Identified Charged Particle Spectra and Yields in Au+Au Collisions at sqrt(s_NN) = 200 GeV

TL;DR

This paper presents a comprehensive measurement of identified charged-hadron spectra in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV with the PHENIX detector, focusing on centrality and mass-dependent trends from low to intermediate $p_T$. It demonstrates mass-dependent spectral shapes consistent with radial flow, shows mean $p_T$ and yields saturating with centrality, and reveals a pronounced baryon enhancement at intermediate $p_T$, with $p/\pi$ and $\bar{p}/\pi$ rising and approaching unity around $p_T\sim2$ GeV/$c$ in central events. The work also highlights a near-constancy of equal-mass particle ratios across $p_T$ and centrality, supports thermal-model expectations for $\mu_B$ and $T_{ch}$, and documents $N_{coll}$-scaling for baryons at intermediate $p_T$, suggesting a transition in hadronization from fragmentation to recombination/junction mechanisms. Collectively, these results illuminate the interplay of collective flow, chemical freeze-out, and partonic energy loss in the hot, dense medium created in heavy-ion collisions.

Abstract

The centrality dependence of transverse momentum distributions and yields for pi^+/-, K^+/-, p and p^bar in Au+Au collisions at sqrt(s_NN) = 200 GeV at mid-rapidity are measured by the PHENIX experiment at RHIC. We observe a clear particle mass dependence of the shapes of transverse momentum spectra in central collisions below ~ 2 GeV/c in p_T. Both mean transverse momenta and particle yields per participant pair increase from peripheral to mid-central and saturate at the most central collisions for all particle species. We also measure particle ratios of pi^-/pi^+, K^-/K^+, p^bar/p, K/pi, p/pi and p^bar/pi as a function of p_T and collision centrality. The ratios of equal mass particle yields are independent of p_T and centrality within the experimental uncertainties. In central collisions at intermediate transverse momenta ~ 1.5-4.5 GeV/c, proton and anti-proton yields constitute a significant fraction of the charged hadron production and show a scaling behavior different from that of pions.

Figures (24)

• Figure 1: BBC versus ZDC analog response. The lines represent the centrality cut boundaries.
• Figure 2: Mass squared versus momentum multiplied by charge distribution in Au+Au collisions at $\hbox{$\sqrt{s_{NN}}$}$ = 200 GeV. The lines indicate the PID cut boundaries for pions, kaons, and protons (anti-protons) from left to right, respectively.
• Figure 3: Track reconstruction efficiency ($\epsilon_{\rm mult}$) as a function of centrality. The error bars on the plot represent the systematic errors.
• Figure 4: The fractional contribution of protons ($\hbox{$\overline{p}$}$) from $\Lambda$ ($\hbox{$\overline{\Lambda}$}$) decays in all measured protons ($\hbox{$\overline{p}$}$), $\delta_{\rm feed}(\hbox{$p_T$})$, as a function of $\hbox{$p_T$}$. The solid (dashed) lines represent the systematic errors for protons ($\hbox{$\overline{p}$}$). The error bars are statistical errors.
• Figure 5: Transverse momentum distributions for pions, kaons, protons and anti-protons in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. The top two figures show $\hbox{$p_T$}$ spectra for the most central 0--5% collisions. The bottom two are for the most peripheral 60--92% collisions. The error bars are statistical only. The $\Lambda$ ($\hbox{$\overline{\Lambda}$}$) feed-down corrections for protons (anti-protons) have been applied.