Hadronization in Heavy-Ion Collisions: Recombination and Fragmentation of Partons

Abstract

We argue that the emission of hadrons with transverse momentum up to about 5 GeV/c in central relativistic heavy ion collisions is dominated by recombination, rather than fragmentation of partons. This mechanism provides a natural explanation for the observed constant baryon-to-meson ratio of about one and the apparent lack of a nuclear suppression of the baryon yield in this momentum range. Fragmentation becomes dominant at higher transverse momentum, but the transition point is delayed by the energy loss of fast partons in dense matter.

Anisotropic flow inCu+Aucollisions atsNN=200GeV

“…The transition from the partonic matter to the hadronic matter is achieved using a simple coalescence model, which combines the two nearest quark and antiquark into mesons and the three nearest quarks or antiquarks into baryons or antibaryons that are close to the invariant mass of these partons. The present coalescence model is thus somewhat different from the ones recently used extensively [33][34][35][36] for studying hadron production at intermediate transverse momenta. By using parton scattering cross sections of 6−10 mb, the AMPT model with string melting was able to reproduce both the centrality and transverse momentum (below 2 GeV/c) dependence of the elliptic flow [10] and pion interferometry [22] measured in Au + Au collisions at √ s = 130A GeV at RHIC [37,38].…”

Hadronic signals of deconfinement at RHIC

“…In the limiting case of equal momentum fractions carried by the constituent quarks these relations simplify to [12,28,29] …”

“…Denoting the quark phase-space distribution by w α (p) (here α denotes color, flavor, and spin degrees of freedom) and employing light-cone coordinates along the direction of the emitted hadron, instantaneous recombination predicts the following meson and baryon spectra, respectively [28]:…”

Relativistic Nuclear Collisions and the QCD Phase Boundary

“…This behavior is quite inconsistent with the standard fragmentation picture of a hadronizing parton, from which much smaller p/π + ratios are expected. Based on these and other data on elliptic flow of mesons and baryons in the transverse momentum region 2 < p t < 5 GeV an alternative picture was developed recently [43,44,45,46] in which hadron production occurs by recombination rather than fragmentation of partons. In fact, if the parton density is high enough, there is, for an exponentially decreasing parton p t spectrum, a critical parton transverse momentum below which "naive" 5 recombination will win over fragmentation.…”