Beam-energy dependence of identified two-particle angular correlations in $\sqrt{s_{NN}}=7.7–200$ GeV $\mathrm{Au}+\mathrm{Au}$ collisions

The two-particle angular correlation functions, $R_2$, of pions, kaons, and protons in Au $+$ Au collisions at $\sqrt{s_{NN}}=7.7,11.5,14.5,19.6,27,39,62.4$, and 200 GeV were measured by the STAR experiment at the BNL Relativistic Heavy Ion Collider. These correlations were measured for both like-sign and unlike-sign charge combinations and versus the centrality. The correlations of pions and kaons show the expected near-side (i.e., at small relative angles) peak resulting from short-range mechanisms. The amplitudes of these short-range correlations decrease with increasing beam energy. However, the proton correlation functions exhibit strong anticorrelations in the near-side region. This behavior is observed for the first time in an $A+A$ collision system. The observed anticorrelation is $p_T$ independent and decreases with increasing beam energy and centrality. The experimental results are also compared to the Monte Carlo models UrQMD, Hijing, and AMPT.

Figure 1

Angular correlation function $R_2(\mathrm{\Delta }y,\mathrm{\Delta }\phi )$ of like-sign (left) and unlike-sign (right) pions in Au $+$ Au collisions at mid centrality 30%–40% and $0.2<p_T<2.0\mathrm{GeV}/c$ in different beam energies from 7.7 GeV (top left) to 200 GeV (bottom right).

Figure 2

Angular correlation function $R_2(\mathrm{\Delta }y,\mathrm{\Delta }\phi )$ of like-sign (left) and unlike-sign (right) protons in Au $+$ Au collisions at mid centrality 30%–40% and $0.4<p_T<2.0\mathrm{GeV}/c$ in different beam energies from 7.7 GeV (top left) to 200 GeV (bottom right). Due to the large statistical fluctuations in large $\mathrm{\Delta }y$ bins, the plots are presented in the range of $\left|\mathrm{\Delta }y\right|\le 1$.

Figure 3

Angular correlation function $R_2(\mathrm{\Delta }y,\mathrm{\Delta }\phi )$ of like-sign (left) and unlike-sign (right) kaons in Au $+$ Au collisions at 200 GeV, mid centrality 30%–40%, and $0.2<p_T<1.6\mathrm{GeV}/c$.

Figure 4

Projection of correlation function $\langle R_2\left(\mathrm{\Delta }y\right)\rangle$ of like-sign (red) and unlike-sign (blue) pions (left) and protons (right) in Au $+$ Au collisions at 30%–40% centrality and eight different energies from 7.7 GeV (top left) to 200 GeV (bottom right). Also shown at the highest beam energies in the right frames are the antiproton-antiproton correlations.

Figure 5

Near-side and away-side $\langle R_2\left(\mathrm{\Delta }y\right)\rangle$ projections of like-sign (red) and unlike-sign (blue) pions (left) and protons (right) in Au $+$ Au collisions at 14.5 GeV (top) and 62.4 GeV (bottom), 30%–40% centrality.

Figure 6

Projection of correlation function $\langle R_2\left(\mathrm{\Delta }y\right)\rangle$ of like-sign (red) and unlike-sign (blue) pions (left) and protons (right) in Au $+$ Au collisions at 14.5 GeV (top) and 62.4 GeV (bottom), 30%–40% centrality compared with the UrQMD (solid line), Hijing (dash-dotted line), and AMPT (dotted line) simulations.

Figure 7

Projection of correlation function $\langle R_2\left(\mathrm{\Delta }y\right)\rangle$ of like-sign (red) and unlike-sign (blue) pions (left) and protons (right) in Au $+$ Au collisions at 14.5 GeV (top) and 62.4 GeV (bottom) for the most central 0%–5%, mid-central 30%–40%, and peripheral 60%–70% events.

Figure 8

Projection of correlation function $\langle R_2\left(\mathrm{\Delta }y\right)\rangle$ of like-sign (red) and unlike-sign (blue) pions (left) and protons (right) in low and high $p_T$ in Au $+$ Au collisions at 14.5 GeV (top) and 62.4 GeV (bottom) in 30%–40% centrality.