Dijet imbalance measurements in $\mathrm{Au}+\mathrm{Au}$ and $pp$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ at STAR

We report the first dijet transverse momentum asymmetry measurements from $\mathrm{Au}+\mathrm{Au}$ and $pp$ collisions at RHIC. The two highest-energy back-to-back jets reconstructed from fragments with transverse momenta above $2\mathrm{GeV}/c$ display a significantly higher momentum imbalance in heavy-ion collisions than in the $pp$ reference. When reexamined with correlated soft particles included, we observe that these dijets then exhibit a unique new feature—momentum balance is restored to that observed in $pp$ for a jet resolution parameter of $R=0.4$, while rebalancing is not attained with a smaller value of $R=0.2$.

Figure 1

Normalized $A_J$ distributions for $\mathrm{Au}+\mathrm{Au}$ HT data (filled symbols) and $pp$ HT $\oplus$ $\mathrm{Au}+\mathrm{Au}\mathrm{MB}$ (open symbols). The red circles are for jets found using only constituents with $p_T^{\mathrm{Cut}}>2\mathrm{GeV}/c$ and the black squares for matched jets found using constituents with $p_T^{\mathrm{Cut}}>0.2\mathrm{GeV}/c$. In all cases $R=0.4$. Stat. errors may be smaller than symbol size for $pp$ HT $\oplus$ $\mathrm{Au}+\mathrm{Au}\mathrm{MB}$.

Figure 2

$A_J$ distributions for $\mathrm{Au}+\mathrm{Au}$ data (filled symbols) and $pp$ HT $\mathrm{Au}+\mathrm{Au}$ MB (open symbols) for low constituent $p_T^{\mathrm{Cut}}$ di-jets from Fig. 1 compared to $A_J$ distributions calculated assuming the RC and EC null hypotheses, respectively, shown as colored bands; see the text for details. Stat. errors may be smaller than symbol size for $pp$ HT $\mathrm{Au}+\mathrm{Au}$ MB.

Figure 3

Repetition of the analysis shown in Fig. 1 with a smaller resolution parameter $R=0.2$. Normalized $A_J$ distributions for $\mathrm{Au}+\mathrm{Au}$ HT data (filled symbols) and $pp$ HT $\oplus$ $\mathrm{Au}+\mathrm{Au}\mathrm{MB}$ (open symbols). The red circles are for jets found using only constituents with $p_T^{\mathrm{Cut}}>2\mathrm{GeV}/c$ and the black squares are for matched jets found using constituents with $p_T^{\mathrm{Cut}}>0.2\mathrm{GeV}/c$. Stat. errors may be smaller than symbol size for $pp$ HT $\oplus$ $\mathrm{Au}+\mathrm{Au}\mathrm{MB}$.