The flow harmonics upsilon sub(2,3) for charged hadrons are studied for a broad range of centrality selections and beam collision energies in Au + Au (radicalS sub(N)N= 7.7-200 Gev) and Pb + Pb ...(radicalS sub(N)N= 2.76 Tev) collisions. They validate the characteristic signature expected for the system size dependence of viscous damping at each collision energy studied. The extracted viscous coefficients that encode the magnitude of the ratio of shear viscosity to entropy density eta/s are observed to decrease to an apparent minimum as the collision energy is increased from radicalS sub(N)N= 7.7 to approximately 62.4 GeV; thereafter, they show a slow increase with radicalS sub(N)Nup to 2.76 TeV. This pattern of viscous damping provides the first experimental constraint for eta/s in the temperature-baryon chemical potential (T, mu sub(B) plane and could be an initial indication for decay trajectories that lie close to the critical end point in the phase diagram for nuclear matter.
Two-pion interferometry measurements in d+Au and Au+Au collisions at sNN=200 GeV are used to extract and compare the Gaussian source radii Rout, Rside and Rlong, which characterize the space–time ...extent of the emission sources. The comparisons, which are performed as a function of collision centrality and the mean transverse momentum for pion pairs, indicate strikingly similar patterns for the d+Au and Au+Au systems. They also indicate a linear dependence of Rside on the initial transverse geometric size R¯, as well as a smaller freeze-out size for the d+Au system. These patterns point to the important role of final-state re-scattering effects in the reaction dynamics of d+Au collisions.
The shear viscosity to entropy ratio (eta/s) is estimated for the hot and dense QCD matter created in Au+Au collisions at BNL Relativistic Heavy Ion Collider (square roots_{NN}=200 GeV). A very low ...value is found; eta/s approximately 0.1, which is close to the conjectured lower bound (1/4pi). It is argued that such a low value is indicative of thermodynamic trajectories for the decaying matter which lie close to the QCD critical end point.
The PHENIX experiment at the BNL Relativistic Heavy Ion Collider has measured second- and third-order Fourier coefficients of the azimuthal distributions of direct photons emitted at midrapidity in ...Au + Au collisions at √ sNN = 200 GeV for various collision centralities. Combining two different analysis techniques, results were obtained in the transverse momentum range of 0.4 < pT < 4.0 GeV/c. At lowpT the second-order coefficients, v2, are similar to the ones observed in hadrons. Third-order coefficients, v3, are nonzero and almost independent of centrality. These new results on v2 and v3, combined with previously published results on yields, are compared to model calculations that provide yields and asymmetries in the same framework. Those models are challenged to explain simultaneously the observed large yield and large azimuthal anisotropies.
The extreme energy densities generated by ultra-relativistic collisions between heavy atomic nuclei produce a state of matter that behaves surprisingly like a fluid, with exceptionally high ...temperature and low viscosity. Non-central collisions have angular momenta of the order of 1,000ћ, and the resulting fluid may have a strong vortical structure that must be understood to describe the fluid properly. The vortical structure is also of particular interest because the restoration of fundamental symmetries of quantum chromodynamics is expected to produce novel physical effects in the presence of strong vorticity. However, no experimental indications of fluid vorticity in heavy ion collisions have yet been found. Since vorticity represents a local rotational structure of the fluid, spin-orbit coupling can lead to preferential orientation of particle spins along the direction of rotation. Here we present measurements of an alignment between the global angular momentum of a non-central collision and the spin of emitted particles (in this case the collision occurs between gold nuclei and produces Λ baryons), revealing that the fluid produced in heavy ion collisions is the most vortical system so far observed. (At high energies, this fluid is a quark-gluon plasma.) We find that Λ and hyperons show a positive polarization of the order of a few per cent, consistent with some hydrodynamic predictions. (A hyperon is a particle composed of three quarks, at least one of which is a strange quark; the remainder are up and down quarks, found in protons and neutrons.) A previous measurement that reported a null result, that is, zero polarization, at higher collision energies is seen to be consistent with the trend of our observations, though with larger statistical uncertainties. These data provide experimental access to the vortical structure of the nearly ideal liquid created in a heavy ion collision and should prove valuable in the development of hydrodynamic models that quantitatively connect observations to the theory of the strong force.
Global polarization of Λ hyperons has been measured to be of the order of a few tenths of a percent in Au+Au collisions at √SNN = 200 GeV, with no significant difference between Λ and Λ¯. These new ...results reveal the collision energy dependence of the global polarization together with the results previously observed √SNN = 7.7 – 62.4 GeV and indicate noticeable vorticity of the medium created in non-central heavy-ion collisions at the highest RHIC collision energy.
A new method is presented for the quantitative measurement of charge separation about the reaction plane. A correlation function is obtained whose shape is concave when there is a net separation of ...positive and negative charges. Correlations not specifically associated with charge, from flow, jets, and momentum conservation, do not influence the shape or magnitude of the correlation function. Detailed simulations are used to demonstrate the effectiveness of the method for the quantitative measurement of charge separation. Such measurements are a prerequisite to the investigation of topological charge effects in the quark-gluon plasma as derived from the 'strong CP problem'.
The charged-particle pseudorapidity density ( d N ch / d η ) for p( p ¯ )+p, p+A and A+A(B) collisions and the mean multiplicity 〈 N ch 〉 for e − +e + , e ± + p , and p( p ¯ )+p collisions are ...studied for a wide range of beam energies ( s ). Characteristic scaling patterns are observed for both d N ch / d η and 〈 N ch 〉 , consistent with a thermal particle production mechanism for the bulk of the soft particles created in all of these systems. The scaling patterns found also validate an essential role for quark participants in these collisions. The measured values for d N ch / d η and 〈 N ch 〉 are observed to factorize into contributions that depend on log ( s ) and the number of nucleon or quark participant pairs N pp . The quantification of these contributions gives expressions that serve to systematize d N ch / d η and 〈 N ch 〉 measurements spanning nearly 4 orders of magnitude in s and to predict their values as a function of s and N pp .
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