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M. van Leeuwen
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These proceedings present a brief overview of the main results on jet-modifications in heavy ion collisions at RHIC. In heavy ion collisions, jets are studied using single hadron spectra and di-hadron correlations with a high-pt{} trigger hadrons. At high pt, a suppression of the yields due to parton energy loss is observed. A quantitative confrontation of the data with various theoretical approaches to energy loss in a dense QCD medium is being pursued. First results from $gamma$-jet events, where the photon balances the initial jet energy, are also presented and compared to expectations from models based on di-hadron measurements. At intermediate pt, two striking modifications of the di-hadron correlation structure are found in heavy ion collisions: the presence of a long-range {it ridge} structure in deta{}, and a large broadening of the recoil jet. Both phenomena seem to indicate an interplay between hard and soft physics.
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The longitudinal asymmetry arises in relativistic heavy ion collisions due to fluctuation in the number of participating nucleons. This asymmetry causes a shift in the center of mass rapidity of the participant zone. The rapidity shift as well as the longitudinal asymmetry have been found to be significant at the top LHC energy for collisions of identical nuclei. We study the longitudinal asymmetry and its effect on charged particle rapidity distribution and anisotropic flow parameters at relatively lower RHIC energies using a model calculation. The rapidity shift is found to be more pronounced for peripheral collisions, smaller systems and also for lower beam energies due to longitudinal asymmetry. A detailed study has been done by associating the average rapidity shift to a polynomial relation where the coefficients of this polynomial characterize the effect of the asymmetry. We show that the rapidity shift may affect observables significantly at RHIC energies.
We present results for the measurement of $phi$ meson production via its charged kaon decay channel $phi to K^+K^-$ in Au+Au collisions at $sqrt{s_{_{NN}}}=62.4$, 130, and 200 GeV, and in $p+p$ and $d$+Au collisions at $sqrt{s_{_{NN}}}=200$ GeV from the STAR experiment at the BNL Relativistic Heavy Ion Collider (RHIC). The midrapidity ($|y|<0.5$) $phi$ meson transverse momentum ($p_{T}$) spectra in central Au+Au collisions are found to be well described by a single exponential distribution. On the other hand, the $p_{T}$ spectra from $p+p$, $d$+Au and peripheral Au+Au collisions show power-law tails at intermediate and high $p_{T}$ and are described better by Levy distributions. The constant $phi/K^-$ yield ratio vs beam species, collision centrality and colliding energy is in contradiction with expectations from models having kaon coalescence as the dominant mechanism for $phi$ production at RHIC. The $Omega/phi$ yield ratio as a function of $p_{T}$ is consistent with a model based on the recombination of thermal $s$ quarks up to $p_{T}sim 4$ GeV/$c$, but disagrees at higher transverse momenta. The measured nuclear modification factor, $R_{dAu}$, for the $phi$ meson increases above unity at intermediate $p_{T}$, similar to that for pions and protons, while $R_{AA}$ is suppressed due to the energy loss effect in central Au+Au collisions. Number of constituent quark scaling of both $R_{cp}$ and $v_{2}$ for the $phi$ meson with respect to other hadrons in Au+Au collisions at $sqrt{s_{_{NN}}}$=200 GeV at intermediate $p_{T}$ is observed. These observations support quark coalescence as being the dominant mechanism of hadronization in the intermediate $p_{T}$ region at RHIC.
Current status of dynamical modeling of relativistic heavy ion collisions and hydrodynamic description of the quark gluon plasma is reported. We find the hadronic rescattering effect plays an important role in interpretation of mass splitting pattern in the differential elliptic flow data observed at RHIC. To demonstrate this, we predict the elliptic flow parameter for phi mesons to directly observe the flow just after hadronisation. We also discuss recent applications of outputs from hydrodynamic calculations to J/psi suppression, thermal photon radiation and heavy quark diffusion.
Reconstructed jets in heavy ion collisions are a crucial tool for understanding the quark-gluon plasma. The separation of jets from the underlying event is necessary particularly in central heavy ion reactions in order to quantify medium modifications of the parton shower and the response of the surrounding medium itself. There have been many methods proposed and implemented for studying the underlying event substructure in proton-proton and heavy ion collisions. In this paper, we detail a method for understanding underlying event contributions in Au+Au collisions at $sqrt{s_{NN}}$ = 200 GeV utilizing the HIJING event generator. This method, extended from previous work by the ATLAS collaboration, provides a well-defined association of truth jets from the fragmentation of hard partons with reconstructed jets using the anti-$k_T$ algorithm. Results presented here are based on an analysis of 750M minimum bias HIJING events. We find that there is a substantial range of jet energies and radius parameters where jets are well separated from the background fluctuations (often termed fake jets) that make jet measurements at RHIC a compelling physics program.
Whether quark- and gluon-initiated jets are modified differently by the quark-gluon plasma produced in heavy-ion collisions is a long-standing question that has thus far eluded a definitive experimental answer. A crucial complication for quark-gluon discrimination in both proton-proton and heavy-ion collisions is that all measurements necessarily average over the (unknown) quark-gluon composition of a jet sample. In the heavy-ion context, the simultaneous modification of both the fractions and substructure of quark and gluon jets by the quark-gluon plasma further obscures the interpretation. Here, we demonstrate a fully data-driven method for separating quark and gluon contributions to jet observables using a statistical technique called topic modeling. Assuming that jet distributions are a mixture of underlying quark-like and gluon-like distributions, we show how to extract quark and gluon jet fractions and constituent multiplicity distributions as a function of the jet transverse momentum. This proof-of-concept study is based on proton-proton and heavy-ion collision events from the Monte Carlo event generator Jewel with statistics accessible in Run 4 of the Large Hadron Collider. These results suggest the potential for an experimental determination of quark and gluon jet modifications.
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