Production of ${\pi }^0$ and $\eta$ mesons in U+U collisions at $\sqrt{s_{NN}}=192$ GeV

The PHENIX experiment at the Relativistic Heavy Ion Collider measured ${\pi }^0$ and $\eta$ mesons at midrapidity in $\mathrm{U}+\mathrm{U}$ collisions at $\sqrt{s_{{}_{NN}}}=192$ GeV in a wide transverse momentum range. Measurements were performed in the ${\pi }^0\left(\eta \right)\to \gamma \gamma$ decay modes. A strong suppression of ${\pi }^0$ and $\eta$ meson production at high transverse momentum was observed in central $\mathrm{U}+\mathrm{U}$ collisions relative to binary scaled $p+p$ results. Yields of ${\pi }^0$ and $\eta$ mesons measured in $\mathrm{U}+\mathrm{U}$ collisions show similar suppression pattern to those measured in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV for similar numbers of participant nucleons. The $\eta /{\pi }^0$ ratios do not show dependence on centrality or transverse momentum and are consistent with previously measured values in hadron-hadron, hadron-nucleus, nucleus-nucleus, and $e^{+}{e}^{-}$ collisions.

Figure 1

Invariant-mass distributions for $\gamma \gamma$ pairs, obtained in 4–4.5 GeV/$c$ $p_T$ interval in 20%–40% centrality $\text{U}+\text{U}$ collisions. Panels (a) and (b) show the signal and normalized mixed-event background invariant-mass distributions in PbSc and PbGl subsystems, respectively. In label captions, $\mathrm{\Delta }{m}_{\mathrm{inv}}$ stands for the invariant-mass bin width and is equal to 10 MeV/$c^2$ for $m_{\mathrm{inv}}<0.3$ GeV/$c^2$ and for 20 MeV/$c^2$ at larger $m_{\mathrm{inv}}$ values. Inserts (c) and (d) show the invariant-mass distributions in ${\pi }^0$ and $\eta$ regions after the mixed-event background subtraction in PbSc, while inserts (e) and (f) show ones in PbGl.

Figure 2

(a) ${\pi }^0$ and (b) $\eta$ invariant $p_T$-spectra measured in different centrality intervals of $\text{U}+\text{U}$ collisions at $\sqrt{s_{{}_{NN}}}=192$ GeV. The dashed curves are fit with a power-law function. Error bars represent a quadratic sum of statistical and type-A systematic uncertainties. Error boxes represent a quadratic sum of type-B and type-C systematic uncertainties. Panels (c) and (d) shows data-to-fit ratios, the markers, error bars, and error boxes are, respectively, the same as for panels (a) and (b).

Figure 3

Panel (a) shows ratios of $\eta$ and ${\pi }^0$ yields measured as a function of $p_T$ in different centrality intervals of $\mathrm{U}+\mathrm{U}$ collisions at $\sqrt{s_{{}_{NN}}}=192$ GeV. Panel (b) compares $\eta$ and ${\pi }^0$ yield ratios measured as a function of $p_T$ in MB $\mathrm{U}+\mathrm{U}$ collisions at $\sqrt{s_{{}_{NN}}}=192$ GeV and $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV [40]. 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 from ${\pi }^0$ and $\eta$ yields.

Figure 4

$R_{AA}$ of ${\pi }^0$ and $\eta$ mesons measured as a function of $p_T$ in different centrality intervals of $\mathrm{U}+\mathrm{U}$ collisions at $\sqrt{s_{{}_{NN}}}=192$ GeV. Error bars represent a quadratic sum of statistical and type-A systematic uncertainties from $\mathrm{U}+\mathrm{U}$ and $p+p$ measurements, respectively. Error boxes represent type-B systematic uncertainties from $\mathrm{U}+\mathrm{U}$ and $p+p$ measurements. Solid and open boxes at unity represent type-C systematic uncertainties from $\mathrm{U}+\mathrm{U}$ (including uncertainties from the $T_{AA}$ values) and $p+p$, respectively.

Figure 5

Comparison at comparable $N_{\mathrm{part}}$ of ${\pi }^0{R}_{AA}$ measured in $\text{U}+\text{U}$ (two Glauber sets) at $\sqrt{s_{{}_{NN}}}=192$ GeV and in $\text{Au}+\text{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV [23]. Error bars represent a quadratic sum of statistical and type-A systematic uncertainties from $\mathrm{U}+\mathrm{U}$ and $p+p$ measurements. Open boxes are type-B systematic uncertainties for $\mathrm{U}+\mathrm{U}$ and $p+p$ collisions. The three boxes at unity are type-C systematic uncertainties from $p+p$ and nucleus-nucleus collisions. The boxes from left to right correspond to $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{U}+\mathrm{U}$ measurements, respectively. Note that while the mean $N_{\mathrm{part}}$ is similar for 40%–60% $\mathrm{U}+\mathrm{U}$ and 40%–50% $\text{Au}+\text{Au}$, its rms is 23.2 for $\mathrm{U}+\mathrm{U}$ while only 13.1 for $\text{Au}+\text{Au}$.

Figure 6

(a) Comparison of integrated $R_{AA}$ for ${\pi }^0$ and $\eta$ measured as a function of $N_{\mathrm{part}}$ in $\text{U}+\text{U}$ (Glauber-1 set) and $\text{Au}+\text{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=192$ GeV and $\sqrt{s_{{}_{NN}}}=200$ GeV, respectively. (b) Comparison of integrated $R_{AA}$ for ${\pi }^0$ measured as a function of $N_{\mathrm{part}}$ in $\text{U}+\text{U}$ (Glauber-1 and Glauber-2 sets) and $\text{Au}+\text{Au}$ collisions. Uncertainties are the same as in Fig. 5. The lower limit of integration is $p_T=5$ GeV/$c$.