Suppressed ${\pi }^0$ Production at Large Transverse Momentum in Central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ Collisions at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$

Transverse momentum spectra of neutral pions in the range $1<p_T<10\mathrm{G}\mathrm{e}\mathrm{V}/c$ have been measured at midrapidity by the PHENIX experiment at BNL RHIC in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$. The ${\pi }^0$ multiplicity in central reactions is significantly below the yields measured at the same $\sqrt{s_{NN}}$ in peripheral $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ and $p+p$ reactions scaled by the number of nucleon-nucleon collisions. For the most central bin, the suppression factor is $\sim 2.5$ at $p_T=2\mathrm{G}\mathrm{e}\mathrm{V}/c$ and increases to $\sim 4–5$ at $p_T\approx 4\mathrm{G}\mathrm{e}\mathrm{V}/c$. At larger $p_T$, the suppression remains constant within errors. The deficit is already apparent in semiperipheral reactions and increases smoothly with centrality.

Figure 1

Invariant ${\pi }^0$ yields at midrapidity as a function of $p_T$ for minimum bias and nine centralities in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$ [0%–10% (80%–92%) is most central (peripheral)].

Figure 2

Nuclear modification factor $R_{AA}(p_T)$ for ${\pi }^0$ in central (closed circles) and peripheral (open circles) $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$. The error bars include all point-to-point experimental ($p+p$, $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$) errors. The shaded bands represent the fractional uncertainties in $⟨T_{\mathrm{A}\mathrm{u}\mathrm{A}\mathrm{u}}⟩$ and in the ${\pi }^0$ yields normalization added in quadrature, which can move all the points up or down together (in the central case the shaded band shown is the fractional error for the first point).

Figure 3

Ratio of $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ over $p+p$ ${\pi }^0$ yields integrated above $4\mathrm{G}\mathrm{e}\mathrm{V}/c$ and normalized using $⟨N_{\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{l}}⟩$ (circles) and $⟨N_{\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{t}}⟩$ (crosses), as a function of centrality given by $⟨N_{\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{t}}⟩$. The errors bands and bars are the same as for Fig. 2.