Giant resonances and giant resonances built on other giant resonances in nuclei are observed with very large cross sections in relativistic heavy ion collisions. A theoretical effort is underway to understand the reaction mechanism which leads to this process, as well as a better understanding of the microscopic properties of multiphonon states, e.g., their strength, energy centroids, widths and anharmonicities.1 Giant Resonances
Single giant resonancesGiant resonances in nuclei were first observed in 1937 by Bothe and Gentner 1 who obtained an unexpectedly large absorption of 17.6 MeV photons (from the 7 Li(p,γ) reaction) in some targets. These observations were later confirmed by Baldwin and Klaiber (1947) with photons from a betatron. In 1948, Goldhaber and Teller 2 interpreted these resonances (called isovector giant dipole resonances (IVGDP)) with a hydrodynamical model in which rigid proton and neutron fluids vibrate against each other, the restoring force resulting from the surface energy. Steinwendel and Jensen 3 later developed the model, considering compressible neutron and proton fluids vibrating in opposite phase in a common fixed sphere, the restoring force resulting from the volume symmetry energy. The standard microscopic basis for the description of giant resonances is the random phase approximation (RPA) in which giant resonances appear as coherent superpositions of one-particle one-hole (1p1h) excitations in closed shell nuclei or two quasiparticle excitations in open shell nuclei (for a review of these techniques, see, for example, ref. 4 ). The isoscalar quadrupole resonances were discovered in inelastic electron scattering by Pitthan and Walcher (1971) and in proton scattering by Lewis and Bertrand (1972). Giant monopole resonances were found later and their properties are closely related to the compression modulus of nuclear matter. Following these, other resonances of higher multipolarities and giant magnetic resonances were investigated. Typical probes for giant resonance studies are (a) γ's and electrons for the excitation of IVGDR, (b) α-particles and electrons for the excitation of isoscalar giant monopole resonance (ISGMR) and giant quadrupole resonance (ISGQR), and (c) (p, n), or ( 3 He, t), for Gamow-Teller resonances, respectively.
Multiphonon resonancesInelastic scattering studies with heavy ion beams have opened new possibilities in the field (for a review of the recent developments, see ref. 5 ). A striking feature was observed when either the beam energy was increased, or heavier projectiles were used, or both 6 . This is displayed in figure a