385 research outputs found
Improvements of the local bosonic algorithm
We report on several improvements of the local bosonic algorithm proposed by
M. Luescher. We find that preconditioning and over-relaxation works very well.
A detailed comparison between the bosonic and the Kramers-algorithms shows
comparable performance for the physical situation examined.Comment: Talk presented at LATTICE96(algorithms), 3 pages, Latex, espcrc
Study of a new simulation algorithm for dynamical quarks on the APE-100 parallel computer
First results on the autocorrelation behaviour of a recently proposed fermion
algorithm by M. L\"uscher are presented and discussed. The occurence of
unexpected large autocorrelation times is explained. Possible improvements are
discussed.Comment: 3 pages, compressed ps-file (uufiles), Contribution to Lattice 9
A new simulation algorithm for lattice QCD with dynamical quarks
A previously introduced multi-boson technique for the simulation of QCD with
dynamical quarks is described and some results of first test runs on a
lattice with Wilson quarks and gauge group SU(2) are reported.Comment: 7 pages, postscript file (166 KB
Adaptive Step Size for Hybrid Monte Carlo Algorithm
We implement an adaptive step size method for the Hybrid Monte Carlo a
lgorithm. The adaptive step size is given by solving a symmetric error
equation. An integr ator with such an adaptive step size is reversible.
Although we observe appreciable variations of the step size, the overhead of
the method exceeds its benefits. We propose an explanation for this phenomenon.Comment: 13 pages, 5 Postscript figures, late
Two-flavour Schwinger model with dynamical fermions in the L\"uscher formalism
We report preliminary results for 2D massive QED with two flavours of Wilson
fermions, using the Hermitean variant of L\"uscher's bosonization technique.
The chiral condensate and meson masses are obtained. The simplicity of the
model allows for high statistics simulations close to the chiral and continuum
limit, both in the quenched approximation and with dynamical fermions.Comment: Talk presented at LATTICE96(algorithms), 3 pages, 3 Postscript
figures, uses twoside, fleqn, espcrc2, epsf, revised version (details of
approx. polynomial
Vector Correlators in Lattice QCD: methods and applications
We discuss the calculation of the leading hadronic vacuum polarization in
lattice QCD. Exploiting the excellent quality of the compiled experimental data
for the e^+e^- --> hadrons cross-section, we predict the outcome of
large-volume lattice calculations at the physical pion mass, and design
computational strategies for the lattice to have an impact on important
phenomenological quantities such as the leading hadronic contribution to
(g-2)mu and the running of the electromagnetic coupling constant. First, the
R(s) ratio can be calculated directly on the lattice in the threshold region,
and we provide the formulae to do so with twisted boundary conditions. Second,
the current correlator projected onto zero spatial momentum, in a Euclidean
time interval where it can be calculated accurately, provides a potentially
critical test of the experimental R(s) ratio in the region that is most
relevant for (g-2)mu. This observation can also be turned around: the vector
correlator at intermediate distances can be used to determine the lattice
spacing in fm, and we make a concrete proposal in this direction. Finally, we
quantify the finite-size effects on the current correlator coming from
low-energy two-pion states and provide a general parametrization of the vacuum
polarization on the torus.Comment: 16 pages, 9 figure files; corrected a factor 2 in Eq. (7) over the
published versio
Virtual effects of light gauginos and higgsinos: a precision electroweak analysis of split supersymmetry
We compute corrections to precision electroweak observables in supersymmetry
in the limit that scalar superpartners are very massive and decoupled. This
leaves charginos and neutralinos and a Standard Model-like Higgs boson as the
only states with unknown mass substantially affecting the analysis. We give
complete formulas for the chargino and neutralino contributions, derive simple
analytic results for the pure gaugino and higgsino cases, and study the general
case. We find that in all circumstances, the precision electroweak fit improves
when the charginos and neutralinos are near the current direct limits. Larger
higgsino and gaugino masses worsen the fit as the theory predictions
asymptotically approach those of the Standard Model. Since the Standard Model
is considered by most to be an adequate fit to the precision electroweak data,
an important corollary to our analysis is that all regions of parameter space
allowed by direct collider constraints are also allowed by precision
electroweak constraints in split supersymmetry.Comment: 22 pages, 5 figures, v2: typos fixed and note adde
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