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Lise Meitner Prize 2002 Of The European Physical Society

August 19, 2002

Berlin, May 2002

The European Physical Society announces that the Lise Meitner Prize 2002 is awarded to

Prof. James Philip Elliott, University of Sussex (UK) Prof. Francesco Iachello, University of Yale (USA)

For their innovative applications of group theoretical methods to the understanding of atomic nuclei.

The physics case

The study of the discrete energy spectra of small quantum systems relates to fundamental symmetries in nature. These symmetries determine the "conserved quantities" describing the intrinsic properties of such systems and are needed in quantum mechanics to characterize the state of a system. Symmetries in nuclei have played a dominant role in the development of the understanding of fundamental properties of matter. Energy spectra of atomic nuclei reflect symmetries related to the properties of the two fundamental building blocks, the proton and the neutron, which carry a half integer spin and are thus characterized by the symmetries of fermions.

The special mathematical tool needed to describe symmetry in quantum systems is known as group theory. The symmetry of the nuclear force under rotations in spin and in isospin has lead to the introduction of an even larger symmetry group (SU(4), introduced by E. Wigner (Nobel Prize, 1963). The major contribution of J. P. Elliott in this field, came in the 1950`s. It provided an understanding of the structure of the spectra of light nuclei in terms of an underlying symmetry, expressed by the symmetry group SU(3). This symmetry reflects the dynamics of a many fermion system. The introduction of this method into nuclear physics opened new ways to the understanding of nuclear structure in general, in particular it allowed for the reconciliation of the spherical shell model of Maria Goeppert-Mayer and H. Jensen (Nobel prize, 1963) with the collective and liquid drop models of A. Bohr and B. Mottelson (Nobel Prize, 1975) which existed as separate and distinct descriptions of the nucleus. The work demonstrated that dynamical symmetries occur in the energy spectra of nuclei, a concept which also influenced the early work of elementary particle physics on the structure of hadrons, and which has led to the application of these concepts in nuclear, atomic and molecular physics.

Following up this work the Interacting Boson Model or Interacting Boson Approximation (IBA) was the next major step in applying the concept of dynamical symmetries to the understanding of the spectra of a large number of atomic nuclei. The Interacting Boson Model has been introduced in 1975 by F. Iachello in collaboration with A. Arima of the University of Tokyo, both at that time at the Kernfysisch Versneller Instituut (KVI) in Groningen (Netherlands). In this model, nuclear structure is described in terms of degrees of freedom involving subunits of integer spins (bosons). The concept of dynamical symmetries used in these studies is based on the fact that a symmetry may be broken in such a way as to lift the degeneracy in the energies but not alter the wave functions. A combination of the fermion and the boson degrees of freedom has lead to the introduction, by F. Iachello, of super-symmetry in nuclei which recently has been confirmed experimentally. Dynamic symmetries have also been applied by him, in the fields of elementary particle physics and molecular physics .

The description of nuclear structure based on the concepts, introduced by the two laureates, continues to play a pivotal role in the present nuclear structure studies in which large g-detector arrays like EUROBALL and Gammasphere are used. These concepts will be very important in the future studies of the newly accessible region of exotic nuclei (proton rich Z>>N, or very neutron rich N>>Z), when a new generation of radioactive beam facilities become operational.

Hahn-Meitner-Institut