Inclusive cross section and double helicity asymmetry for π^0 production in p+p collisions at sqrt(s)=200 GeV: Implications for the polarized gluon distribution in the proton

Abstract

The PHENIX experiment presents results from the RHIC 2005 run with polarized proton collisions at sqrt(s)=200 GeV, for inclusive π^0 production at mid-rapidity. Unpolarized cross section results are given for transverse momenta p_T=0.5 to 20 GeV/c, extending the range of published data to both lower and higher p_T. The cross section is described well for p_T < 1 GeV/c by an exponential in p_T, and, for p_T > 2 GeV/c, by perturbative QCD. Double helicity asymmetries A_LL are presented based on a factor of five improvement in uncertainties as compared to previously published results, due to both an improved beam polarization of 50%, and to higher integrated luminosity. These measurements are sensitive to the gluon polarization in the proton, and exclude maximal values for the gluon polarization.

Figures (3)

• Figure 1: The neutral pion production cross section at $\sqrt{s}=200$ GeV as a function of $p_{T}$ (squares) and the results of NLO pQCD calculations for theory scales $\mu=p_T/2$ (dotted line), $p_T$ (solid line) and $2p_T$ (dashed line), see text for details; note that the error bars are smaller than the points. The inset shows, in addition to $\pi^0$, data for $(\pi^{+}+\pi^{-})/2$ (solid circles), and a fit of charged pion data to an exponential function for $p_T<0.8$ GeV/$c$ (dashed line). The bottom panel shows the relative difference between the data and theory for the three theory scales. Error bars are quadratic sums of experimental statistical and systematic uncertainties (the $9.7\%$ normalization uncertainty is not included).
• Figure 2: The double helicity asymmetry for neutral pion production at $\sqrt{s}=200$ GeV as a function of $p_{T}$ (GeV/$c$). Error bars are statistical uncertainties, with the 9.4% scale uncertainty not shown; other experimental systematic uncertainties are negligible. Four GRSV theoretical calculations based on NLO pQCD are also shown for comparison with the data (see text for details.)
• Figure 3: The $\chi^{2}$ distribution of the measured data plotted versus the value of the first moment of the polarized gluon distribution (solid line) in the $x_g$ range from 0.02 to 0.3 corresponding to our $\pi^0$ data in $p_{T}$ bins from 2 to 9 GeV/$c$. Dashed and dotted lines correspond to $-9.4\%$ and $+9.4\%$ variation in $A_{LL}$ normalization related to the beam polarization uncertainty, the dominant systematical uncertainty of our data. Only statistical uncertainties were used for each curve. Arrows indicate $\Delta G$ corresponding to the different polarized gluon distributions discussed in the text.