2005 Charged Pion

Paper Proposal

Cross section and longitudinal double-spin asymmetry for inclusive charged pion production in polarized proton collisions at sqrt(s) = 200 GeV

Proposed Target Journal

Phys. Rev. Lett.

Principal Authors

A. Kocoloski, L. Ruan, B. Surrow, Y. Xu, Z. Xu

Abstract

We report an extension in transverse momentum to 15 GeV/c for the differential cross section and a first measurement of the longitudinal double-spin asymmetry A_{LL} for inclusive midrapidity charged pion production in polarized proton collisions at sqrt(s) = 200 GeV. The pi-/pi+ ratio shows significant p_{T} dependence from unity at low p_{T} to 0.83±0.01(stat)±0.04(syst) at high p_{T}, an experimental signature of significant valence quark contribution. The next-to-leading order perturbative QCD models incorporating flavor dependence in the fragmentation functions can describe the cross sections and the isospin asymmetry. The charged pion A_{LL} data covering 2 < pT < 13 GeV/c disfavor a large polarized gluon distribution in the proton and provide the first glimpse of a direction of future measurements. The predicted A_{LL} difference in the positively and negatively charged pions can be tested in future RHIC data.

Summary

In summary, we report the differential cross section and longitudinal double-spin asymmetry A_{LL} for inclusive charged pion production at midrapidity in polarized proton collisions at sqrt(s) = 200 GeV. The pion cross sections were determined up to p_{T} = 15 GeV/c and are described by NLO pQCD evaluations over 7 orders of magnitude. The asymmetries A_{LL} cover 2 < p_{T} < 13 GeV/c. They are consistent with NLO pQCD calculations utilizing polarized quark and gluon distributions from inclusive DIS analyses and disfavor large positive values of gluon polarization in the polarized nucleon.

Proposed Figures

Figure 1: “Technical” or “instrumental” figure on the use of a jet patch trigger for this analysis. Final graphics are still to be determined. The left panel shows the minimum-bias and triggered distributions obtained from PYTHIA simulations and used to correct the measured pion spectra. The right panel plots the mean fraction of the jet momentum carried by associated (R=0.4) charged pions in the case of jets that fired the trigger (black filled squares) and jets opposite trigger jets (red open squares). The inset shows the ($\eta$, $\phi$) distribution of charged pions relative to the trigger jet in the data.

Figure 2: Transverse momentum spectra for charged pions at midrapidity ($\left|{y}\right| < 0.5$) (a) and comparison of data to DSS prediction versus $p_{T}$ (b). (c) shows ratio of π-/π+ data compared to DSS and AKK predictions.

Figure 3: The longitudinal double-spin asymmetry $A_{LL}$ in $\vec{p} + \vec{p} \rightarrow \pi^{+/-} + X$ at $\sqrt{s}=200 GeV$ versus pion $p_{T}$. The uncertainties on the data points are statistical. The gray bars indicate the total point-to-point systematic uncertainty, and the curves show predictions based on parameterizations of gluon polarization from global analyses.