Recombination in nuclear collisions

Recombination is a hadronization process that converts partons to hadrons at late time, but the description has no quantitative significance without some meaningful input on the parton distributions at earlier time. Thus observations of particle spectra and correlations have definitive implications on the partonic processes at all transverse momenta. After presenting a general review of the subject at the Workshop, I selected two topics in nuclear collisions for more detailed discussion, which are summarized here. One is on the azimuthal anisotropy at low $p_T$ due to hard or semihard scattering of partons that create ridges with or without triggers. The ridge particles are the products of recombination of thermal partons enhanced by the energy loss of hard or semihard partons. Their $ϕ$ dependence at midrapidity can be determined essentially from geometry. The other topic is on the scaling behavior in $ϕ$ and centrality at high $p_T$, where the hadronization process is dominated by TS and SS recombination. The relevant RHIC data are found to have the same scaling behavior. But at LHC such scaling is badly broken at $p_T \sim 10$ GeV/c if two-jet recombination is important. At the end of this article some comments are made to relate our study of the effects of semihard partons to the observation of minijets in the analyses of STAR data.