20 June 2003
on
Organized by:
Pier Francesco Bortignon, University of Milano, co-ordinator
Nguyen Van Giai, University of Paris-Sud, organizer
Hiroyuki Sagawa, University of Aizu, organizer
Gianluca Colò, University of Milano, organizer
Addresses and phone numbers:
Prof. Pier Francesco Bortignon
Dipartimento di Fisica
Università di Milano
Via Celoria 16, I-20133 Milano
Italy
phone: +39 02 50317215
fax: +39 02 50317487
email: pierfrancesco.bortignon@mi.infn.it
Prof. Nguyen Van Giai
Institut de Physique Nucléaire
Université Paris-Sud
91406 Orsay Cedex
France
phone: +33 1 69157214
fax: +33 1 69157748
email: nguyen@ipno.in2p3.fr
Prof. Hiroyuki Sagawa
Center for Mathematical Sciences
University of Aizu
Aizu-Wakamatsu, Ikkimachi
Fukushima 965-8580
Japan
phone:+81-242-37-2725
fax: +81-242-37-2752
email:sagawa@u-aizu.ac.jp
Dr. Gianluca Colò
Dipartimento di Fisica
Università di Milano
Via Celoria 16, I-20133 Milano
Italy
phone: +39 02 50317261
fax: +39 02 50317487
email: gianluca.colo@mi.infn.it
The study of nuclear structure is facing a new era because of
the development of impressive experimental facilities. The progress
achieved in the production of radioactive beams allow us
to explore phenomena beyond the presently known limits of the valley of
stability. This evolution of nuclear structure physics is natural and
irreversible, and will last for many years, also because of the technological
challenges associated with the new types of detectors and measurements.
It has been often discussed that the structure of the unstable, ``exotic''
nuclei might be different than that of the 
stable nuclei.
For instance, the existence of density profiles characterized by a
``skin'' or a ``halo'' of weakly bound particles is experimentally
confirmed in several light nuclei near the drip lines. Moreover, there are
indications of new magic numbers, like 
16 in the O-isotopes.
The new experiments will provide us with more observables.
It is nowadays feasible to measure the nuclear
response in systems with large isospin and a small separation energy.
Meanwhile, the large particle and 
ray detectors are likely also
to give us precise information of one-, two- and even multi-phonon states.
In the present stage, one has to accompany these measurements with an
appropriate theoretical effort, trying to formulate and perhaps to answer
the relevant physical questions.
As the neutron excess increases near the drip line, we expect the properties of the giant resonances (GR) and of the low-lying collective states, to be dramatically affected by the asymmetry between protons and neutrons. The relative distribution of strength between the GR and the low-lying, so-called ``pygmy'' states, will be an interesting subject also for its astrophysical implications. In unstable nuclei, the coupling to the continuum becomes increasingly important and, together with the pairing interaction in a dilute environment, may change the shell structure and the features of the nuclear response. It is becoming possible to incorporate these effects (continuum and pairing) in consistent response function calculations since new Quasi-particle Random Phase Approximation (QRPA) codes are being developed for this aim. The attempts to extend them to the more complicated case of deformed nuclei are just being performed with different techniques. These novel effects are also addressed by the continuum shell model. In connection with the experimental feasibility of Coulomb excitation and/or inelastic hadron scattering off unstable systems, leaving for the future the possible electron scattering, new interesting examples of coupling microscopic structure models with reaction models are starting.
It seems timely to have a wide discussion to understand to what extent the experiments will be able to provide detailed spectroscopic information on exotic systems (charge and matter densities of ground states, transition probabilities and inelastic cross sections of excited states). Several distinguished experimentalists will be invited for this purpose. Are the microscopic models able to match the experimental data ? An ambitious challenge is the finding of a proper microscopic framework (beyond QRPA) which allows the calculation of both the escape and spreading widths of the vibrational states on equal footing, to describe their line shape in detail and their particle decay. For this purpose, the coupling with configurations of four quasi-particle type should be included in a consistent way. A further question concerns the feedback that the low-lying collective states may have on other properties. The quasi-particle states are known to be modified by particle-vibration coupling; a fraction of the empirical pairing gaps can also be obtained in a self-consistent approach which includes this coupling through self-energy and induced-interaction effects. As already mentioned, another challenge will be a unification between microscopic nuclear structure calculations and reaction models, which can permit a direct comparison with the experimental data in terms of cross sections. The physics of the multi-phonon states like double GR and triple GR will be also included in our discussions.
We would like to review the experimental landscape concerning measurements of the nuclear response in exotic systems, and discuss the technical problems and the future perspectives of the models we have mentioned. In particular, we will address the question whether we can predict and find new exotic systems, displaying haloes or unusual phenomena. We will discuss the status of RPA- and QRPA-type of calculations, and their extensions including the particle-vibration coupling. Are they appropriate ? How do they compare with other models ? Can we extend them from the spherical to the deformed case ? More general questions will be whether we can improve our knowledge of nuclear astrophysics and of the equation of state (EOS) in neutron-rich matter by studying neutron-rich nuclei.
Collaboration meeting: 1 day to progress on
1) coupling to the continuum in spherical and deformed nuclei, without
imposed symmetries;
2) theoretical frameworks to calculate the escape width and spreading width
simultaneously;
3) microscopic models unifying structure and reaction calculations.
Workshop: 4 and 1/2 days, with no more than 6 talks per day.
We will discuss the following subjects from theoretical and experimental
point of view:
1) giant resonances and pygmy resonances;
2) multi-phonon excitations;
3) response in very isospin-asymmetric matter;
4) continuum and pairing effects on the low energy response near the
threshold;
5) proposals from unified structure plus reaction models
for new experiments.
Tom Aumann (GSI, E), Francisco Barranco (Sevilla, T), Karim Bennaceur (IPN-Lyon, J), Yorick Blumenfeld (IPN-Orsay, E), Angela Bracco (Milano, E), Ricardo A. Broglia (Milano, T), Philippe Chomaz (GANIL, T), Tien Khoa Dao (Hanoi, T), Hans Emling (GSI, E), Kouichi Hagino (Kyoto, J), Ikuko Hamamoto (Lund/NBI, T), Muhsin Harakeh (Groningen, E), Denis Lacroix (LPC-Caen, J), Zhongyu Ma (Beijing, T), Jerome Margueron (GANIL, J), Masayuki Matsuo (Niigata, T), Toru Motobayashi (RIKEN, E), Jorge Piekarewicz (Tallahassee, T), Harutaka Sakaguchi (Kyoto, E), Nicu Sandulescu (Bucharest, T), Michael Thonessen (MSU, E), Enrico Vigezzi (Milano, T), Andrea Vitturi (Padova, T), Dario Vretenar (Zagreb, T), Kazuhiro Yabana (Tsukuba, T).
Participation of a total of about 40 people is planned.
It is likely that funds from the Ph. D. School of the Milano University may be used for participants from Universities who signed the Internationalization Agreement with that school. The Co-ordinator and the Director of ECT* will work on the procedure.