Measurements Of High-pT Identified Hadron Spectra In P+p And Au+Au Collisions At RHIC

Many exciting signals from RHIC experiments have indicated the existence of the newmatter created in heavy ion collisions, through jet suppression at high pT etc.. All pQCDmodels predict that energetic partons lose energy when traversing a QGP medium: thejet quenching phenomenon. The effect is experimentally observable through hadrons athigh transverse momentum from jet fragmentation. This offers an excellent tool to probethe QGP properties by measuring the outgoing high-pT hadrons associated with the jetquenching e?ect. Both STAR and PHENIX Collaborations at RHIC have observed asuppression of hadron production at high pT in central heavy ion collisions. The resultwas obtained using the nuclear modification factor (R_AA), which is the hadron yield athigh pT in central Au+Au collisions divided by that in p+p collisions properly scaled bythe underlying number of the nucleon-nucleon binary collisions. Furthermore, the energyloss is expected to depend on the QCD color charge factor: gluons lose more energy thanquarks. The consequence of this mechanism is that the suppression for antiprotons andprotons is expected to be more severe than the pions and kaons. However, we observedsimilar suppression for pion and proton with large uncertainties from STAR, which iscontrast to model expectation. A jet conversion mechanism was proposed, in which theincident jet parton interacts violently with a thermal parton in the QGP matter andconsequently produces an outgoing energetic parton with different flavor (i.e. d,u→s oru,d→g). Based on the jet conversion, R_AA(K) are predicted to be～0.4 at pT > 5.0GeV/c.To further study the evolution of jet chemistry and the jet conversion phenomenon,we need identified hadron spectra in nucleus-nucleus collisions as well as in p+p colli-sions. The identified hadron production in p+p collisions also provides a good test toNext-to-Leading Order (NLO) perturbative-QCD (pQCD) calculations at high pT range.In the framework of models based on pQCD, the inclusive production of single hadron isdescribed by the convolution of parton distribution functions (PDFs), parton interaction cross-sections and fragmentation functions (FFs). However, the flavor-separated fragmen-tation functions of identified hadrons at high Z are not well constrained by the e+e? andother experiments. These measurements of identified hadron at high pT in p+p collisionscan also provide better constraint on the parton fragmentation model and improve theprecisions of their parameters.In this thesis, two new techniques are developed to improve the hadron identificationby re-calibrating the Time Projection Chamber (TPC) measured dE/dx and to enhancethe high-pT hadron yields in the data set using the Barrel Electromagnetic Calorimeter(BEMC) as trigger detector for the charged hadrons. This new method significantlyextend measurements of the identified hadrons cross-section to higher pT at STAR. Thesetechniques are not only applicable to STAR detector, but also provide an useful toolfor experiments at LHC and other facilities. We measured invariant differential cross-sections of chargedπ, K, p (p|-) and neutral kaon up to 15 GeV/c in p + p collisions atsNN^1/2 = 200 GeV. The pT spectra ofπcan be described by NLO pQCD calculationsvery well, such as Albino-Kniehl-Kramer (AKK) and DeFlorian-Sassot-Stratmann (DSS)calculations. However, they fail to describe our proton and kaon spectra at high pT. Basedon the high pT extension of p+p measurements, the nuclear modification factors in Au +Au collisions can also be extended to higher pT. We present R_AA forπ^±, K^±and p(p),compared withρmeasurements in central Au + Au collisions. Our measurements showthat R_AA(π)～R_AA(ρ), which indicates that there is no mass effect on light hadrons fromquark fragmentations. On the contrary, the R_AA of pˉ( p ) and kaons are systematicallyabove that of pions. These features are qualitatively consistent with a jet conversionpicture. In this thesis, we also compare the results to other models, and discuss futuremeasurements with improved precisions and further jet chemistry study at RHIC andLHC.