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| PD32 EXPERIMENT, RESPONSIBLE: Luciano Canton |
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The activity of 2008 has focused on calculations of response functions of 3- and 4-body systems. The ab initio Lorentz Integral Transform (LIT) method has been applied.
The long lasting project of the effects of the two-body currents, consistent with the realistic potential, on the transverse response function of 3He has been brought to an end [1]. Strong effects have been found in low and high energy tails. Particularly important is the study of the validity of the Siegert's theorem. This work represents an important step towards the ab initio calculation of response functions in the 4-body system. To this aim first results on the longitudinal response of the alpha particle with a realistic interaction have been obtained [2]. It has soon been realized that such a response represents a new challenge for the LIT method, due to the presence of a narrow structure (resonance) in the low energy spectrum of 4He. Therefore it has been necessary to explore the applicability of the method in such cases.
It has been found that this is possible, providing that the solution of the LIT equation is accurately solved in the asymptotic region. This is an important finding that opens up the perspective to determine both position and widths of narrow structures of the spectrum in the continuum also for systems with A=4 or larger.
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TITLE: FEW-BODY SYSTEMS IN NUCLEAR PHYSICS
The main goal of this project is the study of hadronic systems, nucleons and light nuclei, with explicit reference to realistic hadron--hadron interactions.
11 researchers divided in three groups (Padua, Pisa, and Trento) are involved in this project, which takes advantage of international collaboration with many universities and research institutions throughout the world (Universities of Mainz, Victoria (Canada), Manitoba, Melbourne, Bochum, the Hungarian Academy of Sciences (Debrecen), Jagellonian Univ. Cracow, S. Petersbourg, the Max Plank Institute in Munich, the Kurchatov Institute in Moscow, the Hebrew University in Jerusalem, the Weizmann Institute of Science, the Thomas Jefferson National Accelerator Facility, Yale, the Triangle University National Laboratory, North Carolina, USA, the Kharkov Institute of Physics and Technology, Ukraine).
The various groups are active in local Universities and research institutions of INFN. The project represents a coordination of the broad range of activities done in the field of few-body theoretical physics by the various groups. These groups developed their own techniques and methods to describe specific aspects of the nuclear and hadronic few-body problem.
In spite of the fact that the techniques are different and somewhat complementary, there are many intersections and interactions amongst the various groups, and between the groups and the experimentalists.
The research activity involves:
I) Bound and scattering states in few--nucleon systems with modern few--body techniques; These techniques range from very advanced Hyperspherical Harmonics expansion methods (Pisa, Trento) to the integral equation approach in momentum space for the solution of Faddeev or AGS equations (Padova).
II) The group obtained important results in ``ab initio calculations'' of electromagnetic and hadronic reactions of light nuclei (A=3-8) with the Lorentz Integral Transform (LIT) method (Trento). The LIT approach has been used to investigate reactions with light sistems. The LIT method has been introduced (and successfully applied) by the Trento group in the last years. It allows to treat "exactly" the many-body problem also in the continuum. The Schroedinger equation and the Schroedinger-like equation required by the LIT are solved by Hyperspherical Harmonics (HH) expansions. In order to reach convergent results the EIHH (Effective Interaction in HH expansion) method has been developed and applied to systems with A=3-6. The aim of the present research is on the one hand to investigate the role of the nuclear potential (the sole ingredient of the calculation) in the lighter systems (A=3,4), and on the other hand to search for the microscopic fundaments of possible collective behaviours in less light nuclei (A=6-8). In particular it is forseen:
1) to study real and virtual photon inclusive reactions with A=4-8 nuclei
2) to study inclusive structure functions and polarization observables for 3-body systems with realistic interactions to investigate in particular the role of 3-body forces.
3) to study exclusive e.m. reactions and scattering reactions involving 4 nucleons.
III) The group obtained important results in the study of the structure and dynamics of few-nucleon systems, with treatment of the Coulomb interaction together with modern realistic nucleon-nucleon (NN) and three-nucleon (3N) potentials (Pisa). The following problems have been addressed:
1) Accurate calculation of scattering observables of various reactions involving 3 or 4 nucleons, with particular attention to the current experimental database in low-energy nuclear physics.
2) Development of advanced models of the nuclear electro-weak transition currents due to meson exchange currents. Analysis of reactions of astrophysical interest (proton-proton and "hep" fusion reactions).
3) Electron scattering from 3He and 4He nuclei, in the quasi elastic and deep inelastic kinematical regions, for the extraction of the neutron form factors from the experimental data.
4) New advancements in methods to solve bound-state and scattering problems for A=3,4 systems (discrete variable representation method).
IV) The group concentrated his past activity also in pion production reactions and has developed a theoretical model that allows to analyze the experimental data for pion production from threshold up to the Delta resonance (Padova). The investigation has been extended to include also pion production in e,e' processes. The conceptual problems on how to deal explicitly with the pion d.o.f. in few-nucleon systems are formidable. These problems have been addressed in the past with generalized AGS equations, which have been derived by the group in Padova to exhibit the origin of irreducible pionic diagrams (some kind of three-nucleon potential effect) in low-energy three-nucleon scattering. Similar problems occur also for the meson-few-quark system, which needs to be better understood in order to describe meson decay of baryon resonances. Preliminary investigations in cooperation with the Graz group have already started within the costituent quark model, and recently a calculation of the pion-baryon vertex has been obtained.
V) The method of sturmian (or Weinberg-state) expansion has been successfully applied to represent scattering in medium-light nuclei, with explicit treatment of low-lying collective excitations. A good reproduction of both bound and resonance spectra has been obtained, once Pauli principle has been taken into account. The plan to extend the study from C-12 to other light nuclei, particularly to the isotopes of helium with neutron excess,
or light nuclei beyond the proton drip-line is on-going. The topic is directly connected with the physics of rare-isotope beams (RIB) and with the calculation of corresponding fusion reactions of medium-light nuclei, also for astrophysical interest.
VI) Also, interdisciplinary applications of coupled-channel quantum scattering methods to mesoscopic systems and to the physics of electron transport in quantum lines and quantum strips will be continued. |
Istituto Nazionale di Fisica Nucleare - Piazza dei Caprettari, 70 - 00186 Roma
tel. +39 066840031 - fax +39 0668307924 - email: presidenza@presid.infn.it
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