8898134
Localized tachyons and RG flows
We study condensation of closed string tachyons living on defects, such as orbifold fixed planes and Neveu-Schwarz fivebranes. We argue that the high energy density of localized states decreases in the process of condensation of such tachyons. In some cases this means that $c_{eff}$ decreases along the flow; in others, $c_{eff}$ remains constant and the decreasing quantity is a closed string analog, $g_{cl}$, of the ``boundary entropy'' of D-branes. We discuss the non-supersymmetric orbifolds $C/Z_n$ and $C^2/Z_n$. In the first case tachyon condensation decreases $n$ and in some cases connects type II and type 0 vacua. In the second case non-singular orbifolds are related by tachyon condensation to both singular and non-singular ones. We verify that $g_{cl}$ decreases in flows between non-singular orbifolds. The main tools in the analysis are the structure of the chiral ring of the perturbed theory, the geometry of the resolved orbifold singularities, and the throat description of singular conformal field theories.
124306496
The M theory three form and E(8) gauge theory
118337669
Symmetries of the Bosonic String S-Matrix
The bracket operation on mutually local BRST classes may be combined with Lorentz invariance and analyticity to write an infinite set of finite difference relations on string scattering amplitudes. When combined with some simple physical criteria these relations uniquely determine the genus zero string $S$-matrix for $N\leq 26$-particle scattering in $\IR^{25,1}$ in terms of a single parameter, $\kappa$, the string coupling. We propose that the high-energy limit of the relations are the Ward identities for the high-energy symmetries of string theory.
122595912
Anomalies in Nonlinear $\sigma$ Models
Certain nonlinear sigma models with fermions suffer from an anomaly similar to the one in non-Abelian gauge theory. We exhibit this anomaly using both perturbative and global methods. The affected theories are ill defined and hence unsuitable for describing low-energy dynamics. They include certain supersymmetric models in four space-time dimensions.
14205277
Melvin Models and Diophantine Approximation
Melvin models with irrational twist parameter provide an interesting example of conformal field theories with non-compact target space, and localized states which are arbitrarily close to being delocalized. We study the torus partition sum of these models, focusing on the properties of the regularized dimension of the space of localized states. We show that its behavior is related to interesting arithmetic properties of the twist parameter γ, such as the Lyapunov exponent. Moreover, for γ in a set of measure one the regularized dimension is in fact not a well-defined number but must be considered as a random variable in a probability distribution.
15868022
Donaldson Invariants for Non-Simply Connected Manifolds
Abstract:We study Coulomb branch (“u-plane”) integrals for supersymmetric SU(2),SO(3) Yang–Mills theory on 4-manifolds X of b1(X)>0, b2+(X)= 1. Using wall-crossing arguments we derive expressions for the Donaldson invariants for manifolds with b1(X)>0, b2+(X)>0. Explicit expressions for , where Fg is a Riemann surface of genus g are obtained using Kronecker's double series identity. The result might be useful in future studies of quantum cohomology.
15369975
Motion of Chern-Simons number at high temperatures under a chemical potential
I investigate the evolution of finite temperature, classical Yang-Mills field equations under the influence of a chemical potential for Chern-Simons number Ncs. The rate of Ncs diffusion,, Γd, and the linear response of Ncs to a chemical potential, Γμ, are both computed; the relation Γd = 2Γμ is satisfied numerically and the results agree with the recent measurement of Γd by Ambjorn and Krasnitz. The response of Ncs under chemical potential remains linear at least to μ = 6T, which is impossible if there is a free energy barrier to the motion of Ncs. The possibility that the result depends on lattice artefacts via hard thermal loops is investigated by changing the lattice action and by examining elongated rectangular lattices; provided that the lattice is fine enough, the result is weakly if at all dependent on the specifics of the cutoff. I also compare SU(2) with SU(3) and find ΓSU(3)∼ 7(αs/αw)4ΓSU(2).
10085298
Defect formation with bulk fields
It has recently been realized that brane-antibrane annihilation (a possible explanation for ending inflation) may result in defect formation, due to the dynamics of the tachyon field. Studies of this possibility have generally ignored the interaction of the brane fields with fields in the bulk; recently it has been argued [G. Dvali and A. Vilenkin, J. Cosmol. Astropart. Phys. 03 (2004) 010.] that interactions with bulk fields suppress or even eliminate defect formation. To investigate the impact of bulk fields on brane defect formation, we construct a toy model that captures the essential features of the tachyon condensation with bulk fields. We study the structure of defects in this toy model, and simulate their formation and evolution on the lattice. We find that, while the energetics and interactions of defects are influenced by the size of the extra dimension and the bulk-brane coupling, the bulk-brane coupling does not prevent the formation of a defect network.
15211002
Jet energy loss and high pT photon production in hot quark-gluon plasma
Abstract Jet-quenching and photon production at high transverse momentum are studied at RHIC energies, together with the correlation between jets and photons. The energy loss of hard partons traversing the hot QGP is evaluated in the AMY formalism, consistently taking into account both induced gluon emission and elastic collisions. The production of high p T photons in Au+Au collisions is calculated, incorporating a complete set of photon-production channels. Putting all these ingredients together with a (3+1)-dimensional ideal relativistic hydrodynamical description of the thermal medium, we achieve a good description of the current experimental data. Our results illustrate that the interaction between hard jets and the soft medium is important for a complete understanding of jet quenching, photon production, and photon-hadron correlations in relativistic nuclear collisions.
16198175
Electromagnetic emission and energy loss in the QGP
I discuss why photon production from the quark-gluon plasma (QGP) presents an interesting problem, both experimentally and theoretically. I show how the photon emission rate can be computed under the simplifying assumption that the QGP fully thermalizes. The theoretical issues are very similar to those for jet energy loss; so it should be possible to treat them in a common formalism and relate the predictions of one phenomenon to those of the other.
11641900
A way to estimate the heavy quark thermalization rate from the lattice
The thermalization rate of a heavy quark is related to its momentum diffusion coefficient. Starting from a Kubo relation and using the framework of the heavy quark effective theory, we argue that in the large-mass limit the momentum diffusion coefficient can be defined through a certain Euclidean correlation function, involving color-electric fields along a Polyakov loop. Furthermore, carrying out a perturbative computation, we demonstrate that the spectral function corresponding to this correlator is relatively flat at small frequencies. Therefore, unlike in the case of several other transport coefficients, for which the narrowness of the transport peak makes analytic continuation from Euclidean lattice data susceptible to severe systematic uncertainties, it appears that the determination of the heavy quark thermalization rate could be relatively well under control.