Centrality Dependence of Two-Particle Correlations in Heavy Ion Collisions

Data from the PHOBOS detector have been used to study two-particle correlations over a broad range of pseudorapidity. A simple cluster model parameterization has been applied to inclusive two-particle correlations over a range of centrality for both Cu+Cu and Au+Au collisions at sqrt(s_NN)=200GeV. Analysis of the data for Au+Au has recently been extended to more peripheral collisions showing that the previously-observed rise in cluster size with decreasing system size eventually reaches a maximum value. Model studies have been used to quantify the significant effect of limited detector acceptance on the extracted cluster parameters. In the case of Au+Au, correlations between a trigger particle with pT>2.5GeV and inclusive associated particles have also been studied. These reveal the presence of a `ridge' at small relative azimuthal angle which extends with roughly constant amplitude out to the largest relative pseudorapidity studied. The large phase-space coverage of the PHOBOS detector has enabled a quantitative understanding of the so-called `ZYAM' parameter used in the subtraction of the contribution of elliptic flow to these triggered correlations.Comment: 4 pages, 4 figures - To appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennessee (version 2: No changes from version 1 other than removing line numbers, version 3: Added full author list and reformatted slightly to keep same number of pages

Initial state nuclear effects for jet production measured in sNN=200GeV\sqrt{s_\mathrm{NN}} = 200 \mathrm{GeV} d+Au collisions by STAR

Full jet reconstruction in heavy-ion collisions is a promising tool for quantitative study of properties of the dense medium produced at RHIC. Measurements of d+Au collisions are important to disentangle initial state nuclear effects from medium-induced $k_\mathrm{T}$ broadening and jet quenching. We report measurements of mid-rapidity ($|\eta_{jet}| < 0.4|$) di-jet correlations in d+Au using high-statistics run 8 RHIC data at $\sqrt{s_\mathrm{NN}} = 200 \mathrm{GeV}$.Comment: 2 pages, 2 figures - To appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennesse

Measurements of jet structure and fragmentation from full jet reconstruction in heavy ion collisions at RHIC

Measurements of inclusive hadron suppression and di-hadron azimuthal correlations have provided important insights into jet quenching in hot QCD matter. However, they do not provide access to the energy of the hard scattering and are limited in their sensitivity since they can be affected by biases toward hard fragmentation and small energy loss. Full jet reconstruction in heavy-ion collisions enables a complete study of the modification of jet structure due to energy loss, but is challenging due to the high-multiplicity environment. We present measurements of fully reconstructed di-jets at mid-rapidity in 200 GeV p+p and central Au+Au collisions performed with the STAR detector. We compare fragmentation functions measured in 200 GeV p+p and central Au+Au collisions and assess the systematic uncertainties of their ratio.Comment: Proceedings of Quark Matter 09 Conferenc

Jet Studies in STAR via Di-jet Triggered (2+1) Multi-hadron Correlations

We explore jet-medium interactions via the recently developed multi-hadron correlation or "2+1" technique. We restrict the di-jet kinematics by selecting a pair of approximately back-to-back high pT hadron triggers and study associated particles. Here we present our study of di-jet systematics comparing the measurements of associate yields and spectra in 200 GeV Au+Au and d+Au collisions in two different scenarios. We present Sum(pT) as an estimate for energy loss. First findings indicate little to no energy loss in the symmetric "2+1" scenario whereas model predictions are about 2 GeV.Comment: 4 pages, 3 figures - To appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennesse

Is the ridge formed by aligned jet propagation and medium flow?

Motivated by the recent observation that the ridge decreases with trigger particle angle ($\phi_s$) relative to the event plane, it is theorized that the ridge is formed by interplay between jet propagation and medium flow. Such interplay may produce asymmetry in the ridge azimuthal correlation at a fixed $\phi_s$. We present an analysis of this asymmetry from STAR data. We found an asymmetric ridge with maximum asymmetry at $\phi_s\approx45^{\circ}$ concurrent with a symmetric jet at all $\phi_s$.Comment: 2 pages, 2 figures, Quark Matter 200

Identifying Heavy-Flavor Jets Using Vectors of Locally Aggregated Descriptors

Jets of collimated particles serve a multitude of purposes in high energy collisions. Recently, studies of jet interaction with the quark-gluon plasma (QGP) created in high energy heavy ion collisions are of growing interest, particularly towards understanding partonic energy loss in the QGP medium and its related modifications of the jet shower and fragmentation. Since the QGP is a colored medium, the extent of jet quenching and consequently, the transport properties of the medium are expected to be sensitive to fundamental properties of the jets such as the flavor of the parton that initiates the jet. Identifying the jet flavor enables an extraction of the mass dependence in jet-QGP interactions. We present a novel approach to tagging heavy-flavor jets at collider experiments utilizing the information contained within jet constituents via the \texttt{JetVLAD} model architecture. We show the performance of this model in proton-proton collisions at center of mass energy $\sqrt{s} = 200$ GeV as characterized by common metrics and showcase its ability to extract high purity heavy-flavor jet sample at various jet momenta and realistic production cross-sections including a brief discussion on the impact of out-of-time pile-up. Such studies open new opportunities for future high purity heavy-flavor measurements at jet energies accessible at current and future collider experiments.Comment: 18 pages, 6 figures and 3 tables. Accepted by JINS

Anisotropic flow background to jet-correlation relative to reaction plane: A revisit of the mathematical framework

We derive the flow-background formula for jet-correlation analysis with high pT trigger particles in any azimuth window relative to reaction plane, extending the mathematic framework of previous study in [J. Bielcikova, et al., Phys. Rev. C 69 (2004) 021901(R)].Comment: 4 pages, 1 figure, published versio

Intra-jet correlations of high-ptp_t hadrons from STAR

Systematic measurements of pseudorapidity ($\Delta\eta$) and azimuthal ($\Delta\phi$) correlations between high-$p_t$ charged hadrons in $\sqrt{s_{NN}}$=200 GeV Au+Au collisions are presented. An enhancement of correlated yield at large $\Delta\eta$ on the near-side is observed. This effect persists up to trigger $p_{t}^{trig}$ $\sim$ 9 GeV/c, indicating that it is associated with jet production. More detailed analysis suggests distinct short-range and long-range components in the correlation.Comment: 4 pages, 5 figures, presented at the 19th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions, "Quark Matter 2006", Shanghai, China, November 14-20, 200