Transverse Single-Spin Asymmetry and Cross-Section for pi0 and eta Mesons at Large Feynman-x in Polarized p+p Collisions at sqrt(s)=200 GeV

Abstract

Measurements of the differential cross-section and the transverse single-spin asymmetry, A_N, vs. x_F for pi0 and eta mesons are reported for 0.4 < x_F < 0.75 at an average pseudorapidity of 3.68. A data sample of approximately 6.3 pb^{-1} was analyzed, which was recorded during p+p collisions at sqrt{s} = 200 GeV by the STAR experiment at RHIC. The average transverse beam polarization was 56%. The cross-section for pi0 is consistent with a perturbative QCD prediction, and the eta/pi0 cross-section ratio agrees with previous mid-rapidity measurements. For 0.55 < x_F < 0.75, A_N for eta (0.210 +- 0.056) is 2.2 standard deviations larger than A_N for pi0 (0.081 +- 0.016).

Figures (5)

• Figure 1: (color online) The distribution of $\sigma_{Log}$, as defined in the text, for $E_{cluster} > 65$ GeV for data and simulation, in units of FPD transverse cell size (3.81 cm). For comparison, the one and two photon cluster peaks from simulation were independently normalized, and uniformly shifted by $+0.01$ transverse cell size (1 bin) to account for the small difference in the average size of clusters between simulation and data.
• Figure 2: (color online) (a) Di-photon invariant mass, $M_{\gamma\gamma}$, distributions in data and simulation for $E_{\gamma\gamma} > 45$ GeV, with the "center cut" as defined in Eq. (\ref{['eq:center']}). The simulation results were normalized to have the same number of events as the data in the $\pi^0$ mass region ($0.08 < M_{\gamma\gamma} < 0.19$ GeV/$c^2$). The symbol $\langle \eta \rangle$ indicates the average pseudorapidity of the photon pair. (b) Same as (a), but plotted using an expanded linear scale to illustrate the $\eta$ mass region. For the dashed line, the $\eta$ signal was removed from the simulation at the pythia level. (c) $A_N$ vs. $M_{\gamma\gamma}$ for the above mass distribution. The error bars are statistical uncertainties only.
• Figure 3: Pseudorapidity vs. tangent of the azimuthal angle of the di-photon center of mass, for $E_{\gamma\gamma} > 50$ GeV. LEFT: $0.08 < M_{\gamma\gamma} < 0.19$ GeV/$c^2$, RIGHT: $0.45 < M_{\gamma\gamma} < 0.65$ GeV/$c^2$. The filled boxes indicate events that pass the center cut (Eq. (\ref{['eq:center']})).
• Figure 4: (color online) Differential production cross-sections for $\pi^0$ and $\eta$ at average pseudorapidity of 3.68. Also shown are the previously published STAR results for similar kinematics Adams:2006uz and a NLO pQCD calculation of the $\pi^0$ cross-section vogel. The error band represents the uncertainty in the calculation due to scale variations. The $\eta$ to $\pi^0$ cross-section ratio is shown in the bottom panel. The error bars indicate the total statistical and systematic uncertainties.
• Figure 5: (color online) $A_N$ vs. $x_F$ at average pseudorapidity of 3.68 for $\pi^0$ and $\eta$. Also shown are the previously published results for $\pi^0$ at lower $x_F$, derived from the same data set but without the center cut :2008qb. The error bars are statistical uncertainties only. The error boxes indicate the systematic uncertainties.