Longitudinal double-spin asymmetry and cross section for inclusive neutral pion production at midrapidity in polarized proton collisions at $\sqrt{s}=200\mathrm{GeV}$

We report a measurement of the longitudinal double-spin asymmetry $A_{LL}$ and the differential cross section for inclusive ${\pi }^0$ production at midrapidity in polarized proton collisions at $\sqrt{s}=200\mathrm{GeV}$. The cross section was measured over a transverse momentum range of $1<p_T<17\mathrm{GeV}/c$ and found to be in good agreement with a next-to-leading order perturbative QCD calculation. The longitudinal double-spin asymmetry was measured in the range of $3.7<p_T<11\mathrm{GeV}/c$ and excludes a maximal positive gluon polarization in the proton. The mean transverse momentum fraction of ${\pi }^0$’s in their parent jets was found to be around 0.7 for electromagnetically triggered events.

Figure 1

(a) Cross section for inclusive ${\pi }^0$ production at midrapidity in $p+p$ collisions at $\sqrt{s}=200\mathrm{GeV}$, compared to a NLO pQCD calculation [8] based on the DSS set of fragmentation functions [25], and to the STAR ${\pi }^{\pm }$ measurement [27]. (b) The ratio of measured cross section and the NLO pQCD calculation. The scale uncertainty is indicated by the dashed curves ($\mu =2p_T$, $p_T/2$). The error bars are statistical and shaded bands are $p_T$-correlated systematic uncertainties. The normalization uncertainty of 11.5% is indicated by a shaded band around unity on the right-hand side.

Figure 2

(a) Mean transverse momentum fraction of ${\pi }^0$’s in their associated jets, as a function of pion $p_T$, for electromagnetically triggered events. Systematic errors are shown by the shaded band around the data points. The curves are results from simulations with the pythia event generator. The solid curve includes detector effects simulated by geant, while the dashed curve uses jet finding at the pythia particle level. (b) The distribution of $z$ for one $p_T$ bin, compared to pythia with a full detector response simulation.

Figure 3

Longitudinal double-spin asymmetry for inclusive ${\pi }^0$ production at midrapidity in $p+p$ collisions at $\sqrt{s}=200\mathrm{GeV}$, compared to NLO pQCD calculations based on the gluon distributions from the GRSV [34], GS-C [33], and DSSV [12] global analyses. Systematic errors are shown by the shaded band. The normalization uncertainty of 9.4% due to the beam polarization measurement is not shown.