Super-Radiance: From Nuclear Physics to Pentaquarks

The phenomenon of super-radiance in quantum optics predicted by Dicke 50 years ago and observed experimentally has its counterparts in many-body systems on the borderline between discrete spectrum and continuum. The interaction of overlapping resonances through the continuum leads to the redistribution of widths and creation of broad super-radiant states and long-lived compound states. We explain the physics of super-radiance and discuss applications to weakly bound nuclei, giant resonances and widths of exotic baryons.Comment: 10 pages, 4 figure

Many-Body Physics on the Border of Nuclear Stability

A brief overview is given of the Continuum Shell Model, a novel approach that extends the traditional nuclear shell model into the domain of unstable nuclei and nuclear reactions. While some of the theoretical aspects, such as role and treatment of one- and two-nucleon continuum states, are discussed more in detail, a special emphasis is made on relation to observed nuclear properties, including definitions of the decay widths and their relation to the cross sections, especially in the cases of non-exponential decay. For the chain of He isotopes we demonstrate the agreement of theoretical results with recent experimental data. We show how the interplay of internal collectivity and coherent coupling to continuum gives rise to the universal mechanism of creating pigmy giant resonances.Comment: 6 pages, 3 figure

Nuclear shape transitions, level density, and underlying interactions

The configuration interaction approach to nuclear structure uses the effective Hamiltonian in a finite orbital space. The various parts of this Hamiltonian and their interplay are responsible for specific features of physics including the shape of the mean field and level density. This interrelation is not sufficiently understood. We intend to study phase transitions between spherical and deformed shapes driven by different parts of the nuclear Hamiltonian and to establish the presence of the collective enhancement of the nuclear level density by varying the shell-model matrix elements. Varying the interaction matrix elements we define, for nuclei in the sd and pf shells, the sectors with spherical and deformed shapes. Using the moments method that does not require the full diagonalization we relate the shape transitions with the corresponding level density. Enhancement of the level density in the low-energy part of the spectrum is observed in clear correlation with a deformation phase transition induced mainly by the matrix elements of single-particle transfer. The single-particle transfer matrix elements in the shell model nuclear Hamiltonian are indeed the carriers of deformation, providing rotational observables and enhanced level densities.Comment: 10 pages, 6 figure