Experimental studies of the collisions of heavy nuclei at relativistic energies have established the properties of the quark–gluon plasma (QGP), a state of hot, dense nuclear matter in which quarks ...and gluons are not bound into hadrons1–4. In this state, matter behaves as a nearly inviscid fluid5 that efficiently translates initial spatial anisotropies into correlated momentum anisotropies among the particles produced, creating a common velocity field pattern known as collective flow. In recent years, comparable momentum anisotropies have been measured in small-system proton–proton (p+p) and proton–nucleus (p+A) collisions, despite expectations that the volume and lifetime of the medium produced would be too small to form a QGP. Here we report on the observation of elliptic and triangular flow patterns of charged particles produced in proton–gold (p+Au), deuteron–gold (d+Au) and helium–gold (3He+Au) collisions at a nucleon–nucleon centre-of-mass energy \\sqrt {s_{{\mathrm{NN}}}\ = 200 GeV. The unique combination of three distinct initial geometries and two flow patterns provides unprecedented model discrimination. Hydrodynamical models, which include the formation of a short-lived QGP droplet, provide the best simultaneous description of these measurements.
Hard-scattered parton probes produced in collisions of large nuclei indicate large partonic energy loss, possibly with collective produced-medium response to the lost energy. We present measurements ...of π^{0} trigger particles at transverse momenta p{T}{t}=4-12 GeV/c and associated charged hadrons (p{T}{a}=0.5-7 GeV/c) vs relative azimuthal angle Δϕ in Au+Au and p+p collisions at sqrts{NN}=200 GeV. The Au+Au distribution at low p{T}{a}, whose shape has been interpreted as a medium effect, is modified for p{T}{t}<7 GeV/c. At higher p{T}{t}, the data are consistent with unmodified or very weakly modified shapes, even for the lowest measured p{T}{a}, which quantitatively challenges some medium response models. The associated yield of hadrons opposing the trigger particle in Au+Au relative to p+p (I{AA}) is suppressed at high p{T} (I{AA}≈0.35-0.5), but less than for inclusive suppression (R{AA}≈0.2).
Recently, multiparticle-correlation measurements of relativistic p/d/^{3}He+Au, p+Pb, and even p+p collisions show surprising collective signatures. Here, we present beam-energy-scan measurements of ...two-, four-, and six-particle angular correlations in d+Au collisions at sqrts_{NN}=200, 62.4, 39, and 19.6 GeV. We also present measurements of two- and four-particle angular correlations in p+Au collisions at sqrts_{NN}=200 GeV. We find the four-particle cumulant to be real valued for d+Au collisions at all four energies. We also find that the four-particle cumulant in p+Au has the opposite sign as that in d+Au. Further, we find that the six-particle cumulant agrees with the four-particle cumulant in d+Au collisions at 200 GeV, indicating that nonflow effects are subdominant. These observations provide strong evidence that the correlations originate from the initial geometric configuration, which is then translated into the momentum distribution for all particles, commonly referred to as collectivity.
The PHENIX collaboration presents first measurements of low-momentum (0.4<p_{T}<3 GeV/c) direct-photon yields from Au+Au collisions at sqrts_{NN}=39 and 62.4 GeV. For both beam energies the ...direct-photon yields are substantially enhanced with respect to expectations from prompt processes, similar to the yields observed in Au+Au collisions at sqrts_{NN}=200. Analyzing the photon yield as a function of the experimental observable dN_{ch}/dη reveals that the low-momentum (>1 GeV/c) direct-photon yield dN_{γ}^{dir}/dη is a smooth function of dN_{ch}/dη and can be well described as proportional to (dN_{ch}/dη)^{α} with α≈1.25. This scaling behavior holds for a wide range of beam energies at the Relativistic Heavy Ion Collider and the Large Hadron Collider, for centrality selected samples, as well as for different A+A collision systems. At a given beam energy, the scaling also holds for high p_{T} (>5 GeV/c), but when results from different collision energies are compared, an additional sqrts_{NN}-dependent multiplicative factor is needed to describe the integrated-direct-photon yield.
PHENIX has measured the e(+)e(-) pair continuum in root s(NN) = 200 GeV Au+Au and p+p collisions over a wide range of mass and transverse momenta. The e(+)e(-) yield is compared to the expectations ...from hadronic sources, based on PHENIX measurements. In the intermediate-mass region, between the masses of the phi and the J/psi meson, the yield is consistent with expectations from correlated c (c) over bar production, although other mechanisms are not ruled out. In the low-mass region, below the phi, the p+p inclusive mass spectrum is well described by known contributions from light meson decays. In contrast, the Au+Au minimum bias inclusive mass spectrum in this region shows an enhancement by a factor of 4.7 +/- 0.4(stat) +/- 1.5(syst) +/- 0.9(model). At low mass (m(ee) < 0.3 GeV/c(2)) and high p(T) (1 < p(T) < 5 GeV/c) an enhanced e(+)e(-) pair yield is observed that is consistent with production of virtual direct photons. This excess is used to infer the yield of real direct photons. In central Au+Au collisions, the excess of the direct photon yield over the p+p is exponential in p(T), with inverse slope T = 221 +/- 19(stat) +/- 19(syst) MeV. Hydrodynamical models with initial temperatures ranging from T-init similar or equal to 300-600 MeV at times of 0.6-0.15 fm/c after the collision are in qualitative agreement with the direct photon data in Au+Au. For low p(T) < 1 GeV/c the low-mass region shows a further significant enhancement that increases with centrality and has an inverse slope of T similar or equal to 100 MeV. Theoretical models underpredict the low-mass, low-p(T) enhancement.
The PHENIX experiment at the BNL Relativistic Heavy Ion Collider (RHIC) has measured electrons with 0.3 < p(T) < 9 GeV/c at midrapidity (y < 0.35) from heavy-flavor (charm and bottom) decays in Au + ...Au collisions at root s(NN) = 200 GeV. The nuclear modification factor R-AA relative to p + p collisions shows a strong suppression in central Au + Au collisions, indicating substantial energy loss of heavy quarks in the medium produced at RHIC energies. A large azimuthal anisotropy v(2) with respect to the reaction plane is observed for 0.5 < p(T) < 5 GeV/c indicating substantial heavy-flavor elliptic flow. Both R-AA and v(2) show a p(T) dependence different from those of neutral pions. A comparison to transport models which simultaneously describe R-AA(p(T)) and v(2)(p(T)) suggests that the viscosity to entropy density ratio is close to the conjectured quantum lower bound, i.e., near a perfect fluid.
Azimuthal angle (Delta phi) correlations are presented for a broad range of transverse momentum (0.4 < p(T) < 10 GeV/c) and centrality (0-92%) selections for charged hadrons from dijets in Au+Au ...collisions at root s(NN) = 200 GeV. With increasing p(T), the away-side Delta phi distribution evolves from a broad and relatively flat shape to a concave shape, then to a convex shape. Comparisons with p + p data suggest that the away-side distribution can be divided into a partially suppressed "head" region centered at Delta phi similar to pi, and an enhanced "shoulder" region centered at Delta phi similar to pi +/- 1.1. The p(T) spectrum for the associated hadrons in the head region softens toward central collisions. The spectral slope for the shoulder region is independent of centrality and trigger p(T). The properties of the near-side distributions are also modified relative to those in p + p collisions, reflected by the broadening of the jet shape in Delta phi and Delta eta, and an enhancement of the per-trigger yield. However, these modifications seem to be limited to p(T)less than or similar to 4 GeV/c, above which both the hadron pair shape and per-trigger yield become similar to p + p collisions. These observations suggest that both the away- and near-side distributions contain a jet fragmentation component which dominates for p(T) greater than or similar to 5 GeV/c and a medium-induced component which is important for p(T) less than or similar to 4 GeV/c. We also quantify the role of jets at intermediate and low p(T) through the yield of jet-induced pairs in comparison with binary scaled p + p pair yield. The yield of jet-induced pairs is suppressed at high pair proxy energy (sum of the p(T) magnitudes of the two hadrons) and is enhanced at low pair proxy energy. The former is consistent with jet quenching; the latter is consistent with the enhancement of soft hadron pairs due to transport of lost energy to lower p(T).
Differential measurements of elliptic flow (v(2)) for Au+Au and Cu+Cu collisions at root s(NN)=200 GeV are used to test and validate predictions from perfect fluid hydrodynamics for scaling of v(2) ...with eccentricity, system size, and transverse kinetic energy (KET). For KET equivalent to m(T)-m up to similar to 1 GeV the scaling is compatible with hydrodynamic expansion of a thermalized fluid. For large values of KET mesons and baryons scale separately. Quark number scaling reveals a universal scaling of v(2) for both mesons and baryons over the full KET range for Au+Au. For Au+Au and Cu+Cu the scaling is more pronounced in terms of KET, rather than transverse momentum.
The PHENIX Electromagnetic Calorimeter (EMCal) is used to measure the spatial position and energy of electrons and photons produced in heavy ion collisions. It covers the full central spectrometer ...acceptance of 70°⩽
θ⩽110° with two walls, each subtending 90° in azimuth. One wall comprises four sectors of a Pb-scintillator sampling calorimeter and the other has two sectors of Pb-scintillator and two of a Pb-glass Cherenkov calorimeter. Both detectors have very good energy, spatial and timing resolution, while the Pb-scintillator excels in timing and the Pb-glass in energy measurements. Also, having two detectors with different systematics increases the confidence level of the physics results. Design and operational parameters of the Pb-scintillator, Pb-glass and special readout electronics for EMCal are presented and running experience during the first year of data taking with PHENIX is discussed. Some examples of data taken during the first run are shown.
The PHENIX experiment has measured the spin alignment for inclusive J=ψ → e+e− decays in protonproton collisions at √s = 510 GeV at midrapidity. The angular distributions have been measured in three ...different polarization frames, and the three decay angular coefficients have been extracted in a full two-dimensional analysis. Previously, PHENIX saw large longitudinal net polarization at forward rapidity at the same collision energy. This analysis at midrapidity, complementary to the previous PHENIX results, sees no sizable polarization in the measured transverse momentum range of 0.0 < pT < 10.0 GeV=c. The results are consistent with a previous one-dimensional analysis at midrapidity at √s = 200 GeV. The transverse-momentum-dependent cross section for midrapidity J=ψ production has additionally been measured, and after comparison to world data, a simple logarithmic dependence of the cross section on √s was found.
PDF
