Kaon and Lambda Production at Intermediate $p_T$: Insights into the Hadronization of the Bulk Partonic Matter Created in Au+Au Collisions at RHIC

TL;DR

This dissertation analyzes identified hadron production in Au+Au collisions at RHIC to probe the early partonic stage and hadronization. By measuring elliptic flow $v_2$ and centrality-dependent suppression $R_{CP}$ for $K_S^0$ and $\Lambda+\overline{\Lambda}$ over broad $p_T$, it reveals a particle-type dependence consistent with coalescence/recombination of bulk partons and constituent-quark-number scaling of $v_2$. The results show that baryons exhibit less suppression and larger $v_2$ at intermediate $p_T$ than mesons, supporting a dense, thermalized partonic medium that hadronizes via multi-parton dynamics rather than simple fragmentation. These findings provide strong evidence for a quark-gluon plasma formed in Au+Au collisions at RHIC and have significant implications for understanding hadronization in dense QCD matter.

Abstract

Measurements of identified particles over a broad $p_T$ range may provide strong evidence for the existence of a thermalized partonic state in heavy-ion collisions (\textit{i.e.} a quark-gluon plasma). Of particular interest are the centrality dependence and the azimuthal anisotropy in the yield of baryons and mesons at intermediate $p_T$. The first measurements of $v_2$ and the number-of-binary-collisions scaled centrality ratio $R_{CP}$ for $K_S^0$ and $Λ+\barΛ$ production in Au+Au collisions at RHIC are presented. These measurements from the STAR detector establish the particle-type dependence of $v_2$ and $R_{CP}$ within $0.4 < p_T < 6.0$. At intermediate $p_T$ (1.5--4.5 GeV/c) $v_2$ of $K_S^0$ and $Λ+\barΛ$ is shown to follow a number-of-constituent-quark scaling with v_2^{kaon}(p_T/2)/2 = v_2^{lambda}(p_T/3)/3. $R_{CP}$ shows that lambda production at intermediate $p_T$ increases more rapidly with system size than kaon production: consistent with multi-parton dynamics in particle production. At $p_T = 5.5$ GeV/c lambda, kaon, and charged hadron production are suppressed by a similar factor ($R_{CP} \approx 0.33$): establishing the extent of the centrality dependent baryon enhancement. The particle- and $p_T$-dependence of $v_2$, and $R_{CP}$ are consistent with expectations based on hadronization of bulk partonic matter by coalescence. As such, the constituent-quark-number scaled $v_2$ may reflect the anisotropy established in a partonic stage providing strong evidence for the existence of a quark-gluon plasma in Au+Au collisions at RHIC.

Figures (58)

• Figure 1.1: Differential cross-sections for single jet production at pseudo-rapidity $\eta = 0$ as a function of the jet transverse momentum $p_T$ in proton (anti-)proton collisions. Jets are somewhat collimated sprays of particles produced when quarks or gluons collide, transfer (and carry away) a lot of momentum, and then fragment into a spray of hadrons. The curves represent pQCD calculations for the collisions at center-of-mass energy $\sqrt{s}=630$ and 1800 GeV.
• Figure 1.2: The energy density in QCD from lattice calculations. When the temperature $T$ reaches the critical temperature $T_c$, the number of degrees of freedom rapidly rises indicating that quarks and gluons become relevant degrees of freedom. The arrows represent the Stefan-Boltzmann values for asymptotically high temperature.
• Figure 1.3: The energy density $\epsilon$ and pressure $P$, scaled by $T^4$ from a statistical model Rafelski:2002ga. The various lines show results for different hadron mass spectra. The results show an increase in the energy density degrees of freedom at a critical temperature near $T = 158$ MeV.
• Figure 1.4: A sketch of the expected evolution of a relativistic heavy-ion collision.
• Figure 1.5: Left: Charged particle rapidity density at mid-rapidity scaled by the number of participating nucleon pairs $0.5 \times$ N$_{\mathrm{part}}$ versus $\sqrt{s_{_{NN}}}$Bazilevsky:2002fz. Right: Scaled pseudo-rapidity density for Au+Au and p+p collisions at $\sqrt{s_{_{NN}}}=$ 19.6, 130, and 200 GeV versus N$_{\mathrm{part}}$Back:2002ft.