581 research outputs found
Setting the scale for the Luescher-Weisz action
We study the quark-antiquark potential of quenched SU(3) lattice gauge theory
with the Luescher-Weisz action. After blocking the gauge fields with the
recently proposed hypercubic transformation we compute the Sommer parameter,
extract the lattice spacing a and set the scale at 6 different values of the
gauge coupling in a range from a = 0.084 fm to 0.136 fm.Comment: Remarks and references added, to appear in Phys. Rev.
The Square-Lattice Heisenberg Antiferromagnet at Very Large Correlation Lengths
The correlation length of the square-lattice spin-1/2 Heisenberg
antiferromagnet is studied in the low-temperature (asymptotic-scaling) regime.
Our novel approach combines a very efficient loop cluster algorithm --
operating directly in the Euclidean time continuum -- with finite-size scaling.
This enables us to probe correlation lengths up to
lattice spacings -- more than three orders of magnitude larger than any
previous study. We resolve a conundrum concerning the applicability of
asymptotic-scaling formulae to experimentally- and numerically-determined
correlation lengths, and arrive at a very precise determination of the
low-energy observables. Our results have direct implications for the
zero-temperature behavior of spin-1/2 ladders.Comment: 12 pages, RevTeX, plus two Postscript figures. Some minor
modifications for final submission to Physical Review Letters. (accepted by
PRL
A Scaling Hypothesis for the Spectral Densities in the O(3) Nonlinear Sigma-Model
A scaling hypothesis for the n-particle spectral densities of the O(3)
nonlinear sigma-model is described. It states that for large particle numbers
the n-particle spectral densities are ``self-similar'' in being basically
rescaled copies of a universal shape function. This can be viewed as a
2-dimensional, but non-perturbative analogue of the KNO scaling in QCD.
Promoted to a working hypothesis, it allows one to compute the two point
functions at ``all'' energy or length scales. In addition, the values of two
non-perturbative constants (needed for a parameter-free matching of the
perturbative and the non-perturbative regime) are determined exactly.Comment: 9 Pages, Latex, 3 Postscript Figure
Boosting Higgs discovery - the forgotten channel
Searches for a heavy Standard Model Higgs boson focus on the 'gold plated
mode' where the Higgs decays to two leptonic Z bosons. This channel provides a
clean signature, in spite of the small leptonic branching ratios. We show that
using fat jets the semi-leptonic ZZ mode significantly increases the number of
signal events with a similar statistical significance as the leptonic mode.Comment: 12 pages, 3 figure
Static quarks with improved statistical precision
We present a numerical study for different discretisations of the static
action, concerning cut-off effects and the growth of statistical errors with
Euclidean time. An error reduction by an order of magnitude can be obtained
with respect to the Eichten-Hill action, for time separations beyond 1.3 fm,
keeping discretization errors small. The best actions lead to a big improvement
on the precision of the quark mass Mb and F_Bs in the static approximation.Comment: 3 pages, 4 figures, Lattice2003(heavy
Finite-size scaling of the helicity modulus of the two-dimensional O(3) model
Using Monte Carlo methods, we compute the finite-size scaling function of the
helicity modulus of the two-dimensional O(3) model and compare it to
the low temperature expansion prediction. From this, we estimate the range of
validity for the leading terms of the low temperature expansion of the
finite-size scaling function and for the low temperature expansion of the
correlation length. Our results strongly suggest that a Kosterlitz-Thouless
transition at a temperature is extremely unlikely in this model.Comment: 4 pages, 3 Postscript figures, to appear in Phys. Rev. B Jan. 1997 as
a Brief Repor
Finite size and temperature effects in the AF Heisenberg model
The low temperature and large volume effects in the d=2+1 antiferromagnetic
quantum Heisenberg model are dominated by magnon excitations. The leading and
next-to-leading corrections are fully controlled by three physical constants,
the spin stiffness, the spin wave velocity and the staggered magnetization.
Among others, the free energy, the ground state energy, the low lying
excitations, staggered magnetization, staggered and uniform susceptibilities
are studied here. The special limits of very low temperature and infinite
volume are considered also.Comment: 44 pages, LATEX, no figure
Heavy Dynamical Fermions in Lattice QCD
It is expected that the only effect of heavy dynamical fermions in QCD is to
renormalize the gauge coupling. We derive a simple expression for the shift in
the gauge coupling induced by flavors of heavy fermions. We compare this
formula to the shift in the gauge coupling at which the
confinement-deconfinement phase transition occurs (at fixed lattice size) from
numerical simulations as a function of quark mass and . We find remarkable
agreement with our expression down to a fairly light quark mass. However,
simulations with eight heavy flavors and two light flavors show that the eight
flavors do more than just shift the gauge coupling. We observe
confinement-deconfinement transitions at induced by a large number of
heavy quarks. We comment on the relevance of our results to contemporary
simulations of QCD which include dynamical fermions.Comment: COLO-HEP-311, 26 pages and 6 postscript figures; file is a shar file
and all macros are (hopefully) include
High density QCD with static quarks
We study lattice QCD in the limit that the quark mass and chemical potential
are simultaneously made large, resulting in a controllable density of quarks
which do not move. This is similar in spirit to the quenched approximation for
zero density QCD. In this approximation we find that the deconfinement
transition seen at zero density becomes a smooth crossover at any nonzero
density, and that at low enough temperature chiral symmetry remains broken at
all densities.Comment: LaTeX, 18 pages, uses epsf.sty, postscript figures include
Perturbative versus Non-perturbative QFT -- Lessons from the O(3) NLS Model
The two-point functions of the energy-momentum tensor and the Noether current
are used to probe the O(3) nonlinear sigma model in an energy range below 10^4
in units of the mass gap . We argue that the form factor approach, with the
form factor series trunctated at the 6-particle level, provides an almost exact
solution of the model in this energy range. The onset of the (2-loop)
perturbative regime is found to occur only at energies around .Comment: 13 pages LaTex, 4 PostScript figures; version published in Physics
Letters
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