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In previous workshops of this series we presented results of the fragmentation observed in collisions of equal-size mercury-drop pairs[1], and a comparison[2] of these data with predictions of a dynamic nuclear reaction model[3], modified to allow the simulation of macroscopic liquid-drop collisions[4]. In the last presentation[2] we also showed preliminary results of an experiment designed to investigate the formation of “exotic shapes” (sheets and donuts) predicted to occur in heavy-ion collisions[5]. The motivation for this seemingly wayward approach, of hoping to learn something about nuclear dynamics from what is observed in macroscopic liquid-drop collisions, can be found in the written versions of our previous presentations. Here we give a progress report on two subjects. The first is a study of the evolution of the surface shape of a system composed by two, equal-size, drops put in contact at the lowest possible relative velocity. The second is related to a qualitative description of the instabilities of the exotic shapes reported previously[2] in central collisions of equal-size drops.

This series of winter workshops is devoted to nuclear dynamics, a term referring to a broad range of physics topics that can be explored by bringing the nuclear system far away from its ordinary tranquil state, usually by means of nuclear collisions. As ever more powerful accelerators and increasingly refined detector systems are becoming available, the scope of these studies has steadily expanded. Not only has the accessible domain of the nuclear chart grown explosively, but prospects have appeared for probing fundamental features of strongly interacting systems. A long-standing goal has been to probe the non-perturbative properties of QCD and the expected transition to a deconfined quark-gluon plasma phase. Of somewhat more recent date is the recognition that high-energy nucleus-nucleus collisions may also provide key insights into chiral symmetry. In particular, the collision may produce extended regions of space within which chiral symmetry is temporarily nearly restored and the subsequent non-equilibrium relaxation towards the normal vacuum may then produce large-amplitude coherent oscillations of the pion field. This hypothetical phenomenon, which has been dubbed disoriented chiral condensates, may lead to an enhanced emission of isospin-polarized soft pions.1–6 We shall briefly review the key DCC features and discuss various possible signals for its actual occurrence. For a recent DCC review, see ref. 7.

We report the multiplicity distributions from the collision of Au + Au at the AGS experiments E866 and E917. During the 1995 run, data was taken for beam energies of 2, 4, and 11 GeV/nucleon (GeV/n). The 1996 run was composed of 6, 8, and more of 11 GeV/n beam,* thus giving us an opportunity to study the nuclear dynamics of the same colliding system at various energies. We were able to obtain the multiplicity distributions from the E866/E917’s global detector New Multiplicity Array (NMA) almost immediately, since it requires very little overhead to generate physics results. Here we share some of these results. Comparisons to models are not done here, mostly due to lack of time between data-taking and reporting of the results.

We are engaged in establishing a bridge between fields where macroscopic liquid-drop collisions are important and heavy-ion reaction modeling, particulary with those theories in which the quantal effects can, somehow, be switched off, allowing a direct access to the bare liquid-drop aspects. Besides providing an interesting testing-ground for those sophysticated models, this represents the oportunity of applying them to fields like meteorology and spray research, where the theoretical situation is less developed. On the 10th Winter Workshop on Nuclear Dynamics, in 1994, we presented [1] the results of collision experiments using mercury drops with the hope of gaining the interest of theoretical nuclear dynamisists. As a result, a collaboration was indeed established which lead to the modification of a nuclear reaction code, developed by N. Cârjan, A. Sierk and R. Nix [2] allowing the simulation of macroscopic liquid-drop collisions. Here we like to make a brief review of what we have learned so far in this line of research. A subject which recently cought our attention was the predictions [3] of nuclear fluid-dynamic simulations concerning the posibility of multifragmentation mechanisms proceeding via the formation of exotic nuclear shapes (sheets, bubbles, donuts, etc.) Since similar predictions have also been reported for macroscopic systems [4] we are now experimenting with liquid-drops colliding at the relative velocities where those exotic shapes are expected [4] to appear. This presentation will also include the first results of that quest, including interesting video images.

Two years ago at the 10th Winter Workshop on Nuclear Dynamics, we reported preliminary results from our measurements of photons in reactions of 200·A GeV 32S ions with Au nuclei.1 The results had been presented earlier at Quark Matter ’93.2 For central collisions we reported a small, but statistically significant, excess of photons over those that can be accounted for by the two-photon decay branches of π0 and η mesons and by the small photon contributions from other radiative decays. This paper is, in a sense, a sequel to the 1994 presentation. Here we present the final WA80 single “direct” photon results,3 and we discuss their implications for various theoretical calculations.

Recent theoretical calculations have raised one of the most intriguing questions today in intermediate energy nuclear physics. This question concerns the formation of exotic shapes in nuclear matter. There have been many theorists, using a rich diversity of models to simulate nuclear dynamics, who have predicted the occurrence of such shapes.[1–5] The title “non-compact geometries” refers to the position-space distribution of the nucleons of a combined nuclear system shortly after a nucleus-nucleus collision, and implies short-lived configurations with novel shapes, e.g. toroids or bubbles. A solid sphere is the geometrical configuration with the minimum surface area for a given volume, i.e., the most compact shape. In this light, any shape other than a sphere can be called “non-compact.”

We consider equilibration in relativistic nuclear dynamics starting from a nonequilibrium Greensfunctions approach. The widely used Boltzmann-Uehling-Uhlenbeck equation is obtained only as the Markovian limit (i.e. negligible memory time). The actual memory time in energetic nuclear collisions turns out to be ~ 2−3 \(\frac{fm}{c}\), which interferes substantially with the time-scale of the relaxation process. The memory kernels of the two-particle collisions and their feedback on the complete dynamics (by an explicite comparison with the standard Boltzmann treatment) will be presented.

The basic structure-function relation in photobiology, which pertains to the mechanism of the primary charge separation in photosynthetic reaction centers (RC), is not yet elucidated, although the first structure of the bacterial photosynthetic RC was determined eight years ago. The major difficulty involved in the understanding of this central energy conversion process in biology is that it requires information on energetics, electronic interactions and nuclear dynamics in electronically excited states, which cannot be readily inferred from the structural data in the ground electronic state. This state of affairs reflects on the intrinsic limitations of structure to infer an excited state dynamics.

These volumes are compelling testimony to the rapid development of heavy-ion collisions as penetrating probes of nuclear structure and as sources of information on nuclear dynamics. But with equal emphasis, the availability of heavy-ion projectiles has had a major impact on other fields of physics. One such particularly fruitful overlap is with atomic physics. Atomic collisions with light projectiles such as photons, electrons, and protons have always been important probes of atomic structure. Most recently, the introduction of a greatly expanded pool of heavy-ion projectiles and the ability to accelerate these to high energies has further extended this field and, more significantly, introduced unique opportunities to study new phenomena. Indeed, during the last decade, paralleling the development of heavy-ion accelerators, a clearly distinguishable field of high-energy atomic and molecular physics has emerged. Recent conferences reflect its scope (Od 80, Gr 81, Cr 82, Da 82).

We investigate the generation of transverse momentum in high energy heavy ion reactions and its relation to the nuclear equation of state. We also study the emission of electron-positron pairs from hot nuclear matter and from nucleon-nucleon collisions. We argue that the study of dilepton spectra will yield valuable information on nuclear dynamics in general and on interacting pions in particular.

Since physicists vary in their degree of interest in the theory of nuclear dynamics, these lectures address several levels of theoretical sophistication. Whereas one might mumble excuses concerning the broad distribution in the background of students, in fact my time and endurance simply gave out before I had produced a completely satisfactory pedagogical sequence which started at the very beginning and derived everything required to reach the end. In addition to sincere apologies to all concerned and a few homework problems designed to fill in the most obvious gaps, I also offer the following brief guide to the reader.