Suppression of High-$p_T$ Neutral Pion Production in Central $\mathrm{Pb}+\mathrm{Pb}$ Collisions at $\sqrt{s_{NN}}=17.3\mathrm{GeV}$ Relative to $p+\mathrm{C}$ and $p+\mathrm{Pb}$ Collisions

Neutral pion transverse momentum spectra were measured in $p+\mathrm{C}$ and $p+\mathrm{Pb}$ collisions at $\sqrt{s_{NN}}=17.4\mathrm{GeV}$ at midrapidity ($2.3\lesssim {\eta }_{\mathrm{lab}}\lesssim 3.0$) over the range $0.7\lesssim p_T\lesssim 3.5\mathrm{GeV}/c$. The spectra are compared to ${\pi }^0$ spectra measured in $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s_{NN}}=17.3\mathrm{GeV}$ in the same experiment. For a wide range of $\mathrm{Pb}+\mathrm{Pb}$ centralities ($N_{\mathrm{part}}\lesssim 300$), the yield of ${\pi }^0$’s with $p_T\gtrsim 2\mathrm{GeV}/c$ is larger than or consistent with the $p+\mathrm{C}$ or $p+\mathrm{Pb}$ yields scaled with the number of nucleon-nucleon collisions ($N_{\mathrm{coll}}$), while for central $\mathrm{Pb}+\mathrm{Pb}$ collisions with $N_{\mathrm{part}}\gtrsim 350$, the ${\pi }^0$ yield is suppressed.

Figure 1

Invariant ${\pi }^0$ yields in minimum bias $p+\mathrm{C}$ and $p+\mathrm{Pb}$ collisions at $\sqrt{s_{NN}}=17.4\mathrm{GeV}$. The error bars represent the quadratic sum of statistical and systematic uncertainties. The lines represent next-to-leading-order pQCD calculation of the ${\pi }^0$ yield in $\sqrt{s}=17.4\mathrm{GeV}$ $p+p$ collisions for three different scales, scaled by $⟨N_{\mathrm{coll}}^{p+\mathrm{C}}⟩=1.7$. The inset shows a comparison of the $p+\mathrm{C}$ spectrum to a parameterization of the yield in $p+p$ collisions at $\sqrt{s}=17.3\mathrm{GeV}$ from [23]. The dark gray box indicates the uncertainty of the parameterization estimated in [18], the light gray box the uncertainty of $⟨N_{\mathrm{coll}}^{p+\mathrm{C}}⟩$.

Figure 2

The $N_{\mathrm{coll}}$-normalized ratio of ${\pi }^0$ yields in $p+\mathrm{Pb}$ and $p+\mathrm{C}$ collisions. The box at $R_{p\mathrm{Pb}}^{\prime }\approx 0.8$ represents the expectation for the case of scaling with $N_{\mathrm{part}}$ rather than $N_{\mathrm{coll}}$. The box at unity indicates the $N_{\mathrm{coll}}$ systematic uncertainty. The error bars represent the quadratic sum of the statistical and remaining systematic uncertainties. The energy scale uncertainty was assumed to cancel.

Figure 3

${\pi }^0$ $R_{AA}^{\prime }$ in $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s_{NN}}=17.3\mathrm{GeV}$ for three centrality classes using $p+\mathrm{C}$ or $p+\mathrm{Pb}$ spectra as a reference. The boxes around unity reflect the systematic uncertainties related to $⟨N_{\mathrm{coll}}⟩$.

Figure 4

Centrality dependence of the average $⟨R_{AA}^{\prime }⟩$ for $2<p_T<2.5\mathrm{GeV}/c$ and $2.5\le p_T<3.0\mathrm{GeV}/c$. The bands around unity indicate the systematic uncertainty of $⟨N_{\mathrm{coll}}⟩$. The error bars represent the quadratic sum of the statistical and systematic uncertainties. The $N_{\mathrm{part}}$ values for the full points were shifted by $+5$ for better visibility.