Rapidity equilibration in d + Au and Au + Au systems

In a Relativistic Diffusion Model (RDM), the evolution of net-proton rapidity spectra with in heavy systems is proposed as an indicator for local equilibration and longitudinal expansion. The broad midrapidity valley recently discovered at RHIC in central Au + Au collisions at = 200 GeV suggests rapid local equilibration which is most likely due to deconfinement, and fast longitudinal expansion. Rapidity spectra of produced charged hadrons in d + Au and Au + Au systems at RHIC energies and their centrality dependence are well described in a three-sources RDM. In central collisions, about 19% of the produced particles are in the equilibrated midrapidity region for d + Au.     

Analysis of non-photonic electrons in Cu + Cu collisions at $\sqrt{s_{NN}}=200$ GeV

Heavy-flavor semileptonic decays are expected to dominate the spectrum of non-photonic electrons produced from collisions at the energies of the Relativistic Heavy Ion Collider. The non-photonic electron yield is suppressed by approximately a factor of 5 in central Au + Au events at  GeV relative to p+p events with the same collision energy. Most theoretical models predict less non-photonic-electron suppression than is observed experimentally. We present a preliminary measurement of the yield of non-photonic electrons in Cu + Cu events at  GeV, as well as the nuclear modification factor.     

Longitudinal broadening of near-side jets due to parton cascade

Longitudinal broadening along the Δη direction on the near side in the two-dimensional (Δφ×Δη) di-hadron correlation distribution has been studied for central Au+Au collisions at  GeV, within a dynamical multi-phase transport model. It was found that longitudinal broadening is generated by a longitudinal flow induced by a strong parton cascade in central Au+Au collisions, to be compared with p+p collisions at  GeV. The longitudinal broadening may shed light on the strongly interacting partonic matter at RHIC.     

Low-mass dielectrons from the PHENIX experiment at RHIC

The production of the low-mass dielectrons is considered to be a powerful tool to study the properties of the hot and dense matter created in the ultra-relativistic heavy-ion collisions. We present the preliminary results on the first measurements of the low-mass dielectron continuum in Au + Au collisions and the φ-meson production measured in Au + Au and d + Au collisions at = 200GeV performed by the PHENIX experiment.     

Charmonium dissociation and recombination at RHIC and LHC

Charmonium production at heavy-ion colliders is considered within the comovers-interaction model. The formalism is extended by including possible secondary J/ψ production through recombination and an estimate of recombination effects is made without adjusting the model parameters. The comovers-interaction model also includes a comprehensive treatment of initial-state nuclear effects, which are discussed in the context of such high energies. With these tools, the model properly describes the centrality and the rapidity dependence of experimental data at RHIC energy,  GeV, for both Au+Au and Cu+Cu collisions. Predictions for LHC,  TeV, are presented and the assumptions and extrapolations involved are discussed.     

<Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis> suppression and <Emphasis Type="Italic">p</Emphasis><Subscript><Emphasis Type="Italic">T</Emphasis></Subscript> spectra in RHIC and LHC energy collisions

In a hydrodynamic model, we have studied J/ψ production in Au+Au/Cu+Cu collisions at RHIC energy, GeV. At the initial time, J/ψ’s are randomly distributed in the fluid. As the fluid evolves in time, the free streaming J/ψ’s are dissolved if the local fluid temperature exceeds a threshold temperature T _J/ψ . Sequential melting of charmonium states (χ _c , ψ ^′ and J/ψ), with melting temperatures , T _J/ψ ≈2T _c and feed-down fraction F≈0.3, explains the PHENIX data on the centrality dependence of J/ψ suppression in Au+Au collisions. J/ψ p _T spectra and the nuclear modification factor in Au+Au collisions are also well explained in the model. The model however overpredicts the centrality dependence of J/ψ suppression in Cu+Cu collisions by 20–30%. The J/ψ p _T spectra are underpredicted by 20–30%. The model predicts that in central Pb+Pb collisions at LHC energy,  GeV, J/ψ’s are suppressed by a factor of ∼10. The model predicted a J/ψ p _T distribution in Pb+Pb collisions at LHC is similar to that in Au+Au collisions at RHIC.     

Diffusion or bounce back in relativistic heavy-ion collisions?

The time evolution of pseudorapidity distributions of produced charged hadrons in d + Au collisions at = 200 GeV is investigated. Results of a nonequilibrium statistical relativistic diffusion model with three sources are compared with a macroscopic “bounce-back” model that does not allow for statistical equilibration at large times, but instead leads to motion reversal. When compared to the data, the results of the diffusion approach are more precise, thus emphasizing that the system is observed to be on its way to thermal equilibrium.     

Medium effects on high p_{\rm T} particle production measured with the PHENIX experiment

Transverse momentum ( ) spectra measured by the PHENIX experiment at RHIC in Au + Au, d + Au and pp collisions at and in Au + Au collisions at are presented. A suppression of the yield of high hadrons in central Au + Au collisions by a factor 4-5 at is found relative to the pp reference scaled by the nuclear overlap function . In contrast, direct photons are not suppressed in central Au + Au collisions and no suppression of high particles can be seen in d + Au collisions. This leads to the conclusion that the dense medium formed in central Au + Au collisions is responsible for the suppression. Arrival of the final proofs: 30 June 2005 PACS: 25.75.Dw     

Energy and system dependence of high-<Emphasis Type="Italic">p</Emphasis><Subscript><Emphasis Type="Italic">T</Emphasis></Subscript> triggered two-particle near-side correlations

Previous studies have indicated that the near-side peak of high-p _T triggered correlations can be decomposed into two parts, the Jet and the Ridge. We present data on the yield per trigger of the Jet and the Ridge from d+Au, Cu+Cu and Au+Au collisions at GeV and 200 GeV and compare data on the Jet to PYTHIA 8.1 simulations for p+p. PYTHIA describes the Jet component up to a scaling factor, meaning that PYTHIA can provide a better understanding of the Ridge by giving insight into the effects of the kinematic cuts. We present collision energy and system dependence of the Ridge yield, which should help distinguish models for the production mechanism of the Ridge.     

Modified fragmentation function in heavy ion collisions at RHIC via direct <Emphasis Type="Italic">γ</Emphasis>-jet measurements

The presented results are the first measurements at RHIC for direct γ-charged hadron azimuthal correlations in heavy ion collisions. We use these correlations to study the color charge density of the medium through the medium-induced modification of high-p _T parton fragmentation. Azimuthal correlations of direct photons at high transverse energy (8<p _T <16 GeV) with away-side charged hadrons of transverse momentum (3<p _T <6 GeV/c) have been measured over a broad range of centrality for Au+Au collisions and p+p collisions at  GeV in the STAR experiment. A transverse shower shape analysis in the STAR Barrel Electromagnetic Calorimeter Shower Maximum Detector is used to discriminate between the direct photons and photons from the decays of high p _T π ⁰. The per-trigger away-side yield of direct γ is smaller than from π ⁰ trigger at the same centrality class. Within the current uncertainty the I _CP of direct γ and π ⁰ are similar.     

High-p T results from PHOBOS

We present results from the PHOBOS experiment on charged hadron production over a large range in , from Data from d + Au collisions at GeV and Au + Au collisions at and 200 GeV are shown. The data indicate that in Au + Au collisions a strongly interacting state of matter is formed, that appears qualitatively different from the system formed in pp or d + Au collisions. The systematics of particle production in d + Au and Au + Au collisions exhibit remarkably simple scaling rules, including an apparent factorization of the energy and centrality dependence over a wide range of collision energies. Arrival of the final proofs: 29 July 2005 PACS: 25.75.-q, 25.75.Dw, 25.75.Gz     

Transverse energy and charged-particle multiplicity at various centralities at RHIC: Statistical model estimates⋆

The transverse energy and the charged-particle multiplicity at midrapidity are evaluated in a single-freeze-out model for different centrality bins at RHIC at = 130 and 200GeV. The predictions of the model are done at the freeze-out parameters determined earlier from measured particle yields and p_T spectra. The results agree qualitatively well with the experimental data.     

Exploring the LHC medium with direct photons

Heavy-ion collisions will enter a new era with the start of the CERN Large Hadron Collider (LHC). A first short run with proton-proton collisions at the injection energy of 0.9 TeV will be followed by a longer one with pp collisions at 10 TeV. First Pb-Pb collisions at  TeV will take place in 2009. Three experiments (ALICE, ATLAS, and CMS) will study both pp and Pb-Pb collisions. A selection of results showing the capabilities of the three experiments for the study of the LHC medium with direct photons is presented.     

Heavy flavor production in PHENIX

The PHENIX experiment at RHIC measured single electron spectra in p + p, d + Au and Au + Au collisions at GeV, and in Au + Au collisions at GeV. In these spectra, electrons from semi-leptonic decays of charmed particles are the dominant contribution after subtraction of all 'photonic' sources (photon conversions, Dalitz decays, decays of light vector mesons). The p + p open charm production cross-section is found to be in good agreement with pQCD NLO calculations. The shape of the distributions obtained for p + p interactions is compared with those observed for nucleus-nucleus collisions. From p + p to d + Au and Au + Au interactions, open charm production is found to scale with the number of binary collisions . Au + Au data at GeV is compatible with the ISR p + p results scaled by . The elliptic flow parameter v₂ of heavy flavor electrons has also been measured, and is found to be non-zero in the intermediate p_T range. Arrival of the final proofs: 29 June 2005 PACS: . ^* Deceased Spokesperson     

On the fluctuations of the force exerted by a lipid nanotubule

After deriving the projected stress tensor in cylindrical geometry for a fluid membrane described by the Helfrich Hamiltonian, we calculate the average force f exerted by a thermally fluctuating nanotubule of radius R , and its standard deviation f . We obtain f and f in terms of the internal membrane tension , the bending rigidity , the temperature k _B T and a molecular cutoff . We find for f a shift ∼ 1/ with respect to the mean field behavior ∼ . We obtain ( f )² ∼ R ln(R/b) where b is a molecular length, f being typically small compared to f . Taking into account the difference between the internal tension and the actual mechanical tension applied to the membrane from which the tubule is drawn, we discuss the amplitude of the fluctuation-induced corrections to the average force. Our results, obtained in the harmonic approximation, hold for tubules with aspect ratio not larger than 200 , of radius significantly smaller than 100nm, that are connected to a large membrane reservoir, e.g., a giant vesicle.     

Kinetics of microphase separation in interpenetrated polymer networks in solution

We present here a theoretical study of the early kinetics of the microphase separation in crosslinked polymer blends, made of two incompatible polymers A and B, dissolved in a common good solvent. Use is made of an extended blob model used previously for the investigation of the static properties of such a transition. We are interested in the variation of the relaxation rate, , versus the wave number q, in the vicinity of the spinodal temperature. We first show that kinetics is entirely dominated by local motions, which are of Rouse type. Slow motions are absent, because of the permanent presence of crosslinks. Second, we find that the characteristic frequency, (q ) = , increases with increasing wave number q according to a sixth power law, that is (q ) q^6{-9/4}, where is the overall monomer volume fraction. Therefore, the swelling of strands due to the excluded-volume forces leads to a renormalization of the characteristic frequency by a multiplicative factor scaling as ^{-9/4}. The main conclusion is that the presence of a good solvent necessitates relaxation rates less important than those relative to crosslinked mixtures in the molten state.     

Effects of phase decoherence on the entanglement of a two-qubit anisotropic Heisenberg XYZ chain with an in-plane magnetic field

We investigate the phase decoherence effects on the entanglement of a two-qubit anisotropic Heisenberg model with a nonuniform magnetic field in the x–z-plane. As a measure of the entanglement, the concurrence of the system is calculated. It is shown that when the magnetic field is along the z-axis, the nonuniform and uniform components of the field have no influence on the entanglement for the cases of and , respectively. But when the magnetic field is not along the z-axis, both the uniform and the nonuniform components of the field will introduce the decoherence effects. It is found that the effects of the Heisenberg chain's anisotropy in the Z-direction on the entanglement are dependent on the direction of the field. Moreover, the larger the initial concurrence is, the higher value it will exhibit during the time evolution of the system for a proper set of the parameters ν, Δ, θ, γ , B and b.     

Effective action for phase fluctuations in <Emphasis Type="Italic">d</Emphasis>-wave superconductors near a Mott transition

Phase fluctuations of a d-wave superconducting order parameter are theoretically studied in the context of high-T_c cuprates. We consider an extended t-J model describing electrons in a layer which also contains long-range Coulomb interactions. The constraint of having at most singly occupied sites is enforced by an additional Hubbard term. The Heisenberg interaction is decoupled by a d-wave order parameter in the particle-particle channel. Assuming first that the equilibrium state has long-range phase order, the effective action is derived perturbatively for small fluctuations within a path integral formalism, in the presence of the Coulomb and Hubbard interaction terms. In a second step, a more general derivation of is performed in terms of a gradient expansion which only assumes that the gradients of the order parameter are small whereas the value of the phase may be large. We show that in the phase-only approximation the resulting reduces in leading order in the field gradients to the perturbative one which thus allows to treat also the case without long-range phase order or vortices. Our result generalizes previous expressions for to the case of interacting electrons, is explicitly gauge invariant, and avoids problematic singular gauge transformations.     

Charm production in the STAR experiment at RHIC

The single electron spectrum over a sufficiently broad range provides an indirect measurement of charm and beauty production at RHIC energies. Heavy-quarks are produced in parton-parton scattering in the initial phase of the collision and thus provide important information about the initial configuration of the colliding nuclei. Even more important, the final state spectra reflect the interactions of these heavy partons with the medium and thus allow us to probe the properties of the hot and dense system created in heavy-ion collisions. We present preliminary measurements of electron and positron spectra in pp and d + Au as well as preliminary elliptic flow measurements in Au + Au collisions at GeV performed by the STAR experiment. We describe the measurement techniques used to discriminate electrons from hadrons and compare the results with theoretical calculations. PACS. 25.75.-q Arrival of the final proofs: 20 June 2005     

Transverse energy per charged particle at relativistic energies from a statistical model with expansion

Transverse-energy and charged-particle pseudorapidity densities at midrapidity and their ratio, dE_T/d|_mid/dN_ch/d|_mid, are evaluated in a statistical model with longitudinal and transverse flows for the wide range of colliders, from AGS to RHIC at = 200 GeV. Evaluations are done at freeze-out parameters obtained from independent fits to observed particle yields and p_T spectra. Decays of hadron resonances are treated thoroughly and are included in derivations of dE_T/d|_mid and dN_ch/d|_mid. The predictions of the model agree well with the experimental data. However, some (explicable) overestimation of the ratio has been observed.