Quantitative Constraints on the Transport Properties of Hot Partonic Matter from Semi-Inclusive Single High Transverse Momentum Pion Suppression in Au+Au collisions at sqrt(s_NN) = 200 GeV

TL;DR

The paper addresses how to extract quantitative constraints on the transport properties of hot partonic matter from semi-inclusive high-$p_T$ $\pi^0$ suppression in $\sqrt{s_{NN}}=200$ GeV Au+Au collisions. It compares four parton-energy-loss formalisms (PQM, GLV, WHDG, ZOWW) to PHENIX $R_{AA}(p_T)$ data using a rigorous uncertainty framework that includes uncorrelated, correlated, and normalization errors. The analysis yields model-parameter constraints at the $\sim$20–25% level for each framework, with the best-fit values: $\langle\hat{q}\rangle\approx 13.2$ GeV$^{2}$/fm (PQM), $dN^{g}/dy\approx 1400$ (GLV/WHDG), and $\epsilon_{0}\approx 1.9$ GeV/fm (ZOWW); however, all models display a somewhat steeper $p_T$-dependence than the data, and the quoted uncertainties do not include theoretical systematics. The work demonstrates the potential of high-$p_T$ jet-quenching observables to constrain the properties of the quark-gluon plasma, while highlighting the need for improved theoretical control over energy-loss dynamics.

Abstract

The PHENIX experiment has measured the suppression of semi-inclusive single high transverse momentum pi^0's in Au+Au collisions at sqrt(s_NN) = 200 GeV. The present understanding of this suppression is in terms of energy-loss of the parent (fragmenting) parton in a dense color-charge medium. We have performed a quantitative comparison between various parton energy-loss models and our experimental data. The statistical point-to-point uncorrelated as well as correlated systematic uncertainties are taken into account in the comparison. We detail this methodology and the resulting constraint on the model parameters, such as the initial color-charge density dN^g/dy, the medium transport coefficient <q^hat>, or the initial energy-loss parameter epsilon_0. We find that high transverse momentum pi^0 suppression in Au+Au collisions has sufficient precision to constrain these model dependent parameters at the +/1 20%-25% (one standard deviation) level. These constraints include only the experimental uncertainties, and further studies are needed to compute the corresponding theoretical uncertainties.

Figures (9)

• Figure 1: (Color online) The $\pi^{0}$ nuclear suppression factor $R_{\rm AA}$ as a function of transverse momentum for 0-5% Au+Au collisions at $\sqrt{s_{NN}}$=200 GeV. Point-to-point uncorrelated statistical and systematic uncertainties are shown as uncertainty bars. Correlated systematic uncertainties are shown as gray boxes around the data points. The global scale factor systematic uncertainty is $\pm$12%.
• Figure 2: (Color online) Left panels show $\pi^{0}$$R_{\rm AA}$ for 0-5% Au+Au collisions at $\sqrt{s_{NN}}$=200 GeV and predictions from PQM PQM, GLV Vitev, WHDG Horowitz, and ZOWW zoww models with (from top to bottom) $\langle$q̂$\rangle$ values of 0.3, 0.9, 1.2, 1.5, 2.1, 2.9, 4.4, 5.9, 7.4, 10.3, 13.2, 17.7, 25.0, 40.5, 101.4 GeV$^{2}$/fm; $dN^{g}/dy$ values of 600, 800, 900, 1050, 1175, 1300, 1400, 1500, 1800, 2100, 3000, 4000; $dN^{g}/dy$ values of 500, 800, 1100, 1400, 1700, 2000, 2300, 2600, 2900, 3200, 3500, 3800; and $\epsilon_{0}$ values of 1.08, 1.28, 1.48, 1.68, 1.88, 2.08, 2.28, 2.68, 3.08 GeV/fm. Red lines indicate the best fit cases of (top) $\langle$q̂$\rangle$ = 13.2, (upper middle) $dN^{g}/dy$ = 1400, (lower middle) $dN^{g}/dy$ = 1400, and (bottom) $\epsilon_{0}$ = 1.88 GeV/fm. Right panels show $R_{\rm AA}$ at $p_{\rm T}=20$ GeV/$c$.
• Figure 3: (Color online) The nuclear suppression factors at $p_{\rm{T}}=20$ GeV/$c$ for PQM as a function of $\langle$q̂$\rangle$ are shown as a blue line with a log-x and log-y display. Also shown is the functional form $\Delta$$\langle$q̂$\rangle$ /$\langle$q̂$\rangle$$\approx$ 2.0 $\cdot~\Delta R_{\rm{AA}}/R_{\rm{AA}}$) over the range $5 <$$\langle$q̂$\rangle$$< 100$ GeV$^{2}$/fm.
• Figure 4: (Color online) The statistical analysis results from the comparison of the PQM model with the $\pi^{0}$$R_{\rm AA}$($p_{\rm T}$) experimental data. The top panel shows the modified $\tilde{\chi}^2$ for different values of the PQM $\langle$q̂$\rangle$. The middle panel shows the computed p-value directly from the modified $\tilde{\chi}^2$ as shown above. The bottom panel shows the number of standard deviations ($\sigma$) away from the minimum (best) $\langle$q̂$\rangle$ parameter value for the PQM model calculations.
• Figure 5: (Color online) The statistical analysis results from the comparison of the GLV model with the $\pi^{0}$$R_{\rm AA}$($p_{\rm T}$) experimental data. The top panel shows the modified $\tilde{\chi}^2$ for different values of the GLV $dN^{g}/dy$. The middle panel shows the computed p-value directly from the modified $\tilde{\chi}^2$ as shown above. The bottom panel shows the number of standard deviations ($\sigma$) away from the minimum (best) $dN^{g}/dy$ parameter value for the GLV model calculations.