Spectra of identified high-$p_T$ ${\pi }^{\pm }$ and $p(\mathrm{p¯})$ in $\mathrm{Cu}+\mathrm{Cu}$ collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV

We report new results on identified (anti)proton and charged pion spectra at large transverse momenta ($3<p_T<10$ GeV$/c$) from $\mathrm{Cu}+\mathrm{Cu}$ collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV using the STAR detector at the Relativistic Heavy Ion Collider (RHIC). This study explores the system size dependence of two novel features observed at RHIC with heavy ions: the hadron suppression at high-$p_T$ and the anomalous baryon to meson enhancement at intermediate transverse momenta. Both phenomena could be attributed to the creation of a new form of QCD matter. The results presented here bridge the system size gap between the available $\mathit{pp}$ and $\mathrm{Au}+\mathrm{Au}$ data, and allow for a detailed exploration of the onset of the novel features. Comparative analysis of all available 200 GeV data indicates that the system size is a major factor determining both the magnitude of the hadron spectra suppression at large transverse momenta and the relative baryon to meson enhancement.

Figure 1

Transverse momentum spectra of pions (a) and protons (b) produced in $\mathrm{Cu}+\mathrm{Cu}$ collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV. Data are presented for four centrality classes: 0–$10\%$, 10–$20\%$, 20–$40\%$, and 40–$60\%$. Closed and open symbols are used for particles and antiparticles, respectively. For clarity, data are separated by powers of four.

Figure 2

Antiparticle to particle ratios, as a function of transverse momentum for pions (a) and protons (b). Data for the four centrality classes show little centrality dependence. Errors are statistical only.

Figure 3

(a) Nuclear modification factor, $R_{\mathit{AA}}$, for charged pions (${\pi }^{+}+{\pi }^{-}$) in $\mathrm{Cu}+\mathrm{Cu}$ (filled symbols) and $\mathrm{Au}+\mathrm{Au}$ (open symbols) collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV. Error bands are shown for most peripheral and most central $\mathrm{Cu}+\mathrm{Cu}$ data to represent evolution of the systematic uncertainties for this dataset. Error boxes at $R_{\mathit{AA}}=1$ represent $\mathrm{Cu}+\mathrm{Cu}$ scale uncertainties due to the number of collisions and from $\mathit{pp}$ spectra normalization. (b) Integrated pion $R_{\mathit{AA}}$ over the range $5<p_T<8$ GeV/$c$ versus $N_{\mathrm{part}}$. The bands represent the systematic uncertainty on ratios. An additional scale error due to $\mathit{pp}$ normalization ($~14\%$) is not shown.

Figure 4

(a) Nuclear modification factor, $R_{\mathit{AA}}$, for protons and antiprotons ($p+\mathrm{p̄}$) in $\mathrm{Cu}+\mathrm{Cu}$ (filled symbols) and $\mathrm{Au}+\mathrm{Au}$ (open symbols) collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV. Error band is shown for most central $\mathrm{Cu}+\mathrm{Cu}$ data to represent characteristic systematic uncertainties for $\mathrm{Cu}+\mathrm{Cu}$ data. Error boxes at $R_{\mathit{AA}}=1$ represent $\mathrm{Cu}+\mathrm{Cu}$ scale uncertainties due to the number of collisions and from $\mathit{pp}$ spectra normalization. (b) Integrated (anti)proton $R_{\mathit{AA}}$ over the range $5<p_T<6$ GeV/$c$ versus $N_{\mathrm{part}}$. The bands represent the systematic uncertainty on ratios. An additional scale error due to $\mathit{pp}$ normalization (about $14\%$) is not shown.

Figure 5

(a) Yield ratio of protons and anti-protons to charged pions, versus transverse momentum, for $\mathrm{Cu}+\mathrm{Cu}$ collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV. For clarity, only one systematic error band is shown for $\mathrm{Cu}+\mathrm{Cu}$ data (most central events), uncertainties on data in other centrality bins are similar in magnitude. Systematic errors for $\mathrm{Au}+\mathrm{Au}$ data are not shown, for details see [9]. (b) Average $(p+\mathrm{p̄})/({\pi }^{+}+{\pi }^{-})$ ratio for $3<p_T<4$ GeV$/c$ is shown as function of centrality ($N_{\mathrm{part}}$) for $\mathrm{Cu}+\mathrm{Cu}$ and $\mathrm{Au}+\mathrm{Au}$ data. The error band represents the $\mathit{pp}$ reference with its uncertainty.