Production of ${\pi }^0$ and $\eta$ mesons in $\mathrm{Cu}+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$

Production of ${\pi }^0$ and $\eta$ mesons has been measured at midrapidity in $\mathrm{Cu}+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$. Measurements were performed in ${\pi }^0\left(\eta \right)\to \gamma \gamma$ decay channel in the $1\left(2\right)-20\text{GeV}/c$ transverse momentum range. A strong suppression is observed for ${\pi }^0$ and $\eta$ meson production at high transverse momentum in central $\mathrm{Cu}+\mathrm{Au}$ collisions relative to the $p+p$ results scaled by the number of nucleon-nucleon collisions. In central collisions the suppression is similar to $\mathrm{Au}+\mathrm{Au}$ with comparable nuclear overlap. The $\eta /{\pi }^0$ ratio measured as a function of transverse momentum is consistent with $m_T$-scaling parametrization down to $p_T=2\mathrm{GeV}/c$, its asymptotic value is constant and consistent with $\mathrm{Au}+\mathrm{Au}$ and $p+p$ and does not show any significant dependence on collision centrality. Similar results were obtained in hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions as well as in $e^{+}{e}^{-}$ collisions in a range of collision energies $\sqrt{s_{{}_{NN}}}=3-1800$ GeV. This suggests that the quark-gluon-plasma medium produced in $\mathrm{Cu}+\mathrm{Cu}$ collisions either does not affect the jet fragmentation into light mesons or it affects the ${\pi }^0$ and $\eta$ the same way.

Figure 1

Drawing showing $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Au}$ collisions with comparable $N_{\mathrm{part}}$. On panel (b) the overlap area is asymmetric, and part of the dilute surfaces of Au and Cu (corona) overlap.

Figure 2

Example of an invariant mass plot ($4.5<p_T<5.0 \mathrm{GeV}/c$). (a) The foreground (photon pairs from the same event) is shown as points, the shaded area is the scaled mixed event background. Insert (b) shows the ${\pi }^0$ peak area after mixed event subtraction; the Gaussian fit to the peak and the first-order polynomial fit to the residual background are also shown. Insert (c) shows the $\eta$ peak area with the Gaussian fit and second-order polynomial for the residual background.

Figure 3

Left: (a) ${\pi }^0$ and (b) $\eta$ invariant $p_T$ spectra measured in different centrality intervals of $\text{Cu}+\text{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$. The dashed curves are a fit with two Hagedorn-type functions with an asymptotic power-law (${p_T}^{-n}$) behavior. Right: ratios of (b)–(d) ${\pi }^0$ and (f)–(h) $\eta$ yields measured in PbSc or PbGl subsystem to the averaged ones. Error bars represent a quadratic sum of statistical and type-A systematic uncertainties. Error boxes in the right panel correspond to the quadratic sum of systematic uncertainties, which are uncorrelated between PbSc and PbGl.

Figure 4

Ratios of $\eta$ and ${\pi }^0$ yields measured as a function of $p_T$ in different centrality intervals of $\mathrm{Cu}+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$. Dashed curve shows the $m_T$ scaling curve normalized to 0.5 at high momentum (see text). Error bars represent a quadratic sum of statistical and type-A systematic uncertainties for ${\pi }^0$ and $\eta$ yields. Error boxes represent a quadratic sum of type-B systematic uncertainties for ${\pi }^0$ and $\eta$ yields.

Figure 5

$R_{AB}$ of ${\pi }^0$ and $\eta$ mesons measured as a function of $p_T$ in different centrality intervals of $\mathrm{Cu}+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$. Error bars represent a quadratic sum of statistical and type-A systematic uncertainties from $\mathrm{Cu}+\mathrm{Au}$ and $p+p$ measurements, respectively. Error boxes represent type-B systematic uncertainties from $\mathrm{Cu}+\mathrm{Au}$ and $p+p$ measurements. Solid and open boxes at unity represent type-C systematic uncertainties from $\mathrm{Cu}+\mathrm{Au}$ (including uncertainties from the $T_{AB}$ values) and $p+p$, respectively. The reference $p+p$ measurements are published in Ref. [12] for ${\pi }^0$ and in Refs. [30, 38] for $\eta$ (see details in the text).

Figure 6

Comparison of ${\pi }^0{R}_{AB}$ measured in $\text{Cu}+\text{Au}, \text{Au}+\text{Au}$ and $\text{Cu}+\text{Cu}$ collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$ and comparable $N_{\mathrm{part}}$. Error bars represent a quadratic sum of statistical and type-A systematic uncertainties from $\mathrm{Cu}+\mathrm{Au}$ and $p+p$ measurements. Open boxes are type-B systematic uncertainties for $\mathrm{Cu}+\mathrm{Au}$ and $p+p$ collisions. The three boxes at unity are type-C systematic uncertainties from $p+p$ and heavy-ion collisions. The boxes from left to right correspond to $\mathrm{Cu}+\mathrm{Au}, \mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ measurements, respectively.

Figure 7

Comparison of integrated $R_{AB}$ for ${\pi }^0$ measured as a function of $N_{\mathrm{part}}$ in $\text{Cu}+\text{Au}$ and $\text{Au}+\text{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$. Uncertainties are the same as in the Fig. 6. The lower limit of integration is $p_T =5 \mathrm{GeV}/c$ on (a), $p_T =10 \mathrm{GeV}/c$ on (b).