Analysis of the Rapid Central Melt Pool Flow in Hybrid Laser-Arc Welding

AbstractHybrid laser arc welding creates a long weld pool tail. By high speed imaging the melt velocity behind the keyhole was measured to be very high, of the order of meters per second. Fluid dynamics simulation was carried out locally in the central axial plane of the pool tail. The high speed melt layer redistributes its momentum to slow movement of the deeper bulk. The consequences of initially high melt speed and its mass flow redistribution on the formation of the central reinforcement peak are discussed

Classical stability of U(1)_A domain walls in dense matter QCD

It was recently shown that there exists metastable U(1)_A domain wall configurations in high-density QCD (\mu >> 1 GeV). In the following we will assess the stability of such non-trivial field configurations at intermediate densities (\mu < 1 GeV). The existence of such configurations at intermediate densities could have interesting consequences for the physics of neutron stars with high core density.Comment: 13 pages, 2 Postscript figures, typos correcte

Chiral 2π2\pi-exchange NN-potentials: Two-loop contributions

We calculate in heavy baryon chiral perturbation theory the local NN-potentials generated by the two-pion exchange diagrams at two-loop order. We give explicit expressions for the mass-spectra (or imaginary parts) of the corresponding isoscalar and isovector central, spin-spin and tensor NN-amplitudes. We find from two-loop two-pion exchange a sizeable isoscalar central repulsion which amounts to $62.3$MeV at $r=1.0$fm. There is a similarly strong isovector central attraction which however originates mainly from the third order low energy constants $\bar d_j$ entering the chiral $\pi N$-scattering amplitude. We also evaluate the one-loop $2\pi$-exchange diagram with two second order chiral $\pi \pi NN$-vertices proportional to the low energy constants $c_{1,2,3,4}$ as well as the first relativistic 1/M-correction to the $2\pi$-exchange diagrams with one such vertex. The diagrammatic results presented here are relevant components of the chiral NN-potential at next-to-next-to-next-to-leading order.Comment: 6 pages, 2 figure

Regularization of Non-commutative SYM by Orbifolds with Discrete Torsion and SL(2,Z) Duality

We construct a nonperturbative regularization for Euclidean noncommutative supersymmetric Yang-Mills theories with four (N= (2,2)), eight (N= (4,4)) and sixteen (N= (8,8)) supercharges in two dimensions. The construction relies on orbifolds with discrete torsion, which allows noncommuting space dimensions to be generated dynamically from zero dimensional matrix model in the deconstruction limit. We also nonperturbatively prove that the twisted topological sectors of ordinary supersymmetric Yang-Mills theory are equivalent to a noncommutative field theory on the topologically trivial sector with reduced rank and quantized noncommutativity parameter. The key point of the proof is to reinterpret 't Hooft's twisted boundary condition as an orbifold with discrete torsion by lifting the lattice theory to a zero dimensional matrix theory.Comment: 36 pages, references added, minor typos fixe

Drum vortons in high density QCD

Recently it was shown that high density QCD supports of number of topological defects. In particular, there are U(1)_Y strings that arise due to K^0 condensation that occurs when the strange quark mass is relatively large. The unique feature of these strings is that they possess a nonzero K^+ condensate that is trapped on the core. In the following we will show that these strings (with nontrivial core structure) can form closed loops with conserved charge and currents trapped on the string worldsheet. The presence of conserved charges allows these topological defects, called vortons, to carry angular momentum, which makes them classically stable objects. We also give arguments demonstrating that vortons carry angular momentum very efficiently (in terms of energy per unit angular momentum) such that they might be the important degrees of freedom in the cores of neutron stars.Comment: 11 pages, accepted for publication in Physical Review

The hyperon-nucleon interaction: conventional versus effective field theory approach

Hyperon-nucleon interactions are presented that are derived either in the conventional meson-exchange picture or within leading order chiral effective field theory. The chiral potential consists of one-pseudoscalar-meson exchanges and non-derivative four-baryon contact terms. With regard to meson-exchange hyperon-nucleon models we focus on the new potential of the Juelich group, whose most salient feature is that the contributions in the scalar--isoscalar (\sigma) and vector--isovector (\rho) exchange channels are constrained by a microscopic model of correlated \pi\pi and KKbar exchange.Comment: 28 pages, 8 figures, submitted to Lecture Notes in Physic

Maximal Neutrino Mixing from a Minimal Flavor Symmetry

We study a number of models, based on a non-Abelian discrete group, that successfully reproduce the simple and predictive Yukawa textures usually associated with U(2) theories of flavor. These models allow for solutions to the solar and atmospheric neutrino problems that do not require altering successful predictions for the charged fermions or introducing sterile neutrinos. Although Yukawa matrices are hierarchical in the models we consider, the mixing between second- and third-generation neutrinos is naturally large. We first present a quantitative analysis of a minimal model proposed in earlier work, consisting of a global fit to fermion masses and mixing angles, including the most important renormalization group effects. We then propose two new variant models: The first reproduces all important features of the SU(5)xU(2) unified theory with neither SU(5) nor U(2). The second demonstrates that discrete subgroups of SU(2) can be used in constructing viable supersymmetric theories of flavor without scalar universality even though SU(2) by itself cannot.Comment: 34 pages LaTeX, 1 eps figure, minor revisions and references adde

U(2)-like Flavor Symmetries and Approximate Bimaximal Neutrino Mixing

Models involving a U(2) flavor symmetry, or any of a number of its non-Abelian discrete subgroups, can explain the observed hierarchy of charged fermion masses and CKM angles. It is known that a large neutrino mixing angle connecting second and third generation fields may arise via the seesaw mechanism in these models, without a fine tuning of parameters. Here we show that it is possible to obtain approximate bimaximal mixing in a class of models with U(2)-like Yukawa textures. We find a minimal form for Dirac and Majorana neutrino mass matrices that leads to two large mixing angles, and show that our result can quantitatively explain atmospheric neutrino oscillations while accommodating the favored, large angle MSW solution to the solar neutrino problem. We demonstrate that these textures can arise in models by presenting a number of explicit examples.Comment: 20 pages RevTex4, 2 figure

Z^* Resonances: Phenomenology and Models

We explore the phenomenology of, and models for, the Z^* resonances, the lowest of which is now well established, and called the Theta. We provide an overview of three models which have been proposed to explain its existence and/or its small width, and point out other relevant predictions, and potential problems, for each. The relation to what is known about KN scattering, including possible resonance signals in other channels, is also discussed.Comment: 29 pages, uses RevTeX4; expanded version (published form

Global Strings in High Density QCD

We show that several types of global strings occur in colour superconducting quark matter due to the spontaneous violation of relevant U(1) symmetries. These include the baryon U(1)_B, and approximate axial U(1)_A symmetries as well as an approximate U(1)_S arising from kaon condensation. We discuss some general properties of these strings and their interactions. In particular, we demonstrate that the U(1)_A strings behave as superconducting strings. We draw some parallels between these strings and global cosmological strings and discuss some possible implications of these strings to the physics in neutron star cores.Comment: LaTeX JHEP-format (26 pages) Option in source for REVTeX4 forma