Event-wise $\langle p_t\rangle$ fluctuations in Au-Au collisions at $\sqrt{s_{\mathit{NN}}}=130$ GeV

We present the first large-acceptance measurement of event-wise mean transverse momentum $\langle p_t\rangle$ fluctuations for Au-Au collisions at nucleon-nucleon center-of-momentum collision energy $\sqrt{s_{\mathit{NN}}}$ = 130 GeV. The observed nonstatistical $\langle p_t\rangle$ fluctuations substantially exceed in magnitude fluctuations expected from the finite number of particles produced in a typical collision. The r.m.s. fractional width excess of the event-wise $\langle p_t\rangle$ distribution is $13.7\pm 0.1$(stat) $\pm 1.3$(syst)% relative to a statistical reference, for the 15% most-central collisions and for charged hadrons within pseudorapidity range $\left|\eta \right|<1,2\pi$ azimuth, and $0.15\le p_t\le 2$ GeV/c. The width excess varies smoothly but nonmonotonically with collision centrality and does not display rapid changes with centrality which might indicate the presence of critical fluctuations. The reported $\langle p_t\rangle$ fluctuation excess is qualitatively larger than those observed at lower energies and differs markedly from theoretical expectations. Contributions to $\langle p_t\rangle$ fluctuations from semihard parton scattering in the initial state and dissipation in the bulk colored medium are discussed.

Figure 1

(Color online) Upper panel: Event frequency distribution on $\sqrt{n}(\langle p_t\rangle -{\stackrel{\wedge }{p}}_t)/{\sigma }_{{\stackrel{\wedge }{p}}_t}$ (see text) for 80% of primary charged hadrons in $\left|\eta \right|<1$ for 183k central events (histogram) compared to gamma reference (dashed curve), Monte Carlo reference (solid curve underlying gamma reference), and broadened distribution (solid curve underlying data, not a fit—see text). Lower panel: Difference in upper panel between data and gamma reference (histogram) or between broadened distribution and gamma reference (solid curve) normalized by the Poisson error $\sqrt{N_{\mathrm{evt}}}$ in each bin.

Figure 2

(Color online) Mean-$p_t$ difference factors $\Delta {\sigma }_{p_t:n}^{(\mathrm{CI})}$ and $\Delta {\sigma }_{p_t:n}^{(\mathrm{CD})}$ for 205k minimum-bias Au-Au events at $\sqrt{s_{\mathit{NN}}}$ = 130 GeV vs relative multiplicity $N/N_0$ [14], which is approximately $N_{\mathrm{part}}/N_{\mathrm{part},\max }$, the relative fraction of participant nucleons [21]. Charge-independent (solid triangular points) and charge-dependent (open triangular points, multiplied by 3 for clarity) difference factors include statistical errors only (smaller than symbols). Parametrizations (dashed curves), extrapolation of parametrizations to true primary particle number (solid curves), and systematic uncertainties (bands) are discussed in the text. Difference factors for the 15% most-central collision events are shown by the solid circle and open circle symbols.