Quark coalescence based on a transport equation

Hadron spectra measured at intermediate transverse momenta, pT=2-6GeV, in 200AGeV Au-Au collisions at the Relativistic Heavy-Ion Collider (RHIC) have revealed a surprisingly large baryon-tomeson ratio ~1 and a so-called constituent quark-number scaling (CQNS) of the elliptic flow, v2(pT). Both phenomena are rather naturally explained within quark coalescence models (QCMs), where constituent quarks are combined into hadrons at the phase boundary between a Quark-Gluon-Plasma (QGP) and a hadron gas. QCMs are typically formulated in a collinear, instantaneous approximation which conserves 3-momentum but not total energy, limiting their applicability to sufficiently high pT [1]. In the present work [2], we employ the Boltzmann equation to compute meson formation via resonant quark-antiquark scattering which improves previous QCMs in that energy is conserved and a proper thermal equilibrium limit can be established. This, in turn, allows a more controlled extension of the approach to low pT and to address the experimentally observed transition from a hydrodynamic regime to CQNS, while 2→1 scattering restricts the applicability to positive values of Q=mM -2mq (mM q: meson, quark mass). We here focus on the φ meson using input strange-quark distributions with collective properties characteristic for RHIC energies. Our results for φ spectra are summarized in Fig. 1, illustrating that we recover CQNS of v2 at sufficiently high pT, but with appreciable sensitivity to the Q value of the