Introduction

Abstract

Quarks (q) and antiquarks ($\bar q$) are confined particles in hadrons. The hadrons (baryons, N, Λ, ∑, Δ…, and mesons, π, p, ω, K ...) are directly observed, whereas the q and the $\bar q$ are not. There may be another phase, and we shall talk about this later, in which q and $\bar q$ are deconfined but we do not live in that phase. So our instruments cannot experimentally detect them in that phase. When they recombine into observable hadrons they are again in the confining phase. So the existence of this deconfined phase must be a pure theory (and it better be a good reliable theory) or they must leave some mark, some signature of their passage into this deconfined phase. Are there such signatures? Experimentalists and theorists all over the world are eagerly searching for such signatures. It is an exciting branch of physics for experiments and also for theory (supercomputers, nationwide computer link-ups etc.). The suppression of the production of an exotic meson J/ψ, (also called gypsy!) in heavy ion collisions may be a candidate. This was predicted from theory by Matsui and Satz (1986) and found in experiments. But there are other possible explanations of this from nuclear physics. This gypsy suppression is typical and we shall discuss it in this chapter, but not with too much detail.