Phantoms of the brain.

8 Theorists calculate that this drastic change in the structure of matter sets in over a narrow range of temperatures centred around 10 12 K. To gauge the change, it is instructive to compare the degrees of freedom — the number of different particles that energy can go to. On the hadronic side of the transition, the important particles at these temperatures are just the pions (the other hadrons being too heavy). These are spinless particles and come in three types — with positive , negative or zero electric charge. On the quark–gluon side, there are three colours of quarks of three different types (up, down and strange — the other quarks are too massive to play a part) and each has two possible spin directions. Taking into account antiquarks, we find 3232222 = 36 quark degrees of freedom. In addition, there are eight gluons each with two possible spin directions — thus 54 degrees of freedom altogether, compared with the previous three. A direct consequence is that a given input of energy will raise the temperature of a quark–gluon plasma much less than it would the hadronic gas, as the energy has to be shared by many more particles. Despite the dramatic nature of these predicted changes, it is not easy to establish experimentally that one has produced a quark–gluon plasma. Difficulties arise because the number of particles reaching the detectors after a heavy ion collision is extremely large, and because the plasma, even if produced, has only a fleeting existence in a very small region. The experimenters are like inspectors who must examine the residue of a great explosion to determine if it was due to conventional or nuclear weapons (or perhaps a meteorite). Ambitious responses to this challenge are being mounted at CERN and at Brookhaven, where the heavy-ion accelerator RHIC will come into operation next summer. Already, CERN has seen what might be the first harbinger of the quark–gluon plasma. Charm–quark/charm–antiquark pairs, making up the J/c family of particles, seem to find it much more difficult to stay paired once the energy in a fireball exceeds a threshold value (Fig. 2c). This certainly suggests the deconfinement mentioned above. It has been advocated for some time as a signature of the quark–gluon plasma. Even though the issue is muddied by the fact that the J/c particles will be buffeted more at higher temperature whether one has hadrons …