Measurement of transverse-single-spin asymmetries for midrapidity and forward-rapidity production of hadrons in polarized $p+p$ collisions at $\sqrt{s}=200$ and 62.4 GeV

Measurements of transverse-single-spin asymmetries ($A_N$) in $p+p$ collisions at $\sqrt{s}=62.4$ and 200 GeV with the PHENIX detector at the Relativistic Heavy Ion Collider are presented. At midrapidity, $A_N$ is measured for neutral pion and eta mesons reconstructed from diphoton decay, and, at forward rapidities, neutral pions are measured using both diphotons and electromagnetic clusters. The neutral-pion measurement of $A_N$ at midrapidity is consistent with zero with uncertainties a factor of 20 smaller than previous publications, which will lead to improved constraints on the gluon Sivers function. At higher rapidities, where the valence quark distributions are probed, the data exhibit sizable asymmetries. In comparison with previous measurements in this kinematic region, the new data extend the kinematic coverage in $\sqrt{s}$ and $p_T$, and it is found that the asymmetries depend only weakly on $\sqrt{s}$. The origin of the forward $A_N$ is presently not understood quantitatively. The extended reach to higher $p_T$ probes the transition between transverse momentum dependent effects at low $p_T$ and multiparton dynamics at high $p_T$.

Figure 1

Uncalibrated energy spectra with and without cuts to isolate minimum ionizing particles (MIP) in the MPC. These cuts include neighboring tower energy deposits and track-matching cuts using the upstream BBC Čerenkov counters. The spectrum with the cut is fit with a power law and a Gaussian. The Gaussian peak position is taken as the most probable MIP energy deposit, $E\approx 234\mathrm{MeV}$.

Figure 2

Two-cluster invariant mass distributions from the 2008 data set at $\sqrt{s}=200\mathrm{GeV}$ for both the north and south MPC detectors. The left panel shows the ${\pi }^0$ peak from the minimum bias triggered data set at low energy while the right side shows the $\eta$ meson peak from the MPC triggered data set at high two-cluster energies $E$. A comparison of the peak position and widths from data and simulation are used to determine the energy scale uncertainty.

Figure 3

Neutral pion $A_N$ at $\sqrt{s}=62.4\mathrm{GeV}$ as a function of $x_F$ in two different pseudorapidity ranges ($3.1<|\eta |<3.5$ and $3.5<|\eta |<3.8$) with statistical and systematic uncertainties. Appendix Table 3 gives the data in plain text. An additional uncertainty from the beam polarization (see Table 1) is not included.

Figure 4

Neutral pion $A_N$ at $\sqrt{s}=62.4\mathrm{GeV}$ as a function of transverse momentum $p_T$. Appendix Table 4 gives the plain text data. An additional uncertainty from the beam polarization (see Table 1) is not included.

Figure 5

Comparison of neutral pion $A_N$ as a function of $x_F$ from $\sqrt{s}=19.4$ to 200 GeV from this publication and [7, 11]. Appendix Table 5 gives the data in plain text.

Figure 6

Isospin comparison of pion $A_N$ as a function of $x_F$ at $\sqrt{s}=62.4\mathrm{GeV}$ from this publication and [10]. Appendix Table 5 gives the data in plain text.

Figure 7

Cluster composition from $p+p$ Monte Carlo event generator studies at $\sqrt{s}=200\mathrm{GeV}$ with a full detector simulation. The kinematic cuts and $p_T$ ranges are the same as those used in the data analysis and directly comparable to Fig. 9, in particular $x_F>0.4$.

Figure 8

The $A_N$ of electromagnetic clusters at $\sqrt{s}=200\mathrm{GeV}$ as a function of $x_F$ and in two different pseudorapidity ranges. Appendix Table 6 gives the data in plain text. An additional uncertainty from the beam polarization (see Table 1) is not included.

Figure 9

The $A_N$ of electromagnetic clusters at $\sqrt{s}=200\mathrm{GeV}$ at large $|x_F|>0.4$ in forward/backward directions as a function of $p_T$. Appendix Table 7 gives the data in plain text. An additional uncertainty from the beam polarization (see Table 1) is not included.

Figure 10

Comparison of $A_N$ of electromagnetic clusters and ${\pi }^0$ mesons [11] at $\sqrt{s}=200\mathrm{GeV}$ as a function of $p_T$ in different ranges of $x_F$. Appendix Table 8 gives the data in plain text. An additional uncertainty from the beam polarization (see Table 1) is not included.

Figure 11

The $A_N$ measured at midrapidity ($|\eta |<0.35$), as a function of $p_T$ for ${\pi }^0$ (a) and $\eta$ (b) mesons (see Tables 9 and 10). Triangles are slightly forward/backward going subsamples of the full data set (circles). These are shifted in $p_T$ for better visibility. An additional uncertainty from the beam polarization (see Table 1) is not included.