Experimental signatures of phase transition to quark matter in high-energy collisions of nuclei

Colliding high-energy heavy nuclei is the only known way to experimentally study the phase transition from nucleonic to quark matter. Observations are, however, frustrated by the fact that secondary $\pi$ mesons are formed in the later stages of the interaction and are therefore insensitive to the early stages when quark matter is formed. We speculate on two types of experimental signatures of phase transition to quark matter. (i) The structure in the rapidity distribution of the secondaries from a near central collision of two heavy nuclei. We estimate that the reduction in shear viscosity between the colored quark-gluon plasma and the peripheral spectator part of each nucleus leads to an observable separation in rapidity of the secondaries from each component. (ii) Abundant production of prompt photons with $\frac{n_{\gamma }}{n_{\pi }}\simeq 30\%$ for central collisions with about 50 GeV energy per nucleon.