SEARCH FOR PRODUCTION OF FRACTIONAL CHARGES, NEW LONG-LIVED PARTICLES, AND SUBTHRESHOLD ANTIPROTONS, IN RELATIVISTIC HEAVY ION COLLISIONS (QUARK, RARE, NUCLEAR)

The results of the experiments involving collision of Relativistic Heavy Ions at Bevalac energies indicate that some degree of thermal and/or collective behavior among the colliding nucleons is present. On the other hand, recent theoretical calculations show that in these reactions exotic phenomena such as transitions to a quark-gluon plasma may take place. It has also been suggested that in such a plasma, quarks or composites with fractional charges could be liberated. Since at the Bevalac energies the creation of heavy, negatively charged particles through direct nucleon-nucleon collisions is not possible, detection of such productions could signal a high degree of collective and/or thermal equilibration among the participant nucleons. Detection of fractional charges on the other hand, would be considered strong evidence for formation of a quark-gluon plasma.;Assuming an exponential momentum spectrum and isotropic production in the center of mass frame, the results of all secondary rigidities were combined to yeild, for a good part of mass range from 0.1 to 5.0 GeV, and for different charges, upper limits of the order of one particle per 10('4) to 10('7) collisions. The combined upper limit for p production was found to be less than one p per 3.5 x 10('7) collisions. Comparison of this upper limit with some models rejects existence of a complete thermal and/or chemical equilibrium among heavy particles in collision of a system as small as ('28)Si.;We performed a search for production of fractional charges, new long-lived particles, and subthreshold antiprotons at 0(DEGREES), in the reaction ('28)Si + ('28)Si at 2.1 GeV/nucleon kinetic energy. Six different secondary rigidities of 0.732, 0.902, 0.98, 1.42, 1.95, 2.37 GeV/cz were searched. In this attempt we obtained null results for production of fractional charges and new long-lived particles. Except for one candidate--having a substantial chance of being a p event--no evidence of subthreshold antiproton production was found. The upper limit yields at all six rigidities for charges: -1/3, -2/3, -1, -4/3, -2, as a function of mass, and for p, are presented.