he state of matter on our planet is not typical of the rest of the universe. In the interior of stars and in supernova explosions, unimaginably high temperatures and pressures predominate, creating particles and nuclei which do not naturally occur here below. In the very early universe quarks and gluons preceded neutrons and protons. Most of the mass of an atom is located in the nucleus, which is unimaginably dense at 14 orders of magnitude greater than that of water. The forces between the individual components of the nucleus—the nucleons—vary according to distance in a manner remarkably similar to those between molecules in a liquid. At very short distances, the binding forces repel; at medium nucleon distances, they attract. In fact, in many ways, atomic nuclei behave very much like drops of liquid. But if the liquid nuclear matter is compressed to ten times its usual density the individual nucleons cease to exist, being replaced by quarks and gluons. Hot Papers #7 and #10 consider the equation of state of nuclear matter when the temperature is raised to nuclear boiling point and the hadrons of ordinary nuclear matter become a plasma of quarks and gluons.

GSI, in Darmstadt, Germany, is the only accelerator laboratory in the world with the capacity to investigate nuclei throughout the entire periodic table including to the heaviest element so far created artificially, the element with atomic number 111. Darmstadt has equipment that can produce beams of any of these nuclei over a broad energy range. Paper #7 from GSI deals with heavy ion collisions at ultra-relativistic energies, in experiments designed to look for signs of a quark-gluon plasma. One of the crucial questions in such experiments is whether thermal and chemical equilibrium is reached at some stage in the collision.

The hypothesis of quark-gluon plasma is still somewhat speculative. The basic idea is that at the energies achieved in ultra-relativistic collisions such as Pb-Pb the quarks and gluons in the individual hadrons become de-confined and a plasma extends through the bulk of nuclear matter. Paper #10 describes the squashy geometry of the Pb-Pb fireballs in which the plasma may make a fleeting appearance. Paper #7 presents a statistical model of the Pb-Pb fireball at the point when the quark-gluon plasma freezes out to produce hadrons. Both papers have important implications for cosmology because when the age of the universe was 1 microsecond, the phase change from quark-gluon plasma to hadrons supposedly took place.

The properties of fundamental particles also feature in #6, on neutrino oscillations. Neutrinos come in three variants which are associated with electrons, muons, and taus. Science Watch has previously highlighted observations that suggest that neutrinos oscillate between these variants. Those earlier results were from neutrinos of solar or atmospheric origin. Paper #6 describes results from the CHOOZ power station in the Ardennes region of France, where electron anti-neutrinos are detected by the positrons produced in an inverse beta-decay. This paper is getting big hits on the citation radar screen because no evidence is found of neutrino oscillations in the electron anti-neutrino mode. However, this startling result does not necessarily conflict with the results from other neutrino experiments because CHOOZ is probing a different energy range for the neutrinos. Nevertheless the limits set out in #6 will challenge proponents of neutrino oscillations.

This period the real world of condensed matter physics is represented by #9, an experimental paper on the ambivalent nature of certain magnetic oxides. Colossal magnetoresistance is a gigantic change of resistance observed in certain materials when a large magnetic field is applied. Materials are normally classed as metals or insulators, but the perovskites can be transformed from a ferromagnetic metal to an insulator simply by raising the temperature and then back to a metal by applying a small magnetic field. This effect might have technological applications, such as allowing an increase in the density of data that can be stored on magnetic media.

Hot Paper #9 shows how manganite crystals can flip from conductor to insulator. These materials have coexisting phases of metal and insulator with identical chemical composition. The stability of one phase over another depends on chemistry. Uehara et al., in #9, study a series of compounds in which La is replaced by Pr while the Mn valence remains fixed. They conclude that both phases coexist in these materials, with the colossal magnetoresistance being explained by percolative transport through the ferromagnetic domains. The capacity for self-organized structures in oxides to surprise continues.