62 research outputs found
Large-N bounds on, and compositeness limit of, gauge and gravitational interactions
In a toy model of gauge and gravitational interactions in
dimensions, endowed with an invariant UV cut-off , and containing a
large number of non-self-interacting matter species, the physical gauge and
gravitational couplings at the cut-off, and
, are shown to be bounded by appropriate
powers of . This implies that the infinite-bare-coupling (so-called
compositeness) limit of these theories is smooth, and can even resemble our
world. We argue that such a result, when extended to more realistic situations,
can help avoid large-N violations of entropy bounds, solve the dilaton
stabilization and GUT-scale problems in superstring theory, and provide a new
possible candidate for quintessence.Comment: 8 pages, Latex, minor modifications in notations and reference
QCD with light Wilson quarks on fine lattices (I): first experiences and physics results
Recent conceptual, algorithmic and technical advances allow numerical
simulations of lattice QCD with Wilson quarks to be performed at significantly
smaller quark masses than was possible before. Here we report on simulations of
two-flavour QCD at sea-quark masses from slightly above to approximately 1/4 of
the strange-quark mass, on lattices with up to 64x32^3 points and spacings from
0.05 to 0.08 fm. Physical sea-quark effects are clearly seen on these lattices,
while the lattice effects appear to be quite small, even without O(a)
improvement. A striking result is that the dependence of the pion mass on the
sea-quark mass is accurately described by leading-order chiral perturbation
theory up to meson masses of about 500 MeV.Comment: TeX source, 17 pages, figures include
QCD with light Wilson quarks on fine lattices (II): DD-HMC simulations and data analysis
In this second report on our recent numerical simulations of two-flavour QCD,
we provide further technical details on the simulations and describe the
methods we used to extract the meson masses and decay constants from the
generated ensembles of gauge fields. Among the topics covered are the choice of
the DD-HMC parameters, the issue of stability, autocorrelations and the
statistical error analysis. Extensive data tables are included as well as a
short discussion of the quark-mass dependence in partially quenched QCD,
supplementing the physics analysis that was presented in the first paper in
this series.Comment: TeX source, 35 pages, figures include
Percolation properties of the 2D Heisenberg model
We analyze the percolation properties of certain clusters defined on
configurations of the 2--dimensional Heisenberg model. We find that, given any
direction \vec{n} in O(3) space, the spins almost perpendicular to \vec{n} form
a percolating cluster. This result gives indications of how the model can avoid
a previously conjectured Kosterlitz-Thouless phase transition at finite
temperature T.Comment: 4 pages, 3 eps figures. Revised version (more clear abstract, some
new references
Stability of lattice QCD simulations and the thermodynamic limit
We study the spectral gap of the Wilson--Dirac operator in two-flavour lattice QCD as a function of the lattice spacing , the space-time volume and the current-quark mass . It turns out that the median of the probability distribution of the gap scales proportionally to and that its width is practically equal to . In particular, numerical simulations are safe from accidental zero modes in the large-volume regime of QCD
Speeding up finite step-size updating of full QCD on the lattice
We propose various improvements of finite step-size updating for full QCD on
the lattice that might turn finite step-size updating into a viable alternative
to the hybrid Monte Carlo algorithm. These improvements are noise reduction of
the noisy estimator of the fermion determinant, unbiased inclusion of the
hopping parameter expansion and a multi-level Metropolis scheme. First
numerical tests are performed for the 2 dimensional Schwinger model with two
flavours of Wilson fermions and for QCD two flavours of Wilson fermions and
Schr"odinger functional boundary conditions.Comment: 22 pages, 1 figur
Topology in 2D CP**(N-1) models on the lattice: a critical comparison of different cooling techniques
Two-dimensional CP**(N-1) models are used to compare the behavior of
different cooling techniques on the lattice. Cooling is one of the most
frequently used tools to study on the lattice the topological properties of the
vacuum of a field theory. We show that different cooling methods behave in an
equivalent way. To see this we apply the cooling methods on classical
instantonic configurations and on configurations of the thermal equilibrium
ensemble. We also calculate the topological susceptibility by using the cooling
technique.Comment: 24 pages, 10 figures (from 16 eps files
Renormalization and topological susceptibility on the lattice: SU(2) Yang-Mills theory
The renormalization functions involved in the determination of the
topological susceptibility in the SU(2) lattice gauge theory are extracted by
direct measurements, without relying on perturbation theory. The determination
exploits the phenomenon of critical slowing down to allow the separation of
perturbative and non-perturbative effects. The results are in good agreement
with perturbative computations.Comment: 12 pages + 4 figures (PostScript); report no. IFUP-TH 10/9
Instanton Corrections to Quark Form Factor at Large Momentum Transfer
Within the Wilson integral formalism, we discuss the structure of
nonperturbative corrections to the quark form factor at large momentum transfer
analyzing the infrared renormalon and instanton effects. We show that the
nonperturbative effects determine the initial value for the perturbative
evolution of the quark form factor and attribute their general structure to the
renormalon ambiguities of the perturbative series. It is demonstrated that the
instanton contributions result in the finite renormalization of the
next-to-leading perturbative result and numerically are characterized by a
small factor reflecting the diluteness of the QCD vacuum within the instanton
liquid model.Comment: Version coincident with the journal publication, 9 pages; REVTe
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