Event-plane Dependent Away-side Jet-like Correlation Shape In Relativistic Heavy-ion Collisions

A strongly coupled quark-gluon plasma(QGP)is be.lieved to be created in rel-ativistic heavy-ion collisions.The interaction between a parton in projectile nucleon and a parton in the target nucleon could occasionally have a large momentum-transfer,which is called the hard scattering.The large longitudinal energy is trans-ferred to the transverse plane.The partons produced under this process thus gain large transverse momenta(pT).These partons interact with the QGP medium and lose their energy.Finally these partons hadronize into high pT hadrons,which called jets.Since je.ts are.produced from the large momentum transfer in the inelastic scat-tering between partons.Jets production can be described by the perturbative QCD.On the other hand,Jets is produced at the early stage of collisions,they have enough time to interact with the medium.Thus people believe that jets is a good probe to study the QGP.Experimentally,one can analyze the high-pT single hadron yield and two-particle jet correlation with respect to high-pT hadrons to study the interaction between jets and me.dium.Be.cause high-pT particles are very likely from the par-tons that emit from the surface region,the suppression of high-pT single hadron yield has limited sensitivity to the core of medium.But this bias provides recoil partons a maximal path-length to experience the QGP medium.So two-particle away-side correlation measurement can provide more valuable information about the properties of medium.In the analysis of two-particle correlation,we first require a high-pT particle(trigger particle),which is likely from a jet.An associated particle is correlated with the trigger particle in a collision and it is usually with a lower pT which may be from the same jet or recoil jet.We need to remove the underlying background in the analysis of two-particle correlations.The second-order harmonic anisotropic flow,so called elliptic flow,which can be removed via the ZYAM(zero yield at minimum)method.The ridge and the away-side double peak correlation maybe mainly caused by higher-oder hydrodynamic flow,particularly v3 that have been overlooked until recently.The subtracting of v2 involves determination of amplitude and shape.The v2 parameters are from the separate flow measurements.The elliptic flow background has been normalized to the data around ??= 1,assuming that the pure signal is zero at minimum.Those measurements are usually coming with large uncertainties.In this thesis,we introduce a data-driven method that we developed for sub-tracting the anisotropic flow background in the analysis of two-particle correlation.The main advantage of this method is no assumptions on amplitudes and shapes of flow background,i.e.,no extra uncertainties induced.We first select events with a large recoil pT(Px)to a high-pT trigger particle to enhance the away-side jet population for a specific ? region.We select a small fraction(10%)of total event with the highest{Px|.We define two region in ? symmetric about the mid-rapidity,-0.5<?<0 and 0<?<1,one called "close region" that is close to the Px region(the ? region where the Px is calculated)and the other called "far region"that is far from the Px region.The anisotropic flow contributes are nearly equal to this two regions because they are symmetric about the mid-rapidity.They should be canceled in the difference of the correlations in close region and far region.The away-side jets,on the other hand,have more significant contribution to close-region that to far-region due to the different ? distances.The difference of close-and far-region two-particle correlation contains only the contribution of away-side jet-like correlations.We use two models to demonstrate the validity of the data-driven method.One is a toy model simulation where only elliptic flow is included but no jets.The other is the PYTHIA model,where jets are present but no anisotropic flows.Our results indicate that our data-driven method can subtract the flow background effectively and retain the jet-like correlation shape on the away side.We employ the data-driven method to analyze the event-plane dependent away-side two-particle correlation shape in 20-60%Au+Au collisions at(?)= 200 GeV from RHIC STAR.The event plane is reconstructed from beam-beam counters(BBC)which are displaced by seve.ral units in ? from the mid-rapidity region.The large ? gap can effectively eliminate the auto-correlation between trigger particles and event plane.We correct for the relatively large resolution from the BBC event plane determination via an unfolding procedure.It is found that the width of un-folded away-side jet-like correlation width increases with longer path-length.Furthermore,we investigate the survival rate of an initial momentum anisotropy(v2ini),not spatial anisotropy,to the final state in a multi-phase transport(AMPT)model in Au+Au collisions at(?)=200 GeV.It is found that both the final-state parton and charged hadron v2 show a linear dependence versus v2ini{PP} with respect to the participant plane(PP).It is found that the slope of this linear dependence(referred to as the survive rate)increases with pT,reaching?100%at pT?2.5 GeV/c for both parton and charged hadron.The survival rate decreases with collision cen-trality and energy,indicating decreasing survival rate with increasing interactions.It is further found that a v2ini{Rnd} with respect to a random direction does not survive in v2{PP} but in the two-particle cumulant v2{2}.The dependence of v2{2}on v2ini{Rnd} is quadratic rather than linear.