7,567 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
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
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
The quark propagator in momentum space
The quark propagator is calculated in the Landau gauge at beta=6.0. A method
for removing the dominant, tree-level lattice artefacts is presented, enabling
a calculation of the momentum-dependent dynamical quark mass.Comment: LATTICE 99(spectrum), 3 pages, 3 figure
Bosonic stringlike behavior and the Ultraviolet filtering of QCD
The gluonic action density is calculated in static mesons at finite
temperature just below the deconfinement point. Our focus is to elucidate the
role of vacuum ultraviolet fluctuations which are filtered using an improved
smearing algorithm. In the intermediate source separation distance, where the
free string picture poorly describes the flux tube width profile, we find upon
reducing the vacuum action towards the classical instanton vacuum, the
characteristics of the flux tube converge and compare favorably with the
predictions of the free bosonic string. This result establishes a connection
between the free string action and vacuum gauge fields and reveals the
important role of ultraviolet physics in understanding the lattice data at this
temperature scale. As a by-product of these calculations, we find the
broadening of the QCD flux tube to be independent of the ultraviolet filtering
at large distances. Our results exhibit a linearly divergent pattern in
agreement with the string picture predictions.Comment: 8 pages, 8 Figures, 1 Tabl
Searching for low-lying multi-particle thresholds in lattice spectroscopy
We explore the Euclidean-time tails of odd-parity nucleon correlation
functions in a search for the S-wave pion-nucleon scattering-state threshold
contribution. The analysis is performed using 2+1 flavor 32^3 x 64 PACS-CS
gauge configurations available via the ILDG. Correlation matrices composed with
various levels of fermion source/sink smearing are used to project low-lying
states. The consideration of 25,600 fermion propagators reveals the presence of
more than one state in what would normally be regarded as an
eigenstate-projected correlation function. This observation is in accord with
the scenario where the eigenstates contain a strong mixing of single and
multi-particle states but only the single particle component has a strong
coupling to the interpolating field. Employing a two-exponential fit to the
eigenvector-projected correlation function, we are able to confirm the presence
of two eigenstates. The lower-lying eigenstate is consistent with a N-pi
scattering threshold and has a relatively small coupling to the three-quark
interpolating field. We discuss the impact of this small scattering-state
contamination in the eigenvector projected correlation function on previous
results presented in the literature.Comment: 8 pages, 4 figures. Manuscript accepted for publicatio
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