The PHENIX experiment discovered a large excess of low-$p_{T}$ direct photons in Au+Au collisions at 200 GeV compared to reference p+p collisions, which has been attributed to thermal radiation from the medium produced in the collisions. At the same time the excess photons show a large azimuthal anisotropy, expressed as Fourier coefficients $v_{2}$ and $v_{3}$. These surprising results have not yet been fully described by theoretical models. We will present the results obtained from real photons in the electromagnetic calorimeter and photons converted on the outer shell of the Hadron Blind Detector. PHENIX has also developed a new technique to identify conversion photons without assuming the radius where the conversion happened. This method greatly increases the available statistics and reduces systematic uncertainties.
Two particle azimuthal correlation functions are presented for charged hadrons produced in Au-Au collisions at RHIC sqrt(s_NN)=200 GeV.The correlation functions indicate sizeable asymmetries and anisotropies. The trend of the asymmetries is compatible with the presence of emission patterns associated with mini-jets. The magnitude and the trend of the differential anisotropies v_2(p_T) and v_2(N_Part), provide important model constraints.
The transverse momentum dependence of the azimuthal anisotropy parameter v_{2}, the second harmonic of the azimuthal distribution, for electrons at mid-rapidity (|eta|<0.35) has been measured with the PHENIX detector in Au+Au collisions at sqrt(s_NN) = 200 GeV with respect to the reaction plane defined at high rapidities (|eta|=3-4). From the result we have calculated non-photonic electron v_{2}, which is expected to reflect the heavy-flavor azimuthal anisotropy, by subtracting v_{2} of electrons from other sources such as photon conversions, Dalitz decay etc.
As in previous analyses at sqrt(s_NN) 200 GeV, correlations in azimuthal angles between inclusive charge particles at intermediate transverse momentum (p_T = 1.0-4.0) GeV/c are studied at sqrt(s_NN) 62.4 GeV. The jet correlations reveal similar modification as in 200 GeV. Specifically large modification, including the "volcano" or "cone" structure, persists in the awayside correlation.
The PHENIX experiment has shown that J/psis are suppressed in central Au+Au collisions at a center of mass energy per nucleon-nucleon collision sqrt s(NN) = 200 GeV, and that the suppression is larger at forward than at mid-rapidity. Part of this difference may be explained by cold nuclear matter effects but the most central collisions suggest that regeneration mechanisms could be at play. In 2007, PHENIX collected almost four times more Au+Au collisions at this energy than used for previous published results. Moreover, the addition of a new reaction plane detector allows a much better analysi…
High-$p_T$ hadron-hadron correlations have been measured with the PHENIX experiment in $\Cu$ and $\pp$ collisions at $\sqrt{s_{NN}}=200$ GeV. A comparison of the jet widths and yields between the two colliding systems allows us to study the medium effect on jets. We also present a first measurement of direct photon-hadron correlations in $\Au$ and $\pp$ collisions. We find that the near-side yields are consistent with zero in both systems. By comparing the jet yields on the away side, we observe a suggestion of the expected suppression of hadrons associated with photons in $\Au$ collisions.
Recent PHENIX measurements of the elliptic (v2) and hexadecapole (v4) Fourier flow coefficients for charged hadrons as a function of transverse momentum (pT), collision centrality and particle species are presented and compared with results from the PHOBOS and STAR collaborations respectively. The status of extensions to future PHENIX measurements at lower beam energies is also discussed.
The azimuthal anisotropy of high $p_{T}$ particle production in heavy ion collisions is a sensitive probe of the jet quenching mechanism. Recent PHENIX measurements for Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV show a $π^{0}$ $v_{2}$ signal that exceeds the pQCD energy loss calculations up to $p_{T} \sim 10$ GeV/c, challenging the traditional perturbative picture of the energy loss process. Here, we present an update and details of that measurement, as well as new high $p_T$ measurements at $\sqrt{s_{NN}}=62$ and 39 GeV. These measurements not only provide an important constraint for underst…
We present azimuthal anisotropy measurements in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$ = 62.4 and 200 GeV. Comparison between reaction plane and cumulant $v_2$ measurements in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV show that non-flow contributions, originating mainly from jets, influence the extracted $v_2$ for $p_T$ $\gtrsim$ 3.5 GeV/c. Number of constituent quark (NCQ) scaling of $v_2$, when studied as a function of transverse kinetic energy $KE_T$, is seen to hold for Au+Au collisions at $\sqrt{s_{NN}}$ = 62.4 and 200 GeV and for Cu+Cu collisions at $\sqrt{s_{NN}}$ = 200 GeV f…
An improved measurement of $v_2$ for $π^0$ in a broad range of $p_T$ and centrality is presented. By combining $v_2$ with the $R_{AA}$, we provide new insights on jet-medium interactions. We show that current pQCD energy loss models cannot describe the suppression of the $π^0$ as a function of the angle with respect to the reaction plane. Our result could help to resolve the factor of 4 differences in the predicted transport coefficients among these models. Alternatively, it may suggest that non-perturbative effects associated with the strongly coupled QGP are important, and new theoretical de…
Recent surveys of multiplicity fluctuations, transverse momentum fluctuations, and two-particle azimuthal correlations are presented for several collision systems as a function of centrality and transverse momentum. Both multiplicity and transverse momentum fluctuations exhibit a power law scaling as a function of the number of participants that is independent of the collision system. Although these observations are consistent with critical behavior, the critical exponent $η$ measured using azimuthal correlations is seen to be independent of centrality and collision system. Also observed in th…
Measurements from the RHIC experiments show a strong suppression in the yield of high-p$_\mathrm{T}$ single hadrons and a clear reduction in strength of the di-jet signal in two-hadron azimuthal-angle correlation functions in central Au+Au collisions at RHIC. These measurements should provide direct information on the properties of the medium within which hard-scattered partons propagate. The PHENIX preliminary results on single high-p$_\mathrm{T}$ hadron production and on particle azimuthal correlations are shown. The properties of the medium created in relativistic heavy ion collisions are d…
We present here v$_2$ of identified and inclusive charged particles measured in the PHENIX central arm detector ($|η| < 0.35$) with respect to the reaction plane defined at $|η| = 3 \sim 4$ in 200 GeV Au+Au collisions. We find that v$_2$ increases from central to mid-central collisions reaching a maximum at about 50% of the geometric cross section and then decreases again for more peripheral collisions. As a function of transverse momentum in minimum-bias collisions, the v$_2$ parameter increases linearly with p$_{\rm T}$ up to p$_{\rm T}$ $\simeq$ 2 GeV/c and then saturates for inclusive char…
PHENIX has measured the transverse momentum ($p_{\rm T}$) spectra and two particle angular correlations for high $p_{\rm T}$ particles in $d$+Au collisions at $\sqrt{s_{_{NN}}}$=200\,GeV using the RHIC Year-2008 run data. The azimuthal angle correlations for two particles with a large rapidity gap exhibit a ridge like structure. Using the $π^0$ reconstructed in the EMCal, we have successfully extended the $p_{\rm T}$ reach of the correlation up to 8\,GeV/$c$. We find that the azimuthal anisotropy of hadrons found at low $p_{\rm T}$ persists up to 6\,GeV/$c$ with a significant centrality and $p…
We analyze the relative azimuthal ($Δϕ$) distribution of same-charge and opposite-charge particle pairs. We then remove elliptic flow background using the ZYAM method. Comparisons between near angle $Δϕ$ peak widths are presented for various centralities.
We present measurements of 2- and 4-particle correlations in d+Au collisions at four different center-of-mass energies: 200, 62.4, 39, and 19.6 GeV. The data were collected in 2016 by the PHENIX experiment at RHIC. The second Fourier coefficient $v_2$ of the particle azimuthal distributions is measured using the Q-cumulant method as a function of event multiplicity. The results give a strong indication of collective behavior down to the lowest energy.
The paper presents recent PHENIX results for the nuclear modification factor R_AA in Au + Au Collisions at sqrt(s_NN) = 200 GeV for the electrons originating from semi-leptonic decays of open heavy flavor carrying particles. We also report the results of the azimuthal asymmetry (v_2) measurement for those electrons, which is directly related to the heavy quark elliptic flow.
Two-particle azimuthal correlations have been shown to be a powerful probe for extracting novel features of the interaction between hard scattered partons and the medium produced in Au+Au collisions at RHIC. At intermediate $p_T$, 2-5GeV/c, jets have been shown to be significantly modified in both particle composition and angular distribution compared to p+pcollisions. We present recent PHENIX results from Au+Au collisions for a variety of $p_T$ and particle combinations.
The azimuthal dependence of 3D HBT radii relative to the event plane gives us information about the source shape at freeze-out. It also provides information on the system's evolution by comparing it to the initial source shape. In recent studies, higher harmonic event planes and flow have been measured at RHIC and the LHC, which result primarily from spatial fluctuations of the initial density across the collision area. If the shape caused by initial fluctuations still exists at freeze-out, the HBT measurement relative to higher order event plane may show these features. We present recent resu…
Two particle azimuthal correlation functions are presented for charged hadrons produced in Au+Au collisions at RHIC ($\sqrt{s_{_{NN}}}=130$ GeV). The measurements allow for the determination of elliptic flow without event-by-event estimation of the reaction plane. The measured correlation functions indicate elliptic flow values ($v_2$) which show significant sensitivity to both the collision centrality and the transverse momenta of emitted hadrons.
The PHENIX Run 4 Au+Au dataset provides a powerful opportunity for exploring the angular anisotropy of identified particle yields at high pT. Complementing traditional v2 measurements, we present pi0 yields as a function of angle with reaction plane, up to pT 10 GeV/c. The centrality dependence of the angular anisotropy allows us to probe the density and path-length dependence of the energy loss of hard-scattered partons. We will discuss various mechanisms for particle production in this high pT region.
Modifications of jet properties resulting from the coupling of jets to the strongly interacting matter produced in RHIC collisions are of great current interest. In recent work, the PHENIX collaboration has applied a novel technique to the analysis of two particle azimuthal correlations which extinguishes the harmonic part of the underlying event revealing the true jet shape. Recent extensions of the method to three particle correlations allow for a more revealing study of jet topologies in Au+Au collisions at ($\sqrt{s_{_{\rm NN}}}$=200 GeV).
Azimuthal two particle correlations at intermediate $p_{T}$ with one of the particles identified have been measured at PHENIX. Trigger ($2.5 < p_{T} < 4.0 GeV/c$) baryons and mesons show little significant difference in the number of associated particles ($1.7 < p_{T} < 2.5 GeV/c$) independent of centrality. For inclusive hadron triggers with $2.5 < p_{T} < 4.0 GeV/c$, associated fragmentation particles with $1.0 < p_{T} < 2.5 GeV/c$ show a higher baryon to meson ratio on the away side.
It has been established that "hard probes", observables involving high-momentum transfer, provide useful tools for studying the hot, dense medium created in nucleus-nucleus collisions at RHIC. The nuclear modification factor, azimuthal correlations, direct photon production, as well as the dependence of the nuclear modificaton factor on centrality and angle with respect to the reaction plane are critical for understanding the early dynamics of such heavy-ion collisions. We will review recent results from PHENIX for particle production at high-pT and discuss their implications.
The measurement of single leptons from the semi-leptonic decay of heavy-flavor hadrons has long been a means for studying heavy-quark production. PHENIX has measured single muons in pp collisions at forward rapidity and single electrons in both pp and AuAu collisions at mid-rapidity at sqrt(s_NN)=200 GeV. The most recent PHENIX single lepton results are presented in the context of state-of-the-art pQCD calculations. An updated azimuthal anisotropy, v2(pT), measurement for heavy-flavor single electrons in AuAu collisions is also presented.
The new forward Muon Piston Calorimeters allow PHENIX to explore low-$x$ parton distributions in d+Au collisions with hopes of observing gluon saturation. We present a two-particle azimuthal $Δϕ$ correlation measurement made between a mid-rapidity particle ($|η_1| < 0.35$) and a forward $π^0$ ($3.1 < η_2 < 3.9$) wherein we compare correlation widths in d+Au to p+p and compute $I_{dA}$.
Bose-Einstein momentum correlation functions of identical bosons reveal the shape and size of the (soft) particle emitting source of the given particle. The widths of these correlation functions are called HBT radii, named after Brown and Twiss who studied the angular diameter of stars via intensity correlations in their radio telescopes. Today, high energy physics experiments measure the HBT radii as a function of many parameters: particle type, transverse momentum, azimuthal angle, collision energy, collision geometry. In this paper we present results from the RHIC PHENIX experiment. These i…
Two-particle azimuthal correlations have allowed detailed study of the modification to di-jets in the hot, dense medium created in RHIC collisions. Light can be shed by such correlations on many novel effects discovered at RHIC such as the enhanced p/pi ratio and conical structure of events with energy loss. Using pi0 triggers to reduce the effect of recombination we utilize high-pT correlations to explore the away-side structure.
Measurements of two particle azimuthal correlations in relativistic heavy ion collisions are useful tools to dissect the interplay between hard-scattered partons and hot dense medium. Correlations with trigger particle selection relative to second order event plane are sensitive to the path-length dependence of parton energy loss and to the influence of the medium on jet for high and intermediate transverse momenta pairs, respectively. To study the parton-medium coupling, it is also crucial to obtain correlations with rejection of contributions from higher harmonic flow. We present current res…
PHENIX has used the versatility of RHIC to map out low $p_{T}$ direct photon production as function of collision system size and beam energy. For systems with a size corresponding to a $dN_{ch}/dη$ larger than 20-30, we observe a large yield of direct photons, a large azimuthal anisotropy with respect to the reaction plane, and a characteristic centrality dependence of $dN_γ/dy \propto (dN_{ch}/dη)^α$, with $α\sim$ 1.2. \\ In this proceeding, we will present new results from Au+Au collisions at $\sqrt{s_{NN}}$ = 39, 62.4 and 200 GeV. After subtracting the prompt photon component, the inverse s…
Measurements of two particle azimuthal correlations in relativistic heavy ion collisions provide information of the possible interplay between hard-scattered partons and the hot-dense medium. Toward an understanding of parton-medium coupling, it is indispensable to obtain correlations where contributions from higher harmonic flow($v_n$) are rejected. It is also important to produce correlation measurements where the trigger particle is selected relative to second and third order event planes. This enables us to explore path-length dependence of parton energy loss and the influence of the mediu…
The origin of the azimuthal anisotropy in particle yields at high pT (pT > 5 GeV/c) in RHIC collisions remains an intriguing puzzle. Traditional flow and parton energy loss models have failed to completely explain the large v2 observed at high pT. Measurement of this parameter at high pT will help to gain an understanding of the interplay between flow, recombination and energy loss, and the role they play in the transition from soft to hard physics. Neutral mesons measured in the PHENIX experiment provide an ideal observable for such studies. We present recent measurements of \piz yields with …
PHENIX has measured many different two-particle azimuthal correlations in several different colliding systems, beam energies, $p_{T}$ windows, etc. The initial striking results from the first full energy Au+Au run have been confirmed, i.e. the observation of away-side suppression at intermediate $p_{T}$ and the shape modification at low $p_{T}$. With the new high-statistics data these results have been extended to further explore the physics resulting in these modified correlations. In this contribution we present a wide variety of correlation results at high-$p_{T}$ ($>$ 5 GeV/c) and at low $…
A method is suggested to deduce the anisotropy in neutral pions by measuring the azimuthal anisotropy of photons in ultra-relativistic nuclear collisions. The ratio of the estimated anisotropy in photons to the anisotropy in neutral pions is seen to scale with a parameter which depends on photon multiplicity and anisotropy. This parameter can be determined from experimental data.
Direct photons are a powerful tool to study the hot and dense matter created in heavy-ion collisions at RHIC, since they are created in the different stages of the collision. Since they do not interact via the strong force, they can travel through the hot and dense matter mostly unaffected. The PHENIX experiment has measured direct photons using different methods, over a broad range of transverse momentum (\pt), in different collision systems, and at different energies. These measurements help interpreting the measurement of hadrons as well as understanding the temperature of the created quark…
At the Relativistic Heavy Ion Collider (RHIC), key insights into the bulk properties of the hot and dense partonic matter arise from the study of azimuthal anisotropy ($v_2$) of the produced particles. These insights include indicating the matter undergoes rapid thermalization and behaves hydrodynamically at low $p_T$. Recently a low energy scan ($\le \sqrt{s_{NN}}$ = 62.4 GeV) began at RHIC to search for the QGP critical point, where a change in $v_2$ from higher energies could play a key role in its identification. Additionally, higher order flow harmonics have recently been shown to provide…
In this paper we show azimuthal particle correlations in three different small-system collisions with different intrinsic initial geometries. The simultaneous constraints of $v_2$ and $v_3$ in $p/d/^3$He$+$Au collisions definitively demonstrate that the $v_n$'s are correlated to the initial geometry. In addition, we find that hydrodynamical models which include QGP formation describe simultaneouly the elliptic and triangular flow data in a statistically acceptable manner in all three systems.
Directed and elliptic event anisotropy parameters measured in the experiments at relativistic heavy-ion collider are presented. The possible origin of the measured elliptic anisotropy parameter $v_2$ and its sensitivity to the early phase of the high-energy heavy-ion collisions are discussed.
The transverse momentum ($p_{\rm T}$) and centrality dependence of the azimuthal anisotropy ($v_{2}$) are measured for charged hadrons in different energies and collision species by the PHENIX experiment at RHIC. We find that $v_{2}$ divided by the participant eccentricity of the initial geometry proportionally increases with the number of participants to the 1/3 power except at small $N_{\rm part}$ in Cu+Cu at $\sqrt{s_{\rm NN}}$ = 62.4 GeV. Taking the eccentricity and quark number scaling into account, there is a universal scaling for $v_{2}$ with different energies and collision sizes. The …
In this analysis we measure the azimuthal angle($ϕ$) dependence of hadrons with respect to the reaction plane ($Ψ_{RP}$) in events that were triggered by a high-$p_T$ $π^0$ of 5-10 GeV. Fitting the distribution of hadrons with a function of the type $ξ(1+2v_2\cos{(2(ϕ-Ψ_{RP})}))$, we observe that there is no significant statistical difference between $v_2$ inclusive hadrons and those hadrons from hard scattering events. We also compare the near-side jet widths using PYTHIA simulations with $h^{\pm}$-$h^{\pm}$ correlations in Au+Au at $\sqrt{s_{NN}}=200$ GeV.
The measurement of J/{psi} azimuthal anisotropy is presented as a function of transverse momentum for different centralities in Au+Au collisions at {sqrt{s_{NN}}} = 200 GeV. The measured J/{psi} elliptic flow is consistent with zero within errors for transverse momentum between 2 and 10 GeV/c. Our measurement suggests that J/{psi} with relatively large transverse momentum are not dominantly produced by coalescence from thermalized charm quarks, when comparing to model calculations.
Detailed measurements of the azimuthal anisotropy (\v2) for identified charged particles are reported as a function of transverse momentum ($p_T$) and centrality for Au+Au collisions at \sqrtsNN = 200 GeV. The measurements indicate clear evidence for eccentricity and particle flavor scaling over a broad range of centralities and transverse rapidity $y_T$, indicating a hydrodynamical origin of the fine structure of azimuthal anisotropy at RHIC. The observed scaling supports the picture of a suddenly hadronizing (recombining) fluid of quarks. An apparent breaking of flavor scaling at relatively …
PHENIX has measured elliptic and triangular flow of charged hadrons in $p/d/^{3}$He$+$Au collisions at a center-of-mass energy of 200 GeV per nucleon pair, and elliptic flow in $d$+Au collisions at 200, 62.4, and 39 GeV. In order to asses the origin of collectivity in the smallest systems, these results are compared with several theoretical models that produce azimuthal particle correlations based on initial and/or final state effects. Hydrodynamical models, which include the formation of a droplet of quark-gluon plasma, provide the best simultaneous description of our observations.
Recent collective flow measurements including higher moment event anisotropy from the PHENIX experiment are presented, and the particle type, beam energy dependence and the relation with jet modification are discussed. The measured higher order event anisotropy with event plane defined at forward rapidities and the long range correlation with large $η$ gaps are both consistent with initial geometrical fluctuation of the participating nuclei. In 200 GeV Au+Au collisions, higher order event anisotropy, especially simultaneous description of v$_2$ and v$_3$, is found to give an additional constra…
We report the first observations of the fourth harmonic (v_4) in the azimuthal distribution of particles at RHIC. The measurement was done taking advantage of the large elliptic flow generated at RHIC. The integrated v_4 is about a factor of 10 smaller than v_2. For the sixth (v_6) and eighth (v_8) harmonics upper limits on the magnitudes are reported.
We report STAR results on the azimuthal anisotropy parameter v2 for strange particles K0S and Lambda at midrapidity in Au+Au collisions at /s_NN = 130 GeV at RHIC. The value of v2 as a function of transverse momentum pt and collision centrality is presented for both particles and compared to model calculations. A strong pt dependence in v2 is observed up to pt ~2.0 GeV/c where v2 begins to saturate.
We report new results on $v_2(p_T)$ for Au+Au collisions at $\sqrt{s_{NN}}$=200 GeV for charged hadrons, pions, kaons, (anti)protons, $K^0_s$, and $Λ$. The analysis is extended to $p_T=12$ GeV/c for charged hadrons and $p_T=4$ GeV/c for identified particles. A comparison of the azimuthal anisotropy of charged hadrons measured at \sqrtsNN=130 and 200 GeV is presented. The $p_T$-dependence of baryon versus meson elliptic flow is discussed.
Azimuthal anisotropy ($v_2$) and two-particle angular correlations of high $p_T$ charged hadrons have been measured in Au+Au collisions at $\sqrt{s_{NN}}$=130 GeV for transverse momenta up to 6 GeV/c, where hard processes are expected to contribute significantly. The two-particle angular correlations exhibit elliptic flow and a structure suggestive of fragmentation of high $p_T$ partons. The monotonic rise of $v_2(p_T)$ for $p_T<2$ GeV/c is consistent with collective hydrodynamical flow calculations. At $\pT>3$ GeV/c a saturation of $v_2$ is observed which persists up to $p_T=6$ GeV/c.
We report the measurement of first (\textit {n} = 1) and higher order(\textit{n} = 2-5) harmonic coefficients ($v_{n}$) of the azimuthal anisotropy in the distribution of the particles produced in U+U collisions at $\sqrt{s_{NN}}$ =193 GeV, recorded with the STAR detector at RHIC. The differential measurement of $(v_{n})$ is presented as a function of transverse momentum ($p_{T}$) and centrality. We also present $v_{n}$ measurement in the ultra-central collisions. These data may provide strong constraints on the theoretical models of the initial condition in heavy ion collisions and the transp…
Heavy quarks are good probes of the hot and dense medium created in relativistic heavy ion collisions since they are mainly generated early in the collision and interact with the medium in all collision stages. In addition, heavy flavor quarkonia production is thought to be uniquely sensitive to the deconfined medium of the Quark Gluon Plasma (QGP) through color screening. Heavy quark production has been studied by the PHENIX experiment at RHIC via measurements of single leptons from semi-leptonic decays, in both the electron channel at mid-rapidity and in the muon channel at forward rapidity.…
Measurements of azimuthal anisotropy for strange and multi-strange hadrons are presented for the first time in their centrality dependence. The high statistics results of v2(pT) allow for a more detailed comparison to hydrodynamical model calculations. Number-of-constituent-quark scaling was tested for different centrality classes separately. Higher order anisotropies like v4(pT) are measured for multi-strange hadrons. While we observe agreement between measured data and models a deeper understanding and refinement of the models seem to be necessary in order to fully understand the details of …
Two-particle azimuthal correlations of high-pT hadrons can serve as a probe of interactions of partons with the dense medium produced in high-energy heavy-ion collisions. First NA49 results on such correlations are presented for central and mid-central Pb+Pb collisions at 158A GeV beam energy, for different centrality bins and charge combinations of trigger and associate particles. These results feature a flattened away-side peak in the most central collisions, which is consistent with expectations of the medium-interaction scenario. A comparison with CERES Pb+Au results at the same energy, as…
Heavy quarks (charm and bottom) are good probes of the hot and dense medium created in relativistic heavy ion collisions since they are mainly generated at the beginning of collisions and interact with the media in all collision stages. In addition, heavy flavor quarkonia production is thought to be uniquely sensitive to the deconfined medium of the Quark Gluon Plasma (QGP) through color screening. Heavy quark production has been studied by the PHENIX experiment at RHIC via measurements of single leptons from semi-leptonic decays in both the electron channel at mid-rapidity and in the muon cha…
Azimuthal angle two particle correlations have been shown to be a powerful probe for extracting novel features of the interaction between hard scattered partons and the medium produced in Au+Au collisions at RHIC. At intermediate $p_T$, 2-5GeV/c, the jets have been shown to be significantly modified in both their particle composition and their angular distribution compared to p+p collisions. Additionally, angular two particle correlations with identified hadrons provide information on the possible role of modified hadronization scenarios such as partonic recombination, which might allow medium…
We report on the first measurement of the azimuthal anisotropy ($v_2$) of dielectrons ($e^{+}e^{-}$ pairs) at mid-rapidity from $\sqrt{s_{_{NN}}} = 200$ GeV Au+Au collisions with the STAR detector at RHIC, presented as a function of transverse momentum ($p_T$) for different invariant-mass regions. In the mass region $M_{ee}\!<1.1$ GeV/$c^2$ the dielectron $v_2$ measurements are found to be consistent with expectations from $π^{0}$, $η$, $ω$ and $ϕ$ decay contributions. In the mass region $1.1\!<M_{ee}\!<2.9$ GeV/$c^2$, the measured dielectron $v_2$ is consistent, within experimental uncertaint…
The J/$ψ$ azimuthal distribution relative to the reaction plane has been measured by the NA50 experiment in Pb-Pb collisions at 158 GeV/nucleon. Various physical mechanisms related to charmonium dissociation in the medium created in the heavy ion collision are expected to introduce an anisotropy in the azimuthal distribution of the observed J/$ψ$ mesons at SPS energies. Hence, the measurement of J/$ψ$ elliptic anisotropy, quantified by the Fourier coefficient v$_2$ of the J/$ψ$ azimuthal distribution relative to the reaction plane, is an important tool to constrain theoretical models aimed at …
Charm and bottom quarks are formed predominantly by gluon fusion in the initial hard scatterings at RHIC, making them good probes of the full medium evolution. Previous measurements at RHIC have shown large suppression and azimuthal anisotropy of open heavy-flavor hadrons in Au+Au collisions at $\sqrt{s_{NN}}=200~{\rm GeV}$. Explaining the simultaneously large suppression and flow of heavy quarks has been challenging. To further understand the heavy-flavor transport in the hot and dense medium, it is imperative to also measure cold nuclear matter effects which affect the initial distribution o…
We have measured the azimuthal anisotropy of pi0's for 1 < pT < 18 GeV/c for Au+Au collisions at sqrt s_NN = 200 GeV. The observed anisotropy shows a gradual decrease in 3 < pT < 7 - 10 GeV/c, but remains positive beyond 10 GeV/c. The magnitude of this anisotropy is under-predicted, up to at least 10 GeV/c, by current perturbative QCD (pQCD) energy-loss model calculations. An estimate of the increase in anisotropy expected from initial-geometry modification due to gluon saturation effects and initial-geometry fluctuations is insufficient to account for this discrepancy. Calculations which impl…
Measurements of the azimuthal anisotropy of high-\pT neutral pion neutral pion production in Au+Au collisions at sqrt(s_NN) = 200 GeV by the PHENIX experiment are presented. The data included in this paper were collected during the 2004 RHIC running period and represent approximately an order of magnitude increase in the number of analyzed events relative to previously published results. Azimuthal angle distributions of pi^0s detected in the PHENIX electromagnetic calorimeters are measured relative to the reaction plane determined event-by-event using the forward and backward beam-beam counter…
