13,414 research outputs found
Nonperturbative QCD, gauge-fixing, Gribov copies, and the lattice
Perturbative QCD uses the Faddeev-Popov gauge-fixing procedure, which leads
to ghosts and the local BRST invariance of the gauge-fixed perturbative QCD
action. In the asymptotic regime, where perturbative QCD is relevant, Gribov
copies can be neglected. In the nonperturbative regime, one must adopt either a
nonlocal Gribov-copy free gauge (e.g., Laplacian gauge) or attempt to maintain
local BRST invariance at the expense of admitting Gribov copies. These issues
are explored. In addition, we discuss the relationship between recent
Dyson-Schwinger based model calculations of the infrared behavior of QCD
Green's functions and the lattice calculation of these quantities.Comment: 9 pages, 2 figures, to appear in Prog. Theor. Phys. Suppl. in the
proceedings of the Tokyo-Adelaide Joint Workshop on Quarks, Astrophysics and
Space Physic
The development of an intelligent interface to a computational fluid dynamics flow-solver code
Researchers at NASA Lewis are currently developing an 'intelligent' interface to aid in the development and use of large, computational fluid dynamics flow-solver codes for studying the internal fluid behavior of aerospace propulsion systems. This paper discusses the requirements, design, and implementation of an intelligent interface to Proteus, a general purpose, 3-D, Navier-Stokes flow solver. The interface is called PROTAIS to denote its introduction of artificial intelligence (AI) concepts to the Proteus code
SOLVING THE BETHE--SALPETER EQUATION IN MINKOWSKI SPACE: SCALAR THEORIES
The Bethe-Salpeter (BS) equation for scalar-scalar bound states in scalar
theories without derivative coupling is formulated and solved in Minkowski
space. This is achieved using the perturbation theory integral representation
(PTIR), which allows these amplitudes to be expressed as integrals over weight
functions and known singularity structures and hence allows us to convert the
BS equation into an integral equation involving weight functions. We obtain
numerical solutions using this formalism for a number of scattering kernels to
illustrate the generality of the approach. It applies even when the na\"{\i}ve
Wick rotation is invalid. As a check we verify, for example, that this method
applied to the special case of the massive ladder exchange kernel reproduces
the same results as are obtained by Wick rotation.Comment: 8 pages, regular latex, no figures. Entire manuscript available as a
ps file at http://www.physics.adelaide.edu.au/theory/home.html Also available
via anonymous ftp at
ftp://adelphi.adelaide.edu.au/pub/theory/ADP-95-28.T182.p
Necessary skills and practices required for effective participation in high bandwidth design team activities
Technology is continually changing, and evolving, throughout the entire construction
industry; and particularly in the design process. One of the principal manifestations of
this is a move away from team working in a shared work space to team working in a
virtual space, using increasingly sophisticated electronic media. Due to the significant
operating differences when working in shared and virtual spaces adjustments to
generic skills utilised by members is a necessity when moving between the two
conditions. This paper reports an aspect of a CRC-CI research project based on
research of ‘generic skills’ used by individuals and teams when engaging with high
bandwidth information and communication technologies (ICT). It aligns with the
project’s other two aspects of collaboration in virtual environments: ‘processes’ and
‘models’. The entire project focuses on the early stages of a project (i.e. design) in
which models for the project are being developed and revised. The paper
summarises the first stage of the research project which reviews literature to identify
factors of virtual teaming which may affect team member skills. It concludes that
design team participants require ‘appropriate skills’ to function efficiently and
effectively, and that the introduction of high band-width technologies reinforces the
need for skills mapping and measurement
Dynamics of merging: Post-merger mixing and relaxation of an Illustris galaxy
During the merger of two galaxies, the resulting system undergoes violent
relaxation and seeks stable equilibrium. However, the details of this evolution
are not fully understood. Using Illustris simulation, we probe two physically
related processes, mixing and relaxation. Though the two are driven by the same
dynamics---global time-varying potential for the energy, and torques caused by
asymmetries for angular momentum---we measure them differently. We define
mixing as the redistribution of energy and angular momentum between particles
of the two merging galaxies. We assess the degree of mixing as the difference
between the shapes of their N(E)s, and their N(L^2)s. We find that the
difference is decreasing with time, indicating mixing. To measure relaxation,
we compare N(E) of the newly merged system to N(E) of a theoretical prediction
for relaxed collisionless systems, DARKexp, and witness the system becoming
more relaxed, in the sense that N(E) approaches DARKexp N(E). Because the
dynamics driving mixing and relaxation are the same, the timescale is similar
for both. We measure two sequential timescales: a rapid, 1 Gyr phase after the
initial merger, during which the difference in N(E) of the two merging halos
decreases by ~80%, followed by a slow phase, when the difference decreases by
~50% over ~8.5 Gyrs. This is a direct measurement of the relaxation timescale.
Our work also draws attention to the fact that when a galaxy has reached Jeans
equilibrium it may not yet have reached a fully relaxed state given by DARKexp,
in that it retains information about its past history. This manifests itself
most strongly in stars being centrally concentrated. We argue that it is
particularly difficult for stars, and other tightly bound particles, to mix
because they have less time to be influenced by the fluctuating potential, even
across multiple merger events.Comment: accepted for publication in JCA
Ubiquity of density slope oscillations in the central regions of galaxy and cluster-sized systems
One usually thinks of a radial density profile as having a monotonically
changing logarithmic slope, such as in NFW or Einasto profiles. However, in two
different classes of commonly used systems, this is often not the case. These
classes exhibit non-monotonic changes in their density profile slopes which we
call oscillations for short. We analyze these two unrelated classes separately.
Class 1 consists of systems that have density oscillations and that are defined
through their distribution function , or differential energy distribution
, such as isothermal spheres, King profiles, or DARKexp, a theoretically
derived model for relaxed collisionless systems. Systems defined through
or generally have density slope oscillations. Class 1 system
oscillations can be found at small, intermediate, or large radii but we focus
on a limited set of Class 1 systems that have oscillations in the central
regions, usually at , where is the largest
radius where . We show that the shape of their
can roughly predict the amplitude of oscillations. Class 2 systems which are a
product of dynamical evolution, consist of observed and simulated galaxies and
clusters, and pure dark matter halos. Oscillations in the density profile slope
seem pervasive in the central regions of Class 2 systems. We argue that in
these systems, slope oscillations are an indication that a system is not fully
relaxed. We show that these oscillations can be reproduced by small
modifications to of DARKexp. These affect a small fraction of systems'
mass and are confined to . The size of these
modifications serves as a potential diagnostic for quantifying how far a system
is from being relaxed.Comment: accepted by the Journal of Cosmology and Astroparticle Physics (JCAP
Gravitational waves and electroweak baryogenesis in a global study of the extended scalar singlet model
We perform a global fit of the extended scalar singlet model with a fermionic
dark matter (DM) candidate. Using the most up-to-date results from the
measured DM relic density, direct detection limits from the
XENON1T (2018) experiment, electroweak precision observables and Higgs searches
at colliders, we constrain the 7-dimensional model parameter space. We also
find regions in the model parameter space where a successful electroweak
baryogenesis (EWBG) can be viable. This allows us to compute the gravitational
wave (GW) signals arising from the phase transition, and discuss the potential
discovery prospects of the model at current and future GW experiments. Our
global fit places a strong upper lower limit on the second
scalar mass, the fermion DM mass and the scalar-fermion DM coupling. In
agreement with previous studies, we find that our model can simultaneously
yield a strong first-order phase transition and saturate the observed DM
abundance. More importantly, the GW spectra of viable points can often be
within reach of future GW experiments such as LISA, DECIGO and BBO.Comment: 42 pages, 10 figures and 2 tables; v2: updated references, submitted
to JHEP; v3: corrected typos and updated references, matches version
published in JHE
LUX likelihood and limits on spin-independent and spin-dependent WIMP couplings with LUXCalc
We present LUXCalc, a new utility for calculating likelihoods and deriving
WIMP-nucleon coupling limits from the recent results of the LUX direct search
dark matter experiment. After a brief review of WIMP-nucleon scattering, we
derive LUX limits on the spin-dependent WIMP-nucleon couplings over a broad
range of WIMP masses, under standard assumptions on the relevant astrophysical
parameters. We find that, under these and other common assumptions, LUX
excludes the entire spin-dependent parameter space consistent with a dark
matter interpretation of DAMA's anomalous signal, the first time a single
experiment has been able to do so. We also revisit the case of spin-independent
couplings, and demonstrate good agreement between our results and the published
LUX results. Finally, we derive constraints on the parameters of an effective
dark matter theory in which a spin-1 mediator interacts with a fermionic WIMP
and Standard Model fermions via axial-vector couplings. A detailed appendix
describes the use of LUXCalc with standard codes to place constraints on
generic dark matter theories.Comment: 16 pages, 6 figures. Software package included as ancillary files.
v2: added references, Baksan limits. v3: clarifications and small
corrections, results unchange
- …