Parton cascade and coalescence

Recent data from Au + Au reactions at √ sNN = 130 and 200 GeV from RHIC show striking difference between baryons and mesons in the intermediate transverse momentum region 2 < p⊥ < 5 GeV. First, elliptic flow scales with constituent quark number[ 1, 2, 3, 4, 5]. The flow per constituent quark as a function of transverse momentum per constituent quark, v 2 (pT/n)/n (n = 2 for mesons, 3 for baryons), is a universal function for all hadron species (within experimental uncertainties). Second, nuclear suppression is weaker (RAA is larger) for baryons than for mesons[ 1, 2, 6, 7, 4, 8, 9]. The most promising mechanism proposed to explain both phenomena is parton coalescence[ 10, 11, 12, 13, 14, 15, 16, 17]. In the coalescence model, mesons form from a quark and antiquark, while baryons from three quarks or three antiquarks (contributions from higher Fock states are small [ 18]). Most versions of the model are based on variants of the simple “coalescence formula” [ 19, 20, 13, 14, 16]