Anomalous Transport Processes in Anisotropically Expanding Quark-Gluon Plasmas

We derive an expression for the anomalous viscosity in an anisotropically expanding quark-gluon-plasma, which arises from interactions of thermal partons with dynamically generated color fields. The anomalous viscosity dominates over the collisional viscosity for large velocity gradients or weak coupling. This effect may provide an explanation for the apparent ``nearly perfect'' liquidity of the matter produced in nuclear collisions at RHIC without the assumption that it is a strongly coupled state.Comment: 31 pages, 1 figure, some typos in published version are correcte

Non-Gaussian fluctuations near the QCD critical point

We study the effect of the QCD critical point on non-Gaussian moments (cumulants) of fluctuations of experimental observables in heavy-ion collisions. We find that these moments are very sensitive to the proximity of the critical point, as measured by the magnitude of the correlation length xi. For example, the cubic central moment of multiplicity ~ xi^4.5 and the quartic cumulant ~ xi^7. We estimate the magnitude of critical point contributions to non-Gaussian fluctuations of pion and proton multiplicities.Comment: 4 pages, 3 figure

Scale Transformations on the Noncommutative Plane and the Seiberg-Witten Map

We write down three kinds of scale transformations {\tt i-iii)} on the noncommutative plane. {\tt i)} is the analogue of standard dilations on the plane, {\tt ii)} is a re-scaling of the noncommutative parameter $\theta$, and {\tt iii)} is a combination of the previous two, whereby the defining relations for the noncommutative plane are preserved. The action of the three transformations is defined on gauge fields evaluated at fixed coordinates and $\theta$. The transformations are obtained only up to terms which transform covariantly under gauge transformations. We give possible constraints on these terms. We show how the transformations {\tt i)} and {\tt ii)} depend on the choice of star product, and show the relation of {\tt ii)} to Seiberg-Witten transformations. Because {\tt iii)} preserves the fundamental commutation relations it is a symmetry of the algebra. One has the possibility of implementing it as a symmetry of the dynamics, as well, in noncommutative field theories where $\theta$ is not fixed.Comment: 20 page

Stringy Derivation of Nahm Construction of Monopoles

We derive the Nahm construction of monopoles from exact tachyon condensation on unstable D-branes. The Dirac operator used in the Nahm construction is identified with the tachyon profile in our D-brane approach, and we provide physical interpretation of the procedures Nahm gave. Crucial is the introduction of infinite number of brane-antibranes from which arbitrary D-brane can be constrcuted, exhibitting a unified view of various D-branes. We explicitly show the equivalence of the D3-brane boundary state with the monopole profile and the D1-brane boundary state with the Nahm data as transverse scalars.Comment: 18 pages, 4 eps figures, JHEP style, comments about low energy limits added, references adde

Center Domains and their Phenomenological Consequences

We argue that the domain structure of deconfined QCD matter, which can be inferred from the properties of the Polyakov loop, can simultaneously explain the two most prominent experimentally verified features of the quark-gluon plasma, namely its large opacity as well as its near ideal fluid properties

Noncommutativity and Tachyon Condensation

We study the fuzzy or noncommutative Dp-branes in terms of infinitely many unstable D0-branes, from which we can construct any Dp-branes. We show that the tachyon condensation of the unstable D0-branes induces the noncommutativity. In the infinite tachyon condensation limit, most of the unstable D0-branes disappear and remaining D0-branes are actually the BPS D0-branes with the correct noncommutative coordinates. For the fuzzy S^2 case, we explicitly show only the D0-branes corresponding to the lowest Landau level survive in the limit. We also show that a boundary state for a Dp-brane satisfying the Dirichlet boundary condition on a curved submanifold embedded in the flat space is not localized on the submanifold. This implies that the Dp-brane on it is ambiguous at the string scale and solves the problem for a spherical D2-brane with a unit flux on the world volume which should be equivalent to one D0-brane. We also discuss the diffeomorphism in the D0-brane picture.Comment: 30 pages, references added, minor corrections and clarifications, version to appear in JHE

Anomalous Viscosity of an Expanding Quark-Gluon Plasma

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.Comment: Final version accepted for publicatio

Supertubes in Matrix model and DBI action

We show the equivalence between the supertube solutions with an arbitrary cross section in two different actions, the DBI action for the D2-brane and the matrix model action for the D0-branes. More precisely, the equivalence between the supertubes in the D2-brane picture and the D0-brane picture is shown in the boundary state formalism which is valid for all order in \alpha'. This is an application of the method using the infinitely many D0-branes and anti-D0-branes which was used to show other equivalence relations between two seemingly different D-brane systems, including the D-brane realization of the ADHM construction of instanton. We also apply this method to the superfunnel type solutions successfully.Comment: 24 pages, references added, version to appear in JHE

Open String Tachyon in Supergravity Solution

We study the tachyon condensation of the D-\bar{D}-brane system with a constant tachyon vev in the context of classical solutions of the Type II supergravity. We find that the general solution with the symmetry ISO(1,p)xSO(9-p) (the three-parameter solution) includes the extremal black p-brane solution as an appropriate limit of the solution with fixing one of the three parameters (c_1). Furthermore, we compare the long distance behavior of the solution with the massless modes of the closed strings from the boundary state of the D-\bar{D}-brane system with a constant tachyon vev. We find that we must fix c_1 to zero and the only two parameters are needed to express the tachyon condensation of the D\={D}-brane system. This means that the parameter $c_1$ does not correspond to the tachyon vev of the D\={D}-brane system.Comment: 20 pages, no figures, LaTeX2e, typos corrected, references added and more general result presente