1. We distinguish two hadronization mechanisms:  Fragmentation Fragmentation builds on the idea of a single quark in the vacuum, it doesn’t consider many quarks. A quark in a jet radiates gluons and creates many quark- antiquark pairs that form hadrons. Fragmentation is described by fragmentation function which give a probability for a given quark to form a certain hadron.  Recombination Recombination describes hadronization of many quarks; it is therefore applicable in a QGP. Quarks close in phase space can recombine into hadrons. Fragmentation is dominant in p+p and electron- positron annihilations. Fragmentation has to win for high pt, but recombination is dominant for intermediate pt, in heavy ion collisions. This was first observed in Au-Au collisions at the Relativistic Heavy Ion Collider (RHIC). Quark recombination in high energy collisions for different energies. Steven Rose, Worcester Polytechnic Institute Mentor: Dr. Rainer Fries, Texas A&M University Cyclotron Institute REU 2007 QGP Phase Transition Collisions of nuclei at high energy result in a heating of the system above a critical temperature where quarks lose association with any particular hadron. In this Quark-Gluon Plasma (QGP), color degrees of freedom are no longer confined. The fireball rapidly expands and cools. Some high momentum quarks will fly outward and leave the plasma as jets. Due to quark confinement, all quark must hadronize again. 200 GeV Au+Au 0-10% 0.56271.72 62.4 GeV Au+Au 0-10% 0.55227.98 62.4 GeV Au+Au 20-40% 0.55226.98 22.5 GeV Cu+Cu 0-10% 0.2595.03 vTvT ATAT Conclusions  Clear excess of data over fragmentation; recombination must be considered  Recombination contributes more to baryon production than meson production  More recombination for 62.4 GeV than 200 GeV  Less recombination for smaller nuclei, larger impact parameter (less densely populated phase space)  Contribution of fragmentation and recombination may be similar for Cu+Cu collisions at 22.5 GeV; not well constrained by data Further Goals  Better fragmentation functions and more data for different energies and system sizes would be helpful for a more systematic study.  Comparison of parameters v T and A T to other models, particularly Hydrodynamics. [1] R.J. Fries, B. Muller, C. Nonaka and S.A. Bass, Phys. Rev. C68, 044902 (2003). [2] S.S. Adler et. al. (PHENIX Collaboration, Preprint nucl- ex/0611007. [3] B.I. Abelev et al. (STAR Collaboration), Phys. Rev. Lett. 97(2006) 152301. [4] B.I. Abelev et al. (STAR Collaboration), Preprint nucl- ex/0703040. References Hadronization Proton over pion ratio measured at RHIC is ~1. Fragmentation Predicts ~0.3 Our Goal: Establish recombination contribution For different collisions energies/systems Methodology  General formulism from [1].  Perform perturbative calculations to create jet spectra for various collisions/energies/nuclei  Calculation is Leading Order, so fits the shape well, but not the size- scale by an appropriate “k-factor”  Use KKP fragmentation functions  Use correct collision geometry as function of impact parameter. Calculate energy loss of jets from path length.  Assume thermal quark spectra, f q, with temperature T and radial flow v T and fireball transverse area A T  Yield of mesons with the meson wave function,  (x): Results AuAu 200 GeV 0-10% AuAu 62.4 GeV 0-10% AuAu 62.4 GeV 20-40% CuCu 22.5 GeV 0-10% Pi 0 s R AA