/ 9 90 60 02 v 1 1 J un 1 99 9 1 DPNU - 99 - 19 Relativistic heavy ion collisions — Where are we now ? Where do we go ?

The construction of RHIC (Relativistic Heavy Ion Collider) at BNL will be completed by the time these proceedings are out and it will start accelerating gold nuclei at 100 GeV/A. What is the purpose of ultrarelativistic heavy ion collisions? The reader may have dreams such as i) study of high density matter, ii) production of the quark-gluon plasma, iii) quasi-reproduction of the big bang, iv) understanding of the history of the universe, and so forth. These are actually what will be pursued at RHIC, and so these are not dreams any more. We, however, have to turn our attention to the other side of reality as well: i) the lifetime of the system created in ultrarelativistic heavy ion collisions is very short, ii) the system is not static, iii) observables are dirty, in other words, the interpretation of observables is not straightforward, iv) there are so many models that claim to describe the results of ultrarelativistic heavy ion collisions successfully, and so on. Thus, in order to fully understand ultrarelativistic heavy ion collisions and QCD, it is really necessary for theorists to attack the following challenges, which were summarized by Matsui at Quark Matter 97 [1]: (i) Compute, as best as one can, expected properties of dense matter and its phase structure and make predictions for signals of new states of matter. (ii) Interpret the data from current fixed-target experiments and identify signals of new physics, if any, from backgrounds of “old physics”. (iii) Describe formation and evolution of dense matter in nuclear collisions and estimate physical conditions to be achieved at future collider experiments. Since the first half of item (i) will be attacked by Yoshié [2], I will concentrate on the second half of item (i) and items (ii) and (iii). I will discuss mainly the recent results of heavy ion collisions at CERN SPS at Elab = 160 200 GeV/A.