all schemes agree very well showing the high accuracy of our present ability to calculate the four-nucleon bound state.

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arXiv2000

Gaussian Sum-Rules and Prediction of Resonance Properties

Techniques for using Gaussian QCD sum-rules to predict hadronic resonance properties are developed for single-resonance and two-resonance phenomenological models, and criteria are developed for determining which of these models is required for analyzing a particular hadronic channel. The vector current sum-rule coupled to the $ρ$ meson is shown to be consistent with a single resonance model, and the Gaussian sum-rule analysis results in an accurate $ρ$ mass prediction which exhibits excellent agreement between the theoretical prediction of the Gaussian sum-rule and the phenomenological model. A two-resonance model is shown to be necessary for the Gaussian sum-rule for the non-strange quark scalar ($\bar n n$) currents. The two-resonance Gaussian sum-rule analysis of the isoscalar and isovector ($I=0,1$) $\bar n n$ scalar mesons exhibits excellent agreement between the theoretical prediction and phenomenological model. The prediction of the resonance properties of the $I=0,1$ $\bar n n$ scalar mesons in this two-resonance model provides valuable information for the interpretation of the scalar mesons, including the X(1775).

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arXiv1999

Electromagnetic response functions of few-nucleon systems

Inclusive electromagnetic reactions in few-nucleon systems are studied basing on accurate three- and four-body calculations. The longitudinal 4He(e,e') response function obtained at q\le 600 MeV/c completely agrees with experiment. The exact 4He spectral function obtained in a semirealistic potential model is presented, and the accuracy of the quasielastic response calculated with its help is assessed, as well as the accuracy of some simpler approximations for the response. The photodisintegration cross section of 3He obtained with the realistic AV14 NN force plus UrbanaVIII NNN force agrees with experiment. It is shown that this cross section is very sensitive to underlying nuclear dynamics in the E_γ\simeq 70-100 MeV region. In particular, the NNN nuclear force clearly manifests itself in this region.

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arXiv1998

Exact 4He Spectral Function in a Semirealistic NN Potential Model

The spectral function of 4He is calculated with the Lorentz integral transform method in a large energy and momentum range. The excitation spectrum of the residual 3N-system is fully taken into account. The obtained spectral function is used to calculate the quasi elastic longitudinal (e,e') response R_l of 4He for q=300, 400, and 500 MeV/c. Comparison with the exact R_l shows a rather sizeable disagreement except in the quasi elastic peak, where the differences reduce to about 10% at q=500 MeV/c. It is shown as well that very simple momentum distribution approximations for the spectral function provide practically the same results for R_l as the exact spectral function.

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arXiv1998

Electric polarizability of nuclei from a longitudinal sum rule

The nuclear electric polarizability is theoretically analyzed using a sum rule derived from the longitudinal part of the forward Compton amplitude. Beyond the leading dipole contribution, this approach leads to the presence of potential-dependent terms that do not show up in previous analyses. The significance of these new contributions is illustrated by performing an explicit calculation for a proton-neutron system interacting via a separable potential.

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arXiv1997

Photodisintegration of the Three-Nucleon Systems and their Polarizabilities

The total photodisintegration cross sections of three-body nuclei are calculated with semirealistic NN potentials below pion threshold. Full final state interaction with Coulomb force is taken into account via the Lorentz integral transform method. The experimental total cross sections are well described and the sum rule $σ_{-1}(^3$H) agrees with elastic electron scattering data. The calculated ^3He polarizability is 0.15 fm^3.

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arXiv1997

Is there a Pronounced Giant Dipole Resonance in ^4He?

A four-nucleon calculation of the total ^4He photodisintegration cross section is performed. The full final-state interaction is taken into account for the first time. This is achieved via the method of the Lorentz integral transform. Semi-realistic NN interactions are employed. Different from the known partial two-body ^4He(γ,n)^3He and ^4He(γ,p)^3H cross sections our total cross section exhibits a pronounced giant resonance. Thus, in contrast to older $(γ,np)$ data, we predict quite a strong contribution of the $(γ,np)$ channel at the giant resonance peak energy.

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arXiv1997

Accurate Four-Body Response Function with Full Final State Interaction: Application to Electron Scattering off 4He

The longitudinal (e,e') response function of 4He is calculated precisely with full final state interaction. The explicit calculation of the four-body continuum states is avoided by the method of integral transforms. Precision tests of the response show the high level of accuracy. Non-relativistic nuclear dynamics are used. The agreement with experimental data is very good over a large energy range for all considered momentum transfers (q=300, 400, 500 MeV/c). Only at higher q the theoretical response overestimates the experimental one beyond the quasi-elastic peak.

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arXiv1995

Lower Bound on the Pion Polarizability from QCD Sum Rules

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 $\langle r_π^2\rangle$.

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