The dynamics of quarkonium production

Quarkonium production is a sensitive probe of the dynamics of hard scatteri ng, which can 'measure' the environment of the heavy quark pair after it is created in a hard process. Quarkonium hadroproduction data indicates that the produced pair experiences a secondary, hard interaction and then ceases to interact at a stage when the pair is still compact (compared to the sir e of the quarkonium wave function). These features differ from scenarios mo stly discussed so far. An approach which relies on an early, perturbative r escattering of the pair with a comoving color field can explain many observ ed features, including the polarization, the chi(1)/chi(2) ratio and the nu clear target A-dependence, which are difficult to understand otherwise.