The propagator for the step potential using the path decomposition expansion

We present a direct path integral derivation of the propagator in the presence of a step potential. The derivation makes use of the Path Decomposition Expansion (PDX), and also of the definition of the propagator as a limit of lattice paths.Comment: To appear in DICE 2008 conference proceeding

Numerically determined transport laws for fingering ("thermohaline") convection in astrophysics

We present the first three-dimensional simulations of fingering convection performed in a parameter regime close to the one relevant for astrophysics, and reveal the existence of simple asymptotic scaling laws for turbulent heat and compositional transport. These laws can straightforwardly be extrapolated to the true astrophysical regime. Our investigation also indicates that thermocompositional "staircases," a key consequence of fingering convection in the ocean, cannot form spontaneously in the fingering regime in stellar interiors. Our proposed empirically-determined transport laws thus provide simple prescriptions for mixing by fingering convection in a variety of astrophysical situations, and should, from here on, be used preferentially over older and less accurate parameterizations. They also establish that fingering convection does not provide sufficient extra mixing to explain observed chemical abundances in RGB stars.Comment: Submitted to ApJ Letters on October 29th. 15 pages, 4 figures. See Garaud 2010 for companion pape

Novel Six-Quark Hidden-Color Dibaryon States in QCD

The recent observation of a hadronic resonance $d^*$ in the proton-neutron system with isospin $I = 0$ and spin-parity $J^P = 3^+$ raises the possibility of producing other novel six-quark dibaryon configurations allowed by QCD. A dramatic example of an exotic six-quark color-singlet system is the charge $Q=+4$, isospin I=3, $I^z=+3$ $|uuuuuu>$ state which couples strongly to $\Delta^{++}$ + $\Delta^{++} .$ The width and decay properties of such six-quark resonances could be regarded as manifestations of "hidden-color" six-quark configurations, a first-principle prediction of QCD -- SU(3)-color gauge theory for the deuteron distribution amplitude. Other implications and possible future experiments are discussed

Higher-Twist Dynamics in Large Transverse Momentum Hadron Production

A scaling law analysis of the world data on inclusive large-pT hadron production in hadronic collisions is carried out. A significant deviation from leading-twist perturbative QCD predictions at next-to-leading order is reported. The observed discrepancy is largest at high values of xT=2pT/sqrt(s). In contrast, the production of prompt photons and jets exhibits the scaling behavior which is close to the conformal limit, in agreement with the leading-twist expectation. These results bring evidence for a non-negligible contribution of higher-twist processes in large-pT hadron production in hadronic collisions, where the hadron is produced directly in the hard subprocess rather than by gluon or quark jet fragmentation. Predictions for scaling exponents at RHIC and LHC are given, and it is suggested to trigger the isolated large-pT hadron production to enhance higher-twist processes.Comment: 5 pages, 4 figures. Extended introduction, additional reference

Periodicity of mass extinctions without an extraterrestrial cause

We study a lattice model of a multi-species prey-predator system. Numerical results show that for a small mutation rate the model develops irregular long-period oscillatory behavior with sizeable changes in a number of species. The periodicity of extinctions on Earth was suggested by Raup and Sepkoski but so far is lacking a satisfactory explanation. Our model indicates that this is a natural consequence of the ecosystem dynamics, not the result of any extraterrestrial cause.Comment: 4 pages, accepted in Phys.Rev.

DATAC bus monitor

The Digital Autonomous Terminal Access Communications (DATAC) bus is a multiple transmitter data bus developed to interconnect various aircraft systems. A typical DATAC bus installation for the NASA B-737 aircraft is shown. A bus monitor unit was developed for the DATAC data bus. The bus monitor unit consists of two systems: A Zilog Z8002 based S100 bus microcomputer system that connects to a DATAC terminal and receives data appearing on the bus, and an IMB PC-compatible computer used to display and interpret that data. Test results of the existing bus monitor have indicated areas for improvement. The most obvious improvement is to increase the speed of the data transfers between the S100 and the MS-DOS systems

An L1 Penalty Method for General Obstacle Problems

We construct an efficient numerical scheme for solving obstacle problems in divergence form. The numerical method is based on a reformulation of the obstacle in terms of an L1-like penalty on the variational problem. The reformulation is an exact regularizer in the sense that for large (but finite) penalty parameter, we recover the exact solution. Our formulation is applied to classical elliptic obstacle problems as well as some related free boundary problems, for example the two-phase membrane problem and the Hele-Shaw model. One advantage of the proposed method is that the free boundary inherent in the obstacle problem arises naturally in our energy minimization without any need for problem specific or complicated discretization. In addition, our scheme also works for nonlinear variational inequalities arising from convex minimization problems.Comment: 20 pages, 18 figure

Light-Front Holography: A First Approximation to QCD

Starting from the Hamiltonian equation of motion in QCD, we identify an invariant light-front coordinate $\zeta$ which allows the separation of the dynamics of quark and gluon binding from the kinematics of constituent spin and internal orbital angular momentum. The result is a single variable light-front Schrodinger equation for QCD which determines the eigenspectrum and the light-front wavefunctions of hadrons for general spin and orbital angular momentum. This light-front wave equation is equivalent to the equations of motion which describe the propagation of spin-$J$ modes on anti-de Sitter (AdS) space.Comment: 4 pages. The limits of validity of the model are further discussed. To appear in Physical Review Letter

Higher-twist contributions to large pT hadron production in hadronic collisions

The scaling behavior of large-pT hadron production in hadronic collisions is investigated. A significant deviation from the NLO QCD predictions is reported, especially at high values of xT=2pT/sqrt(s). In contrast, the prompt photon and jet production data prove in agreement with leading-twist expectations. These results are interpreted as coming from a non-negligible contribution of higher-twist processes, where the hadron is produced directly in the hard subprocess. Predictions for scaling exponents at RHIC are successfully compared to PHENIX preliminary measurements. We suggest to trigger on isolated large-pT hadron production to enhance higher-twist processes, and point that the use of isolated hadrons as a signal for new physics at colliders can be affected by the presence of direct hadron production processes.Comment: 4 pages, 2 figures. Talk given at the 45th Rencontres de Moriond QCD and High Energy Interactions, La Thuile, Italy, 13-20 March 201