706,489 research outputs found
Finite-Size Effects in Lattice QCD with Dynamical Wilson Fermions
As computing resources are limited, choosing the parameters for a full
Lattice QCD simulation always amounts to a compromise between the competing
objectives of a lattice spacing as small, quarks as light, and a volume as
large as possible. Aiming to push unquenched simulations with the Wilson action
towards the computationally expensive regime of small quark masses we address
the question whether one can possibly save computing time by extrapolating
results from small lattices to the infinite volume, prior to the usual chiral
and continuum extrapolations. In the present work the systematic volume
dependence of simulated pion and nucleon masses is investigated and compared
with a long-standing analytic formula by Luescher and with results from Chiral
Perturbation Theory. We analyze data from Hybrid Monte Carlo simulations with
the standard (unimproved) two-flavor Wilson action at two different lattice
spacings of a=0.08fm and 0.13fm. The quark masses considered correspond to
approximately 85 and 50% (at the smaller a) and 36% (at the larger a) of the
strange quark mass. At each quark mass we study at least three different
lattices with L/a=10 to 24 sites in the spatial directions (L=0.85-2.08fm).Comment: 21 pages, 20 figures, REVTeX 4; v2: caption of Fig.7 corrected, one
reference adde
Asymptotic behavior of the ghost propagator in SU3 lattice gauge theory
We study the asymptotic behavior of the ghost propagator in the quenched
SU(3) lattice gauge theory with Wilson action. The study is performed on
lattices with a physical volume fixed around 1.6 fm and different lattice
spacings: 0.100 fm, 0.070 fm and 0.055 fm. We implement an efficient algorithm
for computing the Faddeev-Popov operator on the lattice. We are able to
extrapolate the lattice data for the ghost propagator towards the continuum and
to show that the extrapolated data on each lattice can be described up to
four-loop perturbation theory from 2.0 GeV to 6.0 GeV. The three-loop values
are consistent with those extracted from previous perturbative studies of the
gluon propagator. However the effective \Lambda_{\ms} scale which reproduces
the data does depend strongly upon the order of perturbation theory and on the
renormalization scheme used in the parametrization. We show how the truncation
of the perturbative series can account for the magnitude of the dependency in
this energy range. The contribution of non-perturbative corrections will be
discussed elsewhere.Comment: 26 pages, 7 figure
The Halo Mass Function: High-Redshift Evolution and Universality
We study the formation of dark matter halos in the concordance LCDM model
over a wide range of redshifts, from z=20 to the present. Our primary focus is
the halo mass function, a key probe of cosmology. By performing a large suite
of nested-box N-body simulations with careful convergence and error controls
(60 simulations with box sizes from 4 to 256 Mpc/h, we determine the mass
function and its evolution with excellent statistical and systematic errors,
reaching a few percent over most of the considered redshift and mass range.
Across the studied redshifts, the halo mass is probed over 6 orders of
magnitude (10^7 - 10^13.5 M_sun/h). Historically, there has been considerable
variation in the high redshift mass function as obtained by different groups.
We have made a concerted effort to identify and correct possible systematic
errors in computing the mass function at high redshift and to explain the
discrepancies between some of the previous results. We discuss convergence
criteria for the required force resolution, simulation box size, halo mass
range, initial and final redshift, and time stepping. Because of conservative
cuts on the mass range probed by individual boxes, our results are relatively
insensitive to simulation volume, the remaining sensitivity being consistent
with extended Press-Schechter theory. Previously obtained mass function fits
near z=0, when scaled by linear theory, are in good agreement with our results
at all redshifts, although a mild redshift dependence consistent with that
found by Reed and collaborators exists at low redshifts.Comment: 20 pages, 15 figures. Minor changes to the text and figures; results
and conclusions unchange
SUSY Breaking in Local String/F-Theory Models
We investigate bulk moduli stabilisation and supersymmetry breaking in local
string/F-theory models where the Standard Model is supported on a del Pezzo
surface or singularity. Computing the gravity mediated soft terms on the
Standard Model brane induced by bulk supersymmetry breaking in the LARGE volume
scenario, we explicitly find suppressions by M_s/M_P ~ V^{-1/2} compared to
M_{3/2}. This gives rise to several phenomenological scenarios, depending on
the strength of perturbative corrections to the effective action and the source
of de Sitter lifting, in which the soft terms are suppressed by at least
M_P/V^{3/2} and may be as small as M_P/V^2. Since the gravitino mass is of
order M_{3/2} ~ M_P/V, for TeV soft terms all these scenarios give a very heavy
gravitino (M_{3/2} >= 10^8 GeV) and generically the lightest moduli field is
also heavy enough (m >= 10 TeV) to avoid the cosmological moduli problem. For
TeV soft terms, these scenarios predict a minimal value of the volume to be V ~
10^{6-7} in string units, which would give a unification scale of order M_{GUT}
~ M_s V^{1/6} ~ 10^{16} GeV. The strong suppression of gravity mediated soft
terms could also possibly allow a scenario of dominant gauge mediation in the
visible sector but with a very heavy gravitino M_{3/2} > 1 TeV
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