Boundary K-Matrices for the Six Vertex and the n(2n-1) A_{n-1} Vertex Models

Boundary conditions compatible with integrability are obtained for two dimensional models by solving the factorizability equations for the reflection matrices $K^{\pm}(\theta)$. For the six vertex model the general solution depending on four arbitrary parameters is found. For the $A_{n-1}$ models all diagonal solutions are found. The associated integrable magnetic Hamiltonians are explicitly derived.Comment: 9 pages,latex, LPTHE-PAR 92-4

Finite-temperature Screening and the Specific Heat of Doped Graphene Sheets

At low energies, electrons in doped graphene sheets are described by a massless Dirac fermion Hamiltonian. In this work we present a semi-analytical expression for the dynamical density-density linear-response function of noninteracting massless Dirac fermions (the so-called "Lindhard" function) at finite temperature. This result is crucial to describe finite-temperature screening of interacting massless Dirac fermions within the Random Phase Approximation. In particular, we use it to make quantitative predictions for the specific heat and the compressibility of doped graphene sheets. We find that, at low temperatures, the specific heat has the usual normal-Fermi-liquid linear-in-temperature behavior, with a slope that is solely controlled by the renormalized quasiparticle velocity.Comment: 9 pages, 5 figures, Submitted to J. Phys.

Renormalization of Hamiltonian Field Theory; a non-perturbative and non-unitarity approach

Renormalization of Hamiltonian field theory is usually a rather painful algebraic or numerical exercise. By combining a method based on the coupled cluster method, analysed in detail by Suzuki and Okamoto, with a Wilsonian approach to renormalization, we show that a powerful and elegant method exist to solve such problems. The method is in principle non-perturbative, and is not necessarily unitary.Comment: 16 pages, version shortened and improved, references added. To appear in JHE

Exact Floquet states of a driven condensate and their stabilities

We investigate the Gross-Pitaevskii equation for a classically chaotic system, which describes an atomic Bose-Einstein condensate confined in an optical lattice and driven by a spatiotemporal periodic laser field. It is demonstrated that the exact Floquet states appear when the external time-dependent potential is balanced by the nonlinear mean-field interaction. The balance region of parameters is divided into a phase-continuing region and a phase-jumping one. In the latter region, the Floquet states are spatiotemporal vortices of nontrivial phase structures and zero-density cores. Due to the velocity singularities of vortex cores and the blowing-up of perturbed solutions, the spatiotemporal vortices are unstable periodic states embedded in chaos. The stability and instability of these Floquet states are numerically explored by the time evolution of fidelity between the exact and numerical solutions. It is numerically illustrated that the stable Floquet states could be prepared from the uniformly initial states by slow growth of the external potential.Comment: 14 pages, 3 eps figures, final version accepted for publication in J. Phys.

Polymer quantization of the free scalar field and its classical limit

Building on prior work, a generally covariant reformulation of free scalar field theory on the flat Lorentzian cylinder is quantized using Loop Quantum Gravity (LQG) type `polymer' representations. This quantization of the {\em continuum} classical theory yields a quantum theory which lives on a discrete spacetime lattice. We explicitly construct a state in the polymer Hilbert space which reproduces the standard Fock vacuum- two point functions for long wavelength modes of the scalar field. Our construction indicates that the continuum classical theory emerges under coarse graining. All our considerations are free of the "triangulation" ambiguities which plague attempts to define quantum dynamics in LQG. Our work constitutes the first complete LQG type quantization of a generally covariant field theory together with a semi-classical analysis of the true degrees of freedom and thus provides a perfect infinite dimensional toy model to study open issues in LQG, particularly those pertaining to the definition of quantum dynamics.Comment: 58 page

Stochastic String Motion Above and Below the World Sheet Horizon

We study the stochastic motion of a relativistic trailing string in black hole AdS_5. The classical string solution develops a world-sheet horizon and we determine the associated Hawking radiation spectrum. The emitted radiation causes fluctuations on the string both above and below the world-sheet horizon. In contrast to standard black hole physics, the fluctuations below the horizon are causally connected with the boundary of AdS. We derive a bulk stochastic equation of motion for the dual string and use the AdS/CFT correspondence to determine the evolution a fast heavy quark in the strongly coupled $\N=4$ plasma. We find that the kinetic mass of the quark decreases by $\Delta M=-\sqrt{\gamma \lambda}T/2$ while the correlation time of world sheet fluctuations increases by $\sqrt{\gamma}$.Comment: 27 pages, 5 figures; v2 final version, small changes, references adde

Alternative approach to b−>sγb->s \gamma in the uMSSM

The gluino contributions to the $C'_{7,8}$ Wilson coefficients for $b->s \gamma$ are calculated within the unconstrained MSSM. New stringent bounds on the $\delta^{RL}_{23}$ and $\delta^{RR}_{23}$ mass insertion parameters are obtained in the limit in which the SM and SUSY contributions to $C_{7,8}$ approximately cancel. Such a cancellation can plausibly appear within several classes of SUSY breaking models in which the trilinear couplings exhibit a factorized structure proportional to the Yukawa matrices. Assuming this cancellation takes place, we perform an analysis of the $b->s \gamma$ decay. We show that in a supersymmetric world such an alternative is reasonable and it is possible to saturate the $b->s \gamma$ branching ratio and produce a CP asymmetry of up to 20%, from only the gluino contribution to $C'_{7,8}$ coefficients. Using photon polarization a LR asymmetry can be defined that in principle allows for the $C_{7,8}$ and $C'_{7,8}$ contributions to the $b->s \gamma$ decay to be disentangled. In this scenario no constraints on the ``sign of $\mu$'' can be derived.Comment: LaTeX2e, 23 pages, 7 ps figure, needs package epsfi

AdS and pp-wave D-particle superalgebras

We derive anticommutators of supercharges with a brane charge for a D-particle in AdS(2) x S(2) and pp-wave backgrounds. A coset GL(2|2)/(GL(1))^4 and its Penrose limit are used with the supermatrix-valued coordinates for the AdS and the pp-wave spaces respectively. The brane charges have position dependence, and can be absorbed into bosonic generators by shift of momenta which results in closure of the superalgebras.Comment: 15 page

Closed String Field Theory with Dynamical D-brane

We consider a closed string field theory with an arbitrary matter current as a source of the closed string field. We find that the source must satisfy a constraint equation as a consequence of the BRST invariance of the theory. We see that it corresponds to the covariant conservation law for the matter current, and the equation of motion together with this constraint equation determines the classical behavior of both the closed string field and the matter. We then consider the boundary state (D-brane) as an example of a source. We see that the ordinary boundary state cannot be a source of the closed string field when the string coupling g turns on. By perturbative expansion, we derive a recursion relation which represents the bulk backreaction and the D-brane recoil. We also make a comment on the rolling tachyon boundary state.Comment: 30 pages, LaTeX2e, no figures. Typos are correcte