Bulletin of the American Physical Society
2007 Annual Meeting of the Division of Nuclear Physics
Volume 52, Number 10
Wednesday–Saturday, October 10–13, 2007; Newport News, Virginia
Session HB: Mini-Symposium on Dijets and Correlations in HI Collisions I |
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Chair: Jim Thomas, Lawrence Berkeley National Laboratory Room: Newport News Marriott at City Center Grand Salon III |
Saturday, October 13, 2007 9:00AM - 9:36AM |
HB.00001: QGP Collective Effects and High-Energy Jet Propagation Invited Speaker: In this talk I review recent advances in the understanding of plasma response to the passage of high energy jets. I will discuss our understanding based on the assumption of a thermal isotropic plasma highlighting the importance of both collisional and radiative energy loss. I will also discuss the case of a plasma which has a momentum space anisotropy and the effect of such an anisotropy on jet propagation including the possibility of jet deflection by large-amplitude turbulent chromofields leading to near-side longitudinal jet broadening. Finally, I present recently developed numerical methods for solving the coupled particle-field system in real-time allowing for detailed simulation of jet dynamics in a QGP. [Preview Abstract] |
Saturday, October 13, 2007 9:36AM - 9:48AM |
HB.00002: System size dependence of two particle azimuthal correlations in $Cu+Cu$ and $Au+Au$ collisions at $\sqrt{s_{NN}} = 200 GeV$ at RHIC Christine Nattrass Studies of jets at RHIC have led to exciting results such as jet suppression and long-range pseudorapidity correlations, called the ``Ridge.'' Different mechanisms for both phenomena may be distinguished through the study of identified particles in jets. The intermediate $p_{T}$ region, where recombination and coalescence models have been successful, is of particular interest. Studies of $\Lambda, \bar{\Lambda}, K^0_S$, and $\Xi^{\pm}$ production in jets using azimuthal and pseudorapidity correlations in Cu+Cu collisions at $\sqrt{s_{NN}} = 200 GeV$ are presented. The dependencies of the long range pseudorapidity correlations and near side jet-like correlations on particle type, transverse momentum, system size, and centrality are presented and compared to analyses performed in Au+Au collisions at $\sqrt{s_{NN}} = 200 GeV$. These results help distinguish between particle production mechanisms. [Preview Abstract] |
Saturday, October 13, 2007 9:48AM - 10:00AM |
HB.00003: Decomposition of awayside components of dijet correlation in Au+Au collisions at $\sqrt{S_{NN}}=200$ GeV Chin-Hao Chen A hot dense QCD matter is created by heavy ion collision in Relativistic Heavy Ion Collider (RHIC). Since parton fragments from high-$p_{T}$ scattering form back-to-back hadron pairs, studying the dihadron or photon-hadron angular correlation can probe the medium response to energy deposited by the partons. At intermediate transverse momentum ($p_{T}$), modified away-side dijet correlation has been observed. Unlike p+p collisions, where the away side peaks at $\Delta\phi =\pi$, there is a local minimum at $\Delta\phi = \pi$ in Au+Au. The modified shape suggests a medium response to energy deposited by the transiting parton. We present photon-hadron correlations from $\sqrt{s_{NN}}=200$ GeV Au+Au collisions in the PHENIX run4 data set. We separate the jet components which ``punch through'' the medium from the side peak due to the medium response, counting the number of particles observed in each. These are studied as a function of collision centrality and compared with the p+p dijet correlation. [Preview Abstract] |
Saturday, October 13, 2007 10:00AM - 10:12AM |
HB.00004: Two-Particle Jet-Correlations from STAR: Systematics from Charged Hadrons and First Result for Net-Charges Quan Wang Two-particle jet-like correlations of charged hadrons with a high $p_T$ trigger particle are strongly modified at RHIC, lending strong support for jet quenching and partonic energy loss. We present a systematic study of 2-particle jet-correlations in azimuth ($\Delta\phi$) and pseudo-rapidity ($\Delta\eta$) as a function of trigger and associated particle $p_T$, system size, and collision centrality. In central heavy-ion collisions, significant excess of correlated particles are found on the away-side at about 1 radian away from $\Delta\phi=\pi$, and those correlated particles are observed to possess a larger average $\langle p_T \rangle$ than those at $\pi$. In order to investigate the physics mechanisms underlying these observations, azimuthal correlations of net-charges, reflecting mostly net-protons, are analyzed. First result from the analysis will be reported. [Preview Abstract] |
Saturday, October 13, 2007 10:12AM - 10:24AM |
HB.00005: System size dependence of di-hadron correlations yields and fragmentation functions at RHIC Oana Catu Di-hadron correlations have provided one of the first indications that a strongly interacting medium is formed in ultra-relativistic heavy ion collisions at RHIC. In particular, on the away side of a high $p_T$ trigger hadron the associated jet-like hadron yield is strongly suppressed at high $p_T$, which is interpreted as final-state parton energy loss. Taking the method one step further, we investigate the modification of the away-side di-hadron fragmentation functions for high tranverse momentum particles in $Au-Au$ and $Cu-Cu$ collisions at $\sqrt{s_{NN}} = 200 GeV$ as measured in STAR. A comparison with theoretical predictions using NLO pQCD is also presented and allows the determination of the transport coefficient of the medium and therefore of the initial gluon density. The study of two systems with different geometries allows the testing of the path length dependence. We also present the near- and away-side yields as a function of number of participants, which could provide more information on the influence of the geometry on the observed triggered correlations. [Preview Abstract] |
Saturday, October 13, 2007 10:24AM - 10:36AM |
HB.00006: Energy and System-Size Dependence of Long-Range Multiplicity Correlations from the STAR Experiment Terence Tarnowsky The study of correlations among particles produced in different rapidity regions may provide an understanding of particle production mechanisms. Production of particles in the central rapidity region is dominated at all energies by short range correlations. Correlations that extend over a longer range are observed in hadron-hadron interactions only at higher energies. Results for short and long-range multiplicity correlations (forward-backward) are presented for several systems (Au+Au, Cu+Cu, and pp) and energies (e.g. $\sqrt{s_{NN}}$ = 200 and 62.4 GeV). These correlations are measured with increasing values of a gap in pseudorapidity, from no gap at midrapidity to a separation of 1.6 units (+/- 0.8). For the highest energy, central A+A collisions, the correlation strength maintains a constant value across the measurement region. In peripheral collisions, at lower energies, and in pp data, the maximum appears at midrapidity. Comparison to models with short-range (HIJING) and both short and long-range interactions (Parton String Model) do not fully reproduce central Au+Au data. String fusion as implemented in the Parton String Model is one possibility that has been explored to understand the behavior seen in the data. This result may indicate a reduction in number of particle sources for central Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV and the possible formation of high density matter. [Preview Abstract] |
Saturday, October 13, 2007 10:36AM - 10:48AM |
HB.00007: Jet Correlations after an event-selection of two back-to-back, high-pt particles Craig Ogilvie High-momentum partons lose energy as they travel through the dense QGP that is formed at RHIC. The amount of energy-loss depends on both the density of the medium and the path-length that is traveled by the parton. How to best disentangle these two effects is an open question: work proceeds on multiple fronts; studying energy-loss versus reaction plane, centrality, and for different size colliding systems. Part of the difficulty is the strong energy-loss means that partons, which survive to produce high-pt hadrons, come from hard-collisions that occur predominantly near the surface of the dense matter. One possible method to alter this surface-bias is to select events that have two back-to-back high-pt hadrons. On average these partons will have traveled through similar lengths of dense matter and lost similar amounts of energy. This shifts the surface bias more towards the center of this collision. In this talk, I will present data on jet correlations on behalf of PHENIX using such an event selection. For example, I will report on the angular distribution of particles that are associated with one of the high-pt particles when there is another high-pt particle back-to-back in the same event. Data from A+A collisions using this event selection will be compared with data from p+p collisions. [Preview Abstract] |
Saturday, October 13, 2007 10:48AM - 11:00AM |
HB.00008: Jet Energy Loss Observables in PHENIX Justin Frantz An integral part of the RHIC program has been to use jet probes to study the Heavy Ion Medium. Such measurements have progressed from comparisons of plain particle spectra at high p, to two and three particle opening angle correlations, and currently further jet observables are being explored. Recently for example, PHENIX has found an interesting two-component pt- dependence of the two-particle yields. Extending the 2- particle correlations result by requiring a third hard particle in certain phase space regions, may reveal more detailed information about energy loss and possible resulting geometric biases. Yet another example is moments of the momentum ratio distributions of the jet particles which may also be able to distinguish energy loss models. The status of such analyses in PHENIX and also their relation to Direct Photon-Jet correlations also explored by PHENIX will be discussed. [Preview Abstract] |
Saturday, October 13, 2007 11:00AM - 11:12AM |
HB.00009: Gamma-Jet Analysis in Heavy Ion Collisions with the STAR Detector Martin Codrington One of the most intriguing results from RHIC experiments thus far is the observed suppression of hadrons at high transverse momentum, which is attributed to final state medium-induced energy loss of hard scattered partons. To quantify the energy loss and the response of the medium to the deposited energy and momentum, a probe is needed that has negligible interaction with the medium itself, and thereby can provide a calibration of the momentum scale of the underlying process. One such probe is a prompt photon (i.e. produced from the initial hard-scattering process). Studying correlations of a prompt photon with a jet ($\gamma $+Jet) should allow one to study the attenuation and modification of a jet with well-defined energy quantitatively and thus promises to provide a wealth of information about the energy-loss process. There is, however, a large background of photons from the decay of neutral mesons (mainly the $\pi ^{0})$. Ideally, a large fraction of these decay photons are rejected before a correlation study is undertaken. In the STAR experiment, this can be done using the transverse shower profile measured in the Shower Maximum Detector (SMD) of the Barrel Electromagnetic Calorimeter (BEMC). The latest results of this analysis will be presented. [Preview Abstract] |
Saturday, October 13, 2007 11:12AM - 11:24AM |
HB.00010: Direct measurement of fragmentation photons in p+p collisions at $\sqrt{s_{NN}}=200$ GeV with the PHENIX experiment Ali Hanks Direct photon production is an important observable in heavy ion collisions, as photons are penetrating and therefore largely insensitive to final state effects that lead to jet quenching. Perturbative QCD calculations predict a contribution of up to $30\%$ to the direct photon yield from photons produced through parton fragmentation. In heavy ion collisions this contribution can be modified due to additional stimulated photon bremsstrahlung as well as the energy loss of the partons through gluon radiation prior to fragmentation. A measure of photon-bremsstrahlung would provide direct observation of the scattering of jets in the medium. Thus measurements of the fragmentation component to direct photon yields in both p+p and Au+Au collisions will provide both an important test of pQCD predictions and of predictions for the nuclear modification factor. By selecting photons associated with jets on the near side using hadron-photon correlations, fragmentation photons can be measured directly. However, most photons correlated with jets come from $\pi^{0}$'s, $\eta$'s, and other hadronic decays and must be tagged and subtracted from the inclusive correlations. We present studies of this methodology and its application to recent p+p data at PHENIX. [Preview Abstract] |
Saturday, October 13, 2007 11:24AM - 11:36AM |
HB.00011: Analysis of two-particle jet correlations with a scaling formula Michael Tannenbaum At DNP06, a new formula for the distribution of an associated away-side particle with transverse momentum $p_{T_a}$, which is presumed to be a fragment of an away-jet with $\hat{p}_{T_a}$, triggered by a particle with transverse momentum $p_{T_t}$, presumably from a trigger-side jet with $\hat{p}_{T_t}$, was given: \quad ${dP_{p_{T_a}}/dx_E}|_{p_{T_t}}\approx {{\langle{m}\rangle}\over\hat{x}_h} {(n-1)\over {(1+ {x_E /{\hat{x}_h}})^{n}}}$ \quad where $x_E\approx p_{T_a}/p_{T_t}$ is the ratio of the transverse momenta of the particles, $\hat{x}_h=\hat{p}_{T_a}/\hat{p}_{T_t}$ is the ratio of the transverse momenta of the away-side to trigger-side jets, and $\langle m\rangle$ is the mean multiplicity of particles in the away jet. Many analyses of the away-jet $p_{T_a}$ distributions in Au+Au collisions are available; but these tend to describe the effect of the medium with the variable $I_{AA}(x_E)$, the ratio of the $x_E$ distribution in A+A collisions to that in p-p collisions, which typically shows an enhancement at low values of $x_E$ and a suppression at higher values of $x_E$. Such behavior could be explained as a decrease in $\hat{x}_h$ in A+A collisions due to energy loss of the away jet in the medium. Fits of the above formula to the available data will be presented to establish whether: a) the away-jets simply lose energy; b) some of the away-jets lose energy, others punch-through without losing energy; etc. [Preview Abstract] |
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