A QQ¯ bound state represents a balance between repulsive kinetic and attractive potential energy. In a hot quark–gluon plasma, the interaction potential experiences medium effects. Color screening modifies the attractive binding force between the quarks, while the increase of entropy with QQ¯ separation gives rise to a growing repulsion. We study the role of these phenomena for in-medium QQ¯ binding and dissociation. It is found that the relevant potential for QQ¯ binding is the free energy F ; with increasing QQ¯ separation, further binding through the internal energy U is compensated by repulsive entropic effects.

Germany

Helmut Satz

Quarkonium binding and entropic force

The thermal multihadron production observed in different high energy collisions poses many basic problems: why do even elementary, <math id="M1"><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>-</mo></mrow></msup></math> and hadron-hadron, collisions show thermal behaviour? Why is there in such interactions a suppression of strange particle production? Why does the strangeness suppression almost disappear in relativistic heavy ion collisions? Why in these collisions is the thermalization time less than <math id="M2"><mo>≃</mo><mn>0.5</mn></math> fm/c? We show that the recently proposed mechanism of thermal hadron production through Hawking-Unruh radiation can naturally answer the previous questions. Indeed, the interpretation of quark (q)-antiquark (<math id="M3"><mrow><mover><mrow><mi>q</mi></mrow><mo>̅</mo></mover></mrow></math>) pairs production, by the sequential string breaking, as tunneling through the event horizon of colour confinement leads to thermal behavior with a universal temperature, <math id="M4"><mi>T</mi><mo>≃</mo><mn>170</mn></math> Mev, related to the quark acceleration, a, by <math id="M5"><mi>T</mi><mo>=</mo><mi>a</mi><mo>/</mo><mn>2</mn><mi>π</mi></math>. The resulting temperature depends on the quark mass and then on the content of the produced hadrons, causing a deviation from full equilibrium and hence a suppression of strange particle production in elementary collisions. In nucleus-nucleus collisions, where the quark density is much bigger, one has to introduce an average temperature (acceleration) which dilutes the quark mass effect and the strangeness suppression almost disappears.