31,620 research outputs found
The nature of turbulence in OMC1 at the star forming scale: observations and simulations
Aim: To study turbulence in the Orion Molecular Cloud (OMC1) by comparing
observed and simulated characteristics of the gas motions.
Method: Using a dataset of vibrationally excited H2 emission in OMC1
containing radial velocity and brightness which covers scales from 70AU to
30000AU, we present the transversal structure functions and the scaling of the
structure functions with their order. These are compared with the predictions
of two-dimensional projections of simulations of supersonic hydrodynamic
turbulence.
Results: The structure functions of OMC1 are not well represented by power
laws, but show clear deviations below 2000AU. However, using the technique of
extended self-similarity, power laws are recovered at scales down to 160AU. The
scaling of the higher order structure functions with order deviates from the
standard scaling for supersonic turbulence. This is explained as a selection
effect of preferentially observing the shocked part of the gas and the scaling
can be reproduced using line-of-sight integrated velocity data from subsets of
supersonic turbulence simulations. These subsets select regions of strong flow
convergence and high density associated with shock structure. Deviations of the
structure functions in OMC1 from power laws cannot however be reproduced in
simulations and remains an outstanding issue.Comment: 12 pages, 8 figures, accepted A&A. Revised in response to referee.
For higher resolution, see http://www.astro.phys.au.dk/~maikeng/sim_paper
Pion and Kaon Production in Nucleon - Nucleon Collisions
Inclusive cross section for pion production in proton - proton collisions are
calculated based on unintegrated parton distribution functions (uPDFs). In
addition to purely gluonic terms the present approach includes also quark
degrees of freedom. Phenomenological fragmentation functions from the
literature are used. The new mechanisms are responsible for -
asymmetry. In contrast to standard collinear approach, application of 2 1
- factorization approach can be extended towards much lower transverse
momenta, both at mid and forward rapidity region. The results of the
calculation are compared with SPS and RHIC data.Comment: a talk presented by Marta Tichoruk at the international conference
MESON2006, Cracow, June 2006, 5 pages, 3 figure
Derivation of the Lorentz Force Law, the Magnetic Field Concept and the Faraday-Lenz Law using an Invariant Formulation of the Lorentz Transformation
It is demonstrated how the right hand sides of the Lorentz Transformation
equations may be written, in a Lorentz invariant manner, as 4--vector scalar
products. This implies the existence of invariant length intervals analogous to
invariant proper time intervals. This formalism, making essential use of the
4-vector electromagnetic potential concept, provides a short derivation of the
Lorentz force law of classical electrodynamics, the conventional definition of
the magnetic field, in terms of spatial derivatives of the 4--vector potential
and the Faraday-Lenz Law. An important distinction between the physical
meanings of the space-time and energy-momentum 4--vectors is pointed out.Comment: 15 pages, no tables 1 figure. Revised and extended version of
physics/0307133 Some typos removed and minor text improvements in this
versio
Extension of the Adler-Bobenko-Suris classification of integrable lattice equations
The classification of lattice equations that are integrable in the sense of
higher-dimensional consistency is extended by allowing directed edges. We find
two cases that are not transformable via the 'admissible transformations' to
the lattice equations in the existing classification.Comment: 14 pages, 2 figure
Resonance bifurcations from robust homoclinic cycles
We present two calculations for a class of robust homoclinic cycles with
symmetry Z_n x Z_2^n, for which the sufficient conditions for asymptotic
stability given by Krupa and Melbourne are not optimal.
Firstly, we compute optimal conditions for asymptotic stability using
transition matrix techniques which make explicit use of the geometry of the
group action.
Secondly, through an explicit computation of the global parts of the Poincare
map near the cycle we show that, generically, the resonance bifurcations from
the cycles are supercritical: a unique branch of asymptotically stable period
orbits emerges from the resonance bifurcation and exists for coefficient values
where the cycle has lost stability. This calculation is the first to explicitly
compute the criticality of a resonance bifurcation, and answers a conjecture of
Field and Swift in a particular limiting case. Moreover, we are able to obtain
an asymptotically-correct analytic expression for the period of the bifurcating
orbit, with no adjustable parameters, which has not proved possible previously.
We show that the asymptotic analysis compares very favourably with numerical
results.Comment: 24 pages, 3 figures, submitted to Nonlinearit
The Indirect Limit on the Standard Model Higgs Boson Mass from the Precision FERMILAB, LEP and SLD Data
Standard Model fits are performed on the most recent leptonic and b quark Z
decay data from LEP and SLD, and FERMILAB data on top quark production, to
obtain and . Poor fits are obtained, with confidence levels
2%. Removing the b quark data improves markedly the quality of the fits and
reduces the 95% CL upper limit on by 50 GeV.Comment: 6 pages 3 tables i figur
Symmetry breaking in MAST plasma turbulence due to toroidal flow shear
The flow shear associated with the differential toroidal rotation of tokamak
plasmas breaks an underlying symmetry of the turbulent fluctuations imposed by
the up-down symmetry of the magnetic equilibrium. Using experimental
Beam-Emission-Spectroscopy (BES) measurements and gyrokinetic simulations, this
symmetry breaking in ion-scale turbulence in MAST is shown to manifest itself
as a tilt of the spatial correlation function and a finite skew in the
distribution of the fluctuating density field. The tilt is a statistical
expression of the "shearing" of the turbulent structures by the mean flow. The
skewness of the distribution is related to the emergence of long-lived density
structures in sheared, near-marginal plasma turbulence. The extent to which
these effects are pronounced is argued (with the aid of the simulations) to
depend on the distance from the nonlinear stability threshold. Away from the
threshold, the symmetry is effectively restored
Relating a gluon mass scale to an infrared fixed point in pure gauge QCD
We show that in pure gauge QCD (or any pure non-Abelian gauge theory) the
condition for the existence of a global minimum of energy with a gluon (gauge
boson) mass scale also implies the existence of a fixed point of the
function. We argue that the frozen value of the coupling constant found in some
solutions of the Schwinger-Dyson equations of QCD can be related to this fixed
point. We also discuss how the inclusion of fermions modifies this property.Comment: 4 pages, Revtex - Added some clarifying comments and new reference
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