Directed flow in Au+Au collisions at $\sqrt{s_{\mathit{NN}}}=62.4$ GeV

We present the directed flow ($v_1$) measured in Au+Au collisions at $\sqrt{s_{\mathit{NN}}}=62.4$ GeV in the midpseudorapidity region $\left|\eta \right|<1.3$ and in the forward pseudorapidity region $2.5<\left|\eta \right|<4.0$. The results are obtained using the three-particle cumulant method, the event plane method with mixed harmonics, and for the first time at the Relativistic Heavy Ion Collider, the standard method with the event plane reconstructed from spectator neutrons. Results from all three methods are in good agreement. Over the pseudorapidity range studied, charged particle directed flow is in the direction opposite to that of fragmentation neutrons.

Figure 1

Directed flow of charged particles as a function of pseudorapidity, for centrality 10–70%. The arrows in the upper panel indicate the direction of flow for spectator neutrons. The arrow positions on the pseudorapidity axis corresponds to where the incident ions would lie on a rapidity scale. The lower panel shows the midpseudorapidity region in more detail. The plotted errors are statistical only, and systematic effects are discussed in the text.

Figure 2

$v_1$ versus rapidity for protons and pions. The charged particle $v_1(\eta )$ is plotted as a reference. The different upper end of the $p_t$ range for protons and pions is because of different limits of the $\mathrm{dE}/\mathrm{dx}$ identification method. The solid and dashed lines are results from linear fits described in the text. All results are from analyses using the reaction plane reconstructed by the ZDC-SMD, $v_1\{\text{ZDC-SMD}\}$. The plotted errors are statistical only, and systematic effects are discussed in the text.

Figure 3

Directed flow of charged particles as a function of pseudorapidity for different centralities. The plotted errors are statistical.

Figure 4

The upper panel shows $v_1\{3\}$ versus $p_t$ measured in the main TPC ($\left|\eta \right|<1.3$), for centrality 10–70%. The lower panel shows $v_1\{3\}$ versus $p_t$ measured by the TPC ($2.5<\left|\eta \right|<4.0$) for different centralities. Note the different scales on both axes for the two panels. The differential directed flow of particles with negative η has been changed in sign as stated in the text. The plotted errors are statistical.

Figure 5

Directed flow of charged particles as a function of impact parameter for the midpseudorapidity region ($\left|\eta \right|<1.3$, with the left vertical scale) and the forward pseudorapidity region ($2.5<\left|\eta \right|<4.0$, with the right vertical scale). The differential directed flow of particles with negative η has been changed in sign as stated in the text. The plotted errors are statistical.

Figure 6

Charged particle $v_1$ for Au+Au collisions (10–70%) at 200 GeV [12] (open stars) and 62.4 GeV (solid stars), as a function of $\eta -y_{\mathrm{beam}}$. Also shown are results from NA49 [7] (circles) for pions from 158A GeV midcentral (12.5–33.5%) Pb+Pb collisions as a function of $y-y_{\mathrm{beam}}$. The 62.4- and 200-GeV points are averaged over the positive and negative rapidity regions. All results are from analyses involving three-particle cumulants, $v_1\{3\}$. The plotted errors are statistical.