Cross Sections and Transverse Single-Spin Asymmetries in Forward Neutral-Pion Production from Proton Collisions at $\sqrt{s}=200\mathrm{GeV}$

Measurements of the production of forward high-energy ${\pi }^0$ mesons from transversely polarized proton collisions at $\sqrt{s}=200\mathrm{GeV}$ are reported. The cross section is generally consistent with next-to-leading order perturbative QCD calculations. The analyzing power is small at $x_F$ below about 0.3, and becomes positive and large at higher $x_F$, similar to the trend in data at $\sqrt{s}\le 20\mathrm{GeV}$. The analyzing power is in qualitative agreement with perturbative QCD model expectations. This is the first significant spin result seen for particles produced with $p_T>1\mathrm{GeV}/c$ at a polarized proton collider.

Figure 1

Uncorrected spectra of the diphoton invariant mass in two energy bins. The points are data with statistical errors. The open histograms are reconstructed simulation events, normalized to equal area. The hatched histograms are used to correct the cross section.

Figure 2

Inclusive ${\pi }^0$ production cross section versus leading ${\pi }^0$ energy ($E_{\pi }$). The average transverse momentum ($⟨p_T⟩$) is correlated with $E_{\pi }$, as the PFPD was at a fixed pseudorapidity ($\eta$). The inner error bars are statistical, and are smaller than the symbols for most points. The outer error bars combine these with the $E_{\pi }$-dependent systematic errors. The curves are NLO pQCD calculations evaluated at $\eta =3.8$ [29, 30, 31].

Figure 3

Analyzing powers versus Feynman $x$ ($x_F$). The average transverse momentum ($⟨p_T⟩$) is correlated with $x_F$. The solid points are for identified ${\pi }^0$ mesons. The open points are for the total energy ($E_{\mathrm{tot}}$), shifted by $x_F+0.01$. The inner error bars are statistical, and the outer combine these with the point-to-point systematic errors. The curves are from pQCD models evaluated at $p_T=1.5\mathrm{GeV}/c$ [14, 15, 16, 17]. The $A_N$ values are proportional to $A_N^{\mathrm{CNI}}$, assumed to be 0.013 at 100 GeV.