Onset of ${\pi }^0$ Suppression Studied in $\mathrm{Cu}+\mathrm{Cu}$ Collisions at $\sqrt{s_{NN}}=22.4$, 62.4, and 200 GeV

Neutral pion transverse momentum ($p_T$) spectra at midrapidity ($|y|\lesssim 0.35$) were measured in $\mathrm{Cu}+\mathrm{Cu}$ collisions at $\sqrt{s_{NN}}=22.4$, 62.4, and 200 GeV. Relative to ${\pi }^0$ yields in $p+p$ collisions scaled by the number of inelastic nucleon-nucleon collisions ($N_{\mathrm{coll}}$) the ${\pi }^0$ yields for $p_T\gtrsim 2\mathrm{GeV}/c$ in central $\mathrm{Cu}+\mathrm{Cu}$ collisions are suppressed at 62.4 and 200 GeV whereas an enhancement is observed at 22.4 GeV. A comparison with a jet-quenching model suggests that final state parton energy loss dominates in central $\mathrm{Cu}+\mathrm{Cu}$ collisions at 62.4 and 200 GeV, while the enhancement at 22.4 GeV is consistent with nuclear modifications in the initial state alone.

Figure 1

For $\sqrt{s_{NN}}=22.4$, 62.4, and 200 GeV are plotted (a) invariant ${\pi }^0$ yields in central $\mathrm{Cu}+\mathrm{Cu}$ collisions, (b) invariant ${\pi }^0$ cross sections in $p+p$ collisions [14, 15, 16], and (c) the $p+p$ data plotted as a function of $x_T=2p_T/\sqrt{s}$, which exhibit an approximate $x_T$ scaling. The error bars represent the quadratic sum of the statistical and total systematic uncertainties.

Figure 2

Measured ${\pi }^0$ $R_{AA}$ are compared to a jet quenching calculation [22, 23]. The error bars (here and in Fig. 3) represent the quadratic sum of the statistical and the point-to-point uncorrelated and correlated systematic uncertainties. For $\sqrt{s_{NN}}=22.4\mathrm{GeV}$ the error bars also include the systematic error of the fit of the $p+p$ spectra. The boxes around unity indicate uncertainties related to $⟨N_{\mathrm{coll}}⟩$ and absolute normalization. The bands for the calculation correspond to the assumed range of the initial gluon density $d{N}^g/dy$.

Figure 3

The average $R_{AA}$ in the interval $2.5<p_T<3.5\mathrm{GeV}/c$ as a function of centrality for $\mathrm{Cu}+\mathrm{Cu}$ collisions at $\sqrt{s_{NN}}=22.4$, 62.4, and 200 GeV. The shaded bands represent jet-quenching calculations at three discrete centralities ($N_{\mathrm{part}}\sim 10,50,100$) [22, 23]. The boxes around unity represent the normalization and $⟨N_{\mathrm{coll}}⟩$ uncertainties for a typical $N_{\mathrm{coll}}$ uncertainty of 12%.