Forward Physics at STAR: Status of analysis on forward and mid rapidity correlation measurements in p+p and d+Au

Abstract

Measurements of the production of high energy pi0 mesons at large pseudorapidity, coincident with charged hadrons at mid-rapidity, for proton+proton and deuteron+gold collisions at sqrt(s)_NN=200 GeV are reported. The p+p cross section for inclusive pi0 production follows expectations from next-to-leading order perturbative QCD. Both the inclusive cross section and the di-hadron azimuthal correlations are consistent with a model where parton showers supplement leading-order pQCD. A suppression of the back-to-back azimuthal correlations was observed in d+Au, qualitatively consistent with the gluon saturation picture.

Figures (3)

• Figure 1: Top view of the STAR detector. The scale is such that displacements transverse to the beam ( x) are two times larger than displacements along the beam ( z).
• Figure 2: ( Left) Inclusive $\pi^0$ production cross section versus leading $\pi^0$ energy ($E_\pi$) at average pseudorapidities ($\langle \eta \rangle$) 3.3 and 3.8. The average transverse momentum ($\langle p_T\rangle$) is correlated with $E_\pi$ since the measurements were at a fixed $\eta$. The inner error bars are statistical, and are smaller than the symbols for most data points. The outer error bars combine these with $E_\pi$-dependent systematic errors added in quadrature. The curves are NLO pQCD calculations evaluated at $\eta=3.3$ and 3.8 using different fragmentation functions as described in the text. ( Right) Coincidence probability as function of azimuthal angle difference between $\pi^0$ at forward rapidity ($\langle \eta \rangle$=4.0) and the leading charged particle at mid rapidity ($|\eta|<0.75$, $p_T>0.5$ GeV), in two different $\pi^0$ energy bins for p+p collisions at $\sqrt{s}$=200 GeV. The left column is for PYTHIA simulations and the right column is data. Fits to the data are described in the text. Error bars on the points represent only the statistical uncertainty.
• Figure 3: Coincidence probability as function of azimuthal angle difference between $\pi^0$ at forward rapidity ($\langle \eta \rangle = 4.0$) and leading charged particle at mid rapidity ($|\eta|<0.75$, $p_T>0.5$GeV). ( Left) HIJING 1.381 simulations for p+p and d+Au collisions, including detector effects. HIJING has substantially smaller $k_T$ compared to PYTHIA 6.222, and it gives smaller $\sigma$ in p+p compared to PYTHIA (Fig. \ref{['fig:crosssec']}). ( Right) STAR preliminary data for p+p and d+Au collisions.