Search Results (23 total)

We argue that an expanding quark-gluon plasma has an anomalous viscosity, which arises from interactions with dynamically generated color fields. We derive an expression for the anomalous viscosity in the turbulent plasma domain and apply it to the hydrodynamic expansion phase, when the quark-gluon plasma is near equilibrium. The anomalous viscosity dominates over the collisional viscosity for weak coupling and not too late times. This effect may provide an explanation for the apparent “nearly perfect” liquidity of the matter produced in nuclear collisions at the Relativistic Heavy Ion Collider without the assumption that it is a strongly coupled state.

The effects of mode-mode and isospin-isospin correlations on nonequilibrium chiral dynamics are investigated by using the method of the time-dependent variational approach with squeezed states as trial states. Our numerical simulations show that large domains of the disoriented chiral condensate (DCC) are formed because of the combined effect of the mode-mode and isospin-isospin correlations. Moreover, it is found that, when the mode-mode correlation is included, the DCC domain formation is accompanied by the amplification of the quantum fluctuation, which implies the squeezing of the state. However, neither the DCC domain formation nor the amplification of the quantum fluctuation is observed if only the isospin-isospin correlation is included. This suggests that the mode-mode coupling plays a key role in the DCC domain formation.

We explore possible ways of explaining the net charge event-by-event fluctuations in Au+Au collisions observed in experiments at the Relativistic Heavy Ion Collider within a quark recombination model. We estimate the number of quarks at recombination and their implications for the predicted net charge fluctuations. We also discuss the consequences of diquark and quark-antiquark clustering above the deconfinement temperature.

Phenomenologically, hydrodynamical calculations have been successful in describing global observables in ultrarelativistic heavy-ion collisions, which aim to observe the production of the quark-gluon plasma. On the other hand, theoretically, a lot of evidence that there exists a critical end point (CEP) in the phase diagram of the quantum chromodynamics has been accumulating recently. Nevertheless, so far, no equation of state with the CEP has been employed in hydrodynamical calculations. In this article, we construct the equation of state with the CEP on the basis of the universality hypothesis and show that the CEP acts as an attractor of isentropic trajectories. We also consider the time evolution in the case with the CEP and discuss how the CEP affects the final state observables, such as the correlation length, fluctuation, chemical freeze-out, kinetic freeze-out, and so on. Finally, we argue that the anomalously low kinetic freeze-out temperature at the Relativistic Heavy-Ion Collider at the Brookhaven National Laboratory suggests the possibility of the existence of the CEP.

We propose the measurement of the elliptic flow of hadron resonances at the Relativistic Heavy Ion Collider as a tool to probe the amount of hadronic final state interactions for resonances at intermediate and large transverse momenta. This can be achieved by looking at systematic deviations of the measured flow coefficient v₂ from the scaling law given by the quark recombination formalism. Our method can be generalized to explore the structure of exotic particles, such as the recently found pentaquark Θ⁺(1540).

We investigate the inhomogeneous chiral dynamics of the O(4) linear sigma model in 1+1 dimensions using the time dependent variational approach in the space spanned by the squeezed states. We compare two cases, with and without the Gaussian approximation for the Green’s functions. We show that mode-mode correlation plays a decisive role in the out-of-equilibrium quantum dynamics of domain formation and squeezing of states.

Analyzing correlation functions of charmonia at finite temperature (T) on 32³×(32-96) anisotropic lattices by the maximum entropy method (MEM), we find that J/ψ and η_c survive as distinct resonances in the plasma even up to T≃1.6T_c and that they eventually dissociate between 1.6T_c and 1.9T_c (T_c is the critical temperature of deconfinement). This suggests that the deconfined plasma is nonperturbative enough to hold heavy-quark bound states. The importance of having a sufficient number of temporal data points in MEM analyses is also emphasized.

We discuss characteristic experimental signatures related to the formation of domains of disoriented chiral condensate triggered by the axial anomaly in relativistic heavy-ion collisions. We predict that the enhancement of the fraction of neutral pions compared to all pions depends on the angle of emission with respect to the scattering plane and is concentrated at small transverse momentum and small rapidity in the center-of-mass frame. The anisotropy with respect to the reaction plane is also observable in the inclusive photon distribution.

Extraordinary (X) waves are perpendicularly injected for electron Bernstein (B) wave heating into an Ohmically heated plasma from the inboard side in the WT-3 tokamak. Measurements show that absorption does not take place at the electron cyclotron resonance layer nor the upper hybrid resonance layer, but does happen midway between them. This is consistent with the ray tracing prediction, i.e., the poloidal field and poloidal inhomogeneity of toroidal field lead the B waves to have a large parallel refractive index N_∥ (>1), and the B waves are damped away via the Doppler-shifted cyclotron resonance.

The size of the average fluctuations of net baryon number and electric charge in a finite volume of hadronic matter differs widely between the confined and deconfined phases. These differences may be exploited as indicators of the formation of a quark-gluon plasma in relativistic heavy-ion collisions, because fluctuations created in the initial state survive until freeze-out due to the rapid expansion of the hot fireball.

Huge back-to-back correlations are shown to arise for thermal ensembles of bosonic states with medium-modified masses. The effect is experimentally observable in high energy heavy ion collisions.

QCD spectral functions of hadrons in the pseudoscalar and vector channels are extracted from lattice Monte Carlo data of the imaginary time Green’s functions. The maximum entropy method works well for this purpose, and the resonance and continuum structures in the spectra are obtained in addition to the ground state peaks.

We discuss the effect of the chiral anomaly as a possible mechanism for triggering the formation of domains of disoriented chiral condensate (DCC) in relativistic heavy ion collisions. The anomalous π⁰→2γ coupling and the strong, Lorentz contracted electromagnetic fields of the heavy ions combine to produce the “anomaly kick” to the field configuration of the neutral pion field. We implement the effect of the anomaly kick in our numerical simulation of the linear sigma model in a schematic way which preserves its characteristic features: the effect is coherent over a large region of space but is opposite in sign above and below the ion scattering plane. We demonstrate by detailed simulations with longitudinal expansion that the DCC domain formation is dramatically enhanced by the anomaly kick in spite of its small absolute magnitude. We examine the behavior of various physical quantities such as pion fields, the axial vector currents, and their correlation functions. Our results also provide useful insight into the mechanism and properties of DCC domain formation, in general. Finally, we discuss some experimental observables which can signal the anomaly induced formation of DCC.

Competition among particle evaporation, temperature gradient, and flow is investigated in a phenomenological manner, based on a simultaneous analysis of quantum statistical correlations and momentum distributions for a nonrelativistic, spherically symmetric, three-dimensionally expanding, finite source. The parameters of the model emission function are constrained by fits to neutron and proton momentum distributions and correlation functions in intermediate-energy heavy-ion collisions. The temperature gradient is related to the momentum dependence of the radius parameters of the two-particle correlation function, as well as to the momentum-dependent temperature parameter of the single particle spectrum, while a long duration of particle evaporation is found to be responsible for the low relative momentum behavior of the two-particle correlations.

As long as a rapid change of the entropy density s(T) takes place across the critical temperature T_c of the QCD phase transition, the pressure P(T), and the energy density e(T) normalized by their Stefan-Boltzmann values generally deviate from unity for T≫T_c, even if there are no interactions among quarks and gluons at T>T_c. We shall demonstrate this both analytically and numerically for a general class of s(T) consistent with thermodynamical constraints and make a qualitative comparison of the result with the lattice QCD data. Quantities related to ds(T)/dT such as the specific heat and sound velocity are also discussed.

We demonstrate that fluctuations, their relaxation, and the chiral phase transition are automatically incorporated in the numerical simulations of the classical equations of motion in the linear σ model when longitudinal and transverse expansions are included. We find that domains of disoriented chiral condensate with 4–5 fm in size can form through a quench while an annealing leads to domains of smaller sizes. We also demonstrate that quenching cannot be achieved by relaxing a chirally symmetric system through expansion.

A comment on the Letter by K. Geiger.

In a previous paper, we have shown that a double phi peak structure appears in the dilepton invariant mass spectrum if a first order QCD phase transition occurs in ultrarelativistic heavy-ion collisions. Furthermore, the transition temperature can be determined from the transverse momentum distribution of the low mass phi peak. In this work, we extend the study to the case that a smooth crossover occurs in the quark-gluon plasma to the hadronic matter transition. We find that the double phi peak structure still exists in the dilepton spectrum and thus remains a viable signal for the formation of the quark-gluon plasma in ultrarelativistic heavy-ion collisions.

The property of a rho meson in dense nuclear matter is studied using the QCD sum rule. The spectral function appearing on the hadronic side of the sum is evaluated in the vector dominance model that takes into account the interaction between the rho meson and the pion. Including pion modification by the delta-hole polarization in the nuclear medium, we find that as the nuclear density increases the rho meson peak in the spectral function shifts to smaller invariant masses and its width becomes smaller. We discuss the possibility of studying the rho meson property in dense matter via the dilepton invariant mass spectrum from heavy-ion collisions.

In general, the spectrum of lepton pairs produced in nuclear reactions depends on both invariant mass and momentum. But under a few reasonable assumptions on the time evolution of the system, we show that once the quark-gluon plasma is created in ultrarelativistic heavy ion collisions, the observed dilepton spectrum between the φ and J/ψ peak becomes dependent essentially only on its transverse mass M_T and thus shows M_T scaling. This scaling will not be observed if the quark-gluon plasma is not created in collisions.

The spectral function of a rho meson that is at rest in dense hadronic matter and couples strongly to the pion is studied in the vector dominance model by including the effect of the delta-hole polarization of the pion. With the free rho-meson mass in the Lagrangian, we find that both the rho-meson peak and width increase with increasing nuclear density, and that a low-mass peak appears at invariant mass around three times the pion mass. Including the decreasing density-dependent hadron masses in the Lagrangian as suggested by the scaling law of Brown and Rho, we find instead that the rho peak moves to smaller invariant masses with diminishing strength when the nuclear density increases. The low-mass peak also shifts down with increasing density and becomes more pronounced. The relevance of the rho-meson property in dense matter to delepton production in heavy-ion collisions is discussed.

In the Walecka model, the antilambda mass in dense nuclear matter is smaller than its value in free space. This reduces the threshold for antilambda production in dense matter that forms in the compression stage of ultrarelativistic heavy ion collisions. Because of the large number of mesons produced in the collision, the process KM→Λ¯N, where M denotes either a pion or a rho meson, is shown to be important and provides a plausible explanation for the observed enhancement of antilambda yield in recent experiments carried out at CERN SPS with nuclear beams.

Dilepton production from a nonequilibrium quark-gluon plasma produced in ultrarelativistic nucleus-nucleus collisions is studied, based on the flux-tube model which has been formulated in terms of a model relativistic Boltzmann-Vlasov equation with a boost-invariant particle source term. This model kinetic equation is solved with the collision term in the relaxation-time approximation. In the collisionless limit, the solution of the kinetic equation becomes oscillatory, indicating the spontaneous excitation of the plasma oscillation. We study how such a nonequilibrium evolution of the system is reflected in the dilepton spectrum. It is shown that the dileptons emitted during such a nonequilibrium stage of matter evolution does not necessarily lead to a spectrum which interpolates smoothly the spectrum from thermal emission and that from the Drell-Yan mechanism. Our results show, on the contrary, that the dilepton spectrum is softened significantly in the absence of thermalization. M_T scaling, which is expected in scaling one-dimensional hydrodynamic evolution, is also broken due to the anisotropy in the phase-space distribution of quarks.