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

We argue that the emission of hadrons with transverse momentum up to about $5\mathrm{G}\mathrm{e}\mathrm{V}/c$ 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.

Figure 1

Top: inclusive $P_T$ spectrum of charged hadrons in central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{\mathrm{N}\mathrm{N}}}=200\mathrm{G}\mathrm{e}\mathrm{V}$; data taken from the PHENIX collaboration. Bottom: ratio of protons to ${\pi }^{+}$ as a function of $P_T$. The region below $4\mathrm{G}\mathrm{e}\mathrm{V}/c$ is dominated by recombination, the region above $6\mathrm{G}\mathrm{e}\mathrm{V}/c$ by parton fragmentation.

Figure 2

Ratio $R_{\mathrm{A}\mathrm{A}}$ of hadron yields in central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ to $p+p$ collisions, scaled by the number of binary nucleon-nucleon interactions. Data for pions taken from PHENIX [24].