Study Of The X Dependence Of Leading Hadron Correlations At STAR

The constituents of the nucleon are discovered to be quarks and gluons. The dynamics of quarks and gluons are described by Quantum Chromodynamics (QCD). The gluons are the force carrier of the strong interaction between partons (quarks and gluons), and the strong force is characterized by color charge. The confined quarks together form protons, neutrons and other hadrons (e.g. pion). The mass of gluons is zero; the sum of quark mass inside a proton only occupies a small fraction of the proton mass. This means the strong interactions propagated by the gluons are predicted to play an important role in forming the mass of a proton. The constituents of a proton can be probed via high energy experiments. For example, Deep Inelastic Scattering (DIS) experiments scatter a lepton or neutrino off a proton (or nucleus) to probe partons with longitudinal momentum fraction xBJ at resolution scale Q2. The hard scattering processes can be factorized into convolutions between parton distribution function, parton scattering and parton fragmentation functions.The quark distribution function is well determined, while the gluon distribution function derived from the structure function F2(xBJ,Q2) increases rapidity as xBJ decreases at fixed Q2. As xBJ decreases, the proton gluon density increases rapidly as a gluou splits into two gluons or a quark can emit a gluon (gluon emission). The gluon distribution can not increase indefinitely due to the unitarity of the scattering amplitude. When the gluon splitting processes balances the gluon recombination processes, saturation is expected. The nuclear parton distribution is little known at low xBJ region as the current fixed nucleus target DIS experiment only provides constraints to the nuclei gluon distribution function at xBJ>0.02. The forward particle production at RHIC can probe low xBJ gluons. Forward inclusive neu-tral pion production measured at STAR experiment is found to be suppressed in d+Au collisions compared to p+p collisions, which is consistent with a Color Glass Condensate (CGC) description. However, inclusive production is a measure of the integral on a broad range of xBJ value. To select a certain low xBJ region, di-hadron azimuthal correlations with the leading particle triggered in the forward rapidity are further studied in d+Au collisions to probe the gluon distribution function of Au nuclei.The STAR experiment at RHIC has a nearly continuous electromagnetic system spaning pseudo-rapidity-1<η<4with full azimuthal angle coverage. The anal-ysis in this thesis focuses on the azimuthal correlations between a leading neutral pion triggered in the Forward Meson Spectrometer (FMS,2.5<η<4.0) and an associated neutral pion or jet-like cluster measured in the Endcap ElectorMagnetic Calorimeter (EEMC,1.083<η<2.0) during RHIC run8p+p collisions and d+Au collisions at (?)=200GeV. The correlation studies in p+p collisions are taken as reference for d+Au results. The low x gluons in the dense gold nuclei are scattered by the deuteron nuclear probe, and their fragments can be measured by the FMS and EEMC detectors. The FMS-EEMC correlations provide sensitivity for the gold nuclei gluon distribution function within0.003<xBJ <0.02region.A threshold bounded cluster finder is developed to search the photon signal in the EEMC. We first looked at the FMS π0-EEMC π0azimuthal correlations [4]. The statistics of FMS π0-EEMC π0are low. The direction of the initial partons does not rely on the type of final state particles. For this matter, jets are considered as more direct probes than inclusive hadrons. We use the EEMC to reconstruct jet-like clusters based on cone radius algorithm. Details of excluding detector accep-tance effects, suppressing the underlying event contributions to the jet-like cluster are studied. The transverse momentum and collision centrality dependence of the azimuthal correlations are also studied. To understand the pedestal underneath the correlation peak, a p+Au collision is approximated by requiring a neutron to be observed in the deuteron beam direction in d+Au collisions. The comparison between the FMS-EEMC correlation in p+Au collisions and d+Au collisions will be discussed as well.After a series of systematic check, the back-to-back azimuthal correlations of the FMS π0-EEMC jet-like clusters are found to be broader in d+Au collisions than in that p+p collisions. The width differences between p+p and d+Au colli-sions are not dependent on the underlying event contributions to the jet-like clus-ters. The FMS-EEMC azimuthal correlations probe the intermediate x region for nuclei gluon distribution function the forward+mid-rapidity correlations and the forward+forward correlations. The forward di-hadron correlation studies at STAR prefers a smooth transition from dilute parton gas to CGC state.