Photon detector with PbWO4 crystals and APD readout

Quantum chromodynamics (QCD), the theory of strong interactions, with the statistical treatment predicts a phase transition from a confined state of quarks and gluons as normal nuclear matter into a deconfined state called quark gluon plasma (QGP). The universe on the current cosmology has evolved from an initial state of the extreme energy density to the present state through a rapid expansion and cooling. We speculate about the phase transition described by QCD at the early universe. The only one is the relativistic heavy-ion collisions in the current experimental technique for raising the phase transition in our laboratory. We should search and discover some signatures of QGP with heavy-ion collisions in order to understand the thermal history of our universe. The most promising probe in heavy-ion collisions is expected to be photons because of their large mean free paths in the strongly interacting reaction regime. Photons can escape from the reaction region and carry pure information on it. A larger amount of particles is, however, produced in relativistic heavy-ion collisions than that of elementary reactions such as electron-position and proton-antiproton collisions in a unit solid angle. (J.P.N.)