D. D. Warner, M. A. Bentley and
P. Van Isacker
Nature Physics VOL 2 May 2006
The nucleus is a
unique laboratory in physics — a quantum many-body system comprising
two types of fermion, the neutron and proton, differing in charge but otherwise
essentially identical in their behaviour. The fact that the strong interaction
between these fermions is largely independent of charge results in striking
symmetries in nuclei. This neutron–proton exchange invariance is encompassed in
the elegant concept and formalism of Wigner's isotopic spin — or
isospin. The impact of isospin symmetry is maximal near the N=Z
line where nuclei have equal numbers of neutrons and protons, and studies
involving isospin effects have undergone a resurgence in recent years as such
nuclei become more readily accessible. In this review we discuss three
isospin-related phenomena: the elegant isospin symmetry of excited analogue
states in nuclei, the origin of the extra binding for nuclei with equal numbers
of neutrons and protons and the exotic phenomenon of neutron–proton pairing.
These three topics, all of considerable current interest, demonstrate the
power, simplicity and modern relevance of the isospin concept.