Jet-hadron correlations in STAR

Advancements in full jet reconstruction have made it possible to use jets as triggers in azimuthal angular correlations to study the modification of hard-scattered partons in the medium created in ultrarelativistic heavy-ion collisions. This increases the range of parton energies accessible in these analyses and improves the signal-to-background ratio compared to dihadron correlations. Results of a systematic study of jet-hadron correlations in central Au-Au collisions at sqrt(s_NN) = 200 GeV are indicative of a broadening and softening of jets which interact with the medium. Furthermore, jet-hadron correlations suggest that the suppression of the associated hadron yield at high-pT is balanced in large part by low-pT enhancement.Comment: 4 pages, 2 figures, proceedings for Quark Matter 201

Low- and Intermediate-pT_T Di-hadron Distributions in Au+Au Collisions at sNN=200\sqrt{s_{NN}}=200 GeV from STAR

We present a study of low- and intermediate-p$_T$ correlated azimuthal angular distributions in Au+Au collisions at $\sqrt{s_{NN}}=200 \rm{GeV}$ from STAR. The near-side associated yields in Au+Au collisions are found to be strongly enhanced, due to contributions from large $\Delta\eta$. The enhancement is reduced for high \pTtrig. We show a strong broadening and enhancement of the away-side yield. The evolution of the away-side shape may be explained as the sum of a broad structure from bulk response and a narrow peak from jet fragmentation.Comment: 4 pages, 3 figure

Novel Bose-Einstein Interference in the Passage of a Fast Particle in a Dense Medium

When an energetic particle collides coherently with many medium particles at high energies, the Bose-Einstein symmetry with respect to the interchange of the exchanged virtual bosons leads to a destructive interference of the Feynman amplitudes in most regions of the phase space but a constructive interference in some other regions of the phase space. As a consequence, the recoiling medium particles have a tendency to come out collectively along the direction of the incident fast particle, each carrying a substantial fraction of the incident longitudinal momentum. Such an interference appearing as collective recoils of scatterers along the incident particle direction may have been observed in angular correlations of hadrons associated with a high-$p_T$ trigger in high-energy AuAu collisions at RHIC.Comment: 10 pages, 2 figures, invited talk presented at the 35th Symposium on Nuclear Physics, Cocoyoc, Mexico, January 3, 2012, to be published in IOP Conference Serie

STAR Results on High Transverse Momentum, Heavy Flavor and Electromagnetic Probes

We summarize here recent results from the STAR collaboration focusing on processes involving large momentum transfers. Measurements of angular correlations of di-hadrons are explored in both the pseudorapidity (eta) and azimuthal (phi) projections. In central Au+Au, an elongated structure is found in the eta projection which persists up to the highest measured pT. After quantifying the particle yield in this structure and subtracting it from the near-side yield, we observe that the remainder exhibits a behavior strikingly similar to that of the near-side yield in d+Au. For heavy flavor production, using electron-hadron correlations in p+p collisions, we obtain an estimate of the b-quark contribution to the non-photonic electrons in the pT region 3-6 GeV/c, and find it consistent with FONLL calculations. Together with the observed suppression of non-photonic electrons in Au+Au, this strongly suggests suppression of b-quark production in Au+Au collisions. We discuss results on the mid-rapidity Upsilon cross-section in p+p collisions. Finally, we present a proof-of-principle measurement of photon-hadron correlations in p+p collisions, paving the way for the tomographic study of the matter produced in central Au+Au via gamma-jet measurements.Comment: 8 pages, 4 figures. Proceedings of "Quark Matter 2006", 19th International Conference on Ultra-Relativistic Nucleus-Nucleus Collision

Reconstructed Jets at RHIC

To precisely measure jets over a large background such as pile up in high luminosity p+p collisions at LHC, a new generation of jet reconstruction algorithms is developed. These algorithms are also applicable to reconstruct jets in the heavy ion environment where large event multiplicities are produced. Energy loss in the medium created in heavy ion collisions are already observed indirectly via inclusive hadron distributions and di-hadron correlations. Jets can be used to study this energy loss in detail with reduced biases. We review the latest results on jet-medium interactions as seen in A+A collisions at RHIC, focusing on the recent progress on jet reconstruction in heavy ion collisions.Comment: Proceedings for the 26th Winter Workshop on Nuclear Dynamic

Bulk properties and flow

In this report, I summarize the experimental results on {\bf bulk properties and flow} presented at Quark Matter 2004. It is organized in four sections: 1) Initial condition and stopping; 2) Particle spectra and freeze-outs; 3) Anisotropic flow; 4) Outlook for future measurements.Comment: 10 pages, 4 figures, "Rapporteur-Conference Highlights", Quark Matter 2004, Oakland, January 11-1

Freeze-out dynamics via charged kaon femtoscopy in √ sNN = 200 GeV central Au + Au collisions

We present measurements of three-dimensional correlation functions of like-sign, low-transverse-momentum kaon pairs from √sNN=200 GeV Au+Au collisions. A Cartesian surface-spherical harmonic decomposition technique was used to extract the kaon source function. The latter was found to have a three-dimensional Gaussian shape and can be adequately reproduced by Therminator event-generator simulations with resonance contributions taken into account. Compared to the pion one, the kaon source function is generally narrower and does not have the long tail along the pair transverse momentum direction. The kaon Gaussian radii display a monotonic decrease with increasing transverse mass mT over the interval of 0.55≤mT≤1.15 GeV/c2. While the kaon radii are adequately described by the mT -scaling in the outward and sideward directions, in the longitudinal direction the lowest mT value exceeds the expectations from a pure hydrodynamical model prediction

Exploring Early Parton Momentum Distribution with the Ridge from the Near-Side Jet

In a central nucleus-nucleus collision at high-energies, medium partons kicked by a near-side jet acquire a momentum along the jet direction and subsequently materialize as the observed ridge particles. They carry direct information on the early parton momentum distribution which can be extracted by using the ridge data for central AuAu collisions at \sqrt{s_{NN}}=200 GeV. The extracted parton momentum distribution has a thermal-like transverse momentum distribution but a non-Gaussian, relatively flat rapidity distribution at mid-rapidity with sharp kinematic boundaries at large rapidities that depend on the transverse momentum.Comment: In Proceedings of 20th International Conference on Ultra-Relativistic Nucleus Nucleus Collisions, Jaipur, India, Feb. 4-10, 200

STAR results on medium properties and response of the medium to energetic partons

We report new STAR results on the consequences of highly energetic partons propagating through the medium formed in heavy ion collisions using correlations as an experimental probe. The recent results providing insights about color factor effects and path length dependence of parton energy loss, system size dependence of di-hadron fragmentation functions, conical emission and ridge formation in heavy ion collisions are presented.Comment: STAR Plenary talk at QM2008. Manuscript for the Proceedings of Quark Matter 2008, Jaipur, Indi

Higher Flow Harmonics in Heavy Ion Collisions from STAR

We report STAR measurements relating to higher flow harmonics including the centrality dependence of two- and four-particle cumulants for harmonics 1 to 6. Two-particle correlation functions vs. \Delta\eta and \Delta\phi are presented for pT and number correlations. We find the power spectra (Fourier Transforms of the correlation functions) for central collisions drop quickly for higher harmonics. The \Delta\eta dependence of v3{2}2 and the pT and centrality dependence of v2 and v3 are studied. Trends are conistent with expectations from models including hot-spots in the initial energy density and an expansion phase. We also present v3 and v2{2}2 - v2{4}2 vs. \surdsNN .Comment: 8 pages. Conference proceedings for Quark Matter 201