Di-hadron Azimuthal Correlations Relative To Reaction Plane In Au+Au Collisions At (~SNN)～(1/2)=200GeV

Quarks and gluons are the basic building blocks of the matter.And Quantum ChromoDynamics (QCD)is believed to be a successful theory to describe the strong force between the color charge carriers- quarks and gluons.According to asymptotic freedom - one of key features of QCD,quarks and gluons are confined in hadrons and no free quarks and gluons are observed.Lattice QCD predicts a phase transition at high temperature or high density from the normal hadron gas state to a state with quarks and gluons which are deconfined from hadrons-the Quark Gluon Plasma (QGP).The Relativistic Heavy Ion Collider(RHIC)located at Brookhaven National Laboratory(BNL)was designed to collide high energy heavy ions to create such a high temperature and high density matter,simulating the evolution of the early universe in the first few seconds after the Big Bang.After several years of measurements accumulation, the matter created RHIC has been proved to be more like a medium most resemble of properties of a perfect liquid of strongly interacting quark gluon plasma.The two pillars for this discovery are the observed strong elliptic flow and jet quenching.To quantify the property of the medium created at RHIC,high transverse momentum(PT) particles,which are presumably from jet fragmentations,are ideal penetrating probes. Usually people use high PT single hadron yields and di-hadron correlations with a high PT trigger particle to study the medium created at RHIC.While suppression of high PT single hadron yields has limited sensitivity to the medium core,di-hadron correlation measurements will provide richer and more valuable information about the properties of the created medium.The disappearance of back-to-back jet from the first high PT di-hadron correlation measurement at RHIC has been recognized as an evidence of the jet quenching discovery. The following analysis by correlating low PT particles associated with the high PT trigger particle shows the broadening in the away side,which is consistent with the jet energy dissipation in the medium.The first di-hadron analysis with respect to the reaction plane shows the away-side suppression happens out-of-plane while it is not significant in-plane.This indicates the possible path length effect in the jet energy loss.Recently, a new phenomenon of the long range△ηcorrelation——so called "Ridge" has been observed in the near side di-hadron correlation,but the origin of ridge phenomenon is still not understood yet[Arm04,Vol06,Maj04,Won07,Hwa05].In this thesis,we present STAR results of di-hadron azimuthal correlations with a high PT trigger particle relative to reaction plane and a lower pT particle associated with the trigger particle in(?)= 200 GeV Au + Au collisions at RHIC.The large data sample(which is about 10 times more than previous analysis):Au + Au minimum bias events and Au + Au central events collected during year 2004 run were used in this analysis.The minimum bias d + Au collision data collected in year 2003 were used for the baseline comparison.The di-hadron azimuthal angle correlation was calculated according to the azimuthal angle of the trigger particle relative to the reaction plane(φ_s=|φ^trig-ψ_EP|).Reaction plane was reconstructed using the standard event plane method.Particles falling in the PT bin of associated particles(p_T^assoc)were eliminated in the event plane reconstruction to avoid the auto-correlation.The v₄ contribution in the flow background,which had not been taken into account,was found to be not negligible in the di-hadron correlation analysis relative to the reaction plane.We re-evaluated the flow background formula to include the v₄ contribution.The flow parameters were taken from the averaged STAR measurements using the standard event plane method and the four particle cumulant method.The difference between two methods is the most significant systematic error contribution in the results.The normalization of the flow background was calculated using an improved method based on the Zero-Yield-At-Minimum(ZYAM).The flow background was calculated in each p_T^assocbin andφ_s bin,and it was subtracted from the raw di-hadron correlation distributions to extract the correlation function we are interested.Both the mid-central 20-60%and the top 5%central Au + Au collisions were investigated,and the minimum bias d + Au collision data were presented for baseline comparison.The correlation functions are also obtained from small and large |△ηregions separately,in attempt to isolate the jet and ridge contributions to the nearside correlation strength and study their behavior inφ_s.The di-hadron correlations are strongly modified in Au + Au collisions with respect to minimum bias d + Au collisions.The modification strongly depend on the trigger particle orientation relative to the event plane and evolve with associated p_T^assoc.The qualitative trend of the correlation function withφ_s appears similar in central and midcentral collisions;quantitatively,theφ_s dependence of the correlation function is stronger in the middle central collisions.The p_T^assocdependences of the correlation function are similar in the two centrality selections.The away-side correlation broadens from in-plane to out-of-plane,and broadens with increasing associated p_T^assocfor mostφ_s slices.For 20-60%Au + Au collisions,the awayside correlation starts as a single peak in the reaction plane,and becomes a stronger and stronger double-peak structure as the trigger particle moves from in-plane to out-of-plane. For top 5%central Au + Au,the away-side correlation already shows a doublepeak structure in the reaction plane,and the double-peak becomes stronger and stronger from in-plane to out-of-plane.The trends of the away-side modification underscore the importance of the path-length that the away-side parton transverses in the medium. The away-side medium path-length in the reaction plane direction in 20-60%Au + Au collisions is quite modest and not enough to generate significant modification to jet correlation,while that in the top 5%collisions is long enough to cause significant jet modification.The strongest modification is found for trigger particles perpendicular to the reaction plane where the away-side medium path-length is the longest,and this path-length appears to be not very different in 20-60%and top 5%Au + Au collisions.The near-side correlation amplitude decreases from in-plane to out-of-plane.The decrease was found to entirely come from the decrease in the long range△ηcorrelation (ridge).The ridge yield decreases to～0 at out-of-plane in the 20-60%centrality,while significant ridge yields persist from in-plane to out-of-plane in top 5%collisions.The jet contribution to the near-side correlation is extracted from the difference of small and large△ηcorrelations,subject to small experimental systematic uncertainties.The jet contribution in the 20-60%centrality appears to somehow increase from in-plane to out-of-plane.The near-side jet parallel to the reaction plane appears to have suffered significant interactions with the medium,which reduce the real jet correlated multiplicity and produce a long range△ηridge containing a large number of hadrons.The near-side jet perpendicular to the reaction plane,on the other hand,appears to suffer minimal medium modification,generating small amount of ridge.The top 5%results are qualitatively similar,but the significant ridge contribution persists over allφ_s,and the variations of the jet and ridge magnitudes inφ_s is significantly smaller,consistent with the more spherical collision geometry.The main results presented in this thesis are with 3＜p_T^trig＜4 GeV/c.The results from trigger particles with higher PT is necessary to check the results since they are more probably coming from jet fragmentation.In the future RHIC run with the TPC DAQ1000 upgrade,we are able to accumulate more than ten times statistics of the data sample in Runâ…£,used in this thesis.It provides us the possibility of investigating the correlations with high PT trigger particles in the future.