6,608 research outputs found
Cost of QCD simulations with n_f=2 dynamical Wilson fermions
Cost estimates for simulations of full QCD with n_f=2 Wilson fermions by
hybrid Monte Carlo are presented. The extrapolations are based on the average
number of iterations of the iterative solver within the fermionic part of the
HMC molecular dynamics, which is closely related to the minimal eigenvalue of
M^+M The cost formula is determined as a product of the scaling functions of
iterative solver and integrated autocorrelation time as function of the inverse
lattice pseudoscalar mass. Timings by SESAM/TxL allow to fix the pre-factor. It
is demonstrated that a 2-flavor dynamical determination of light hadron masses
with a statistical precision comparable to the corresponding quenched results
from CP-PACS can be an appropriate task for a 100 Tflops system.Comment: Contribution to Lattice2001 (panel discussion), 2 pages, 2 figures,
eq 1 correcte
QCD on \alpha-Clusters
It is shown that the 21264 Alpha processor can reach about 20% sustained
efficiency for the inversion of the Wilson-Dirac operator. Since fast ethernet
is not sufficient to get balancing between computation and communication on
reasonable lattice- and system-sizes, an interconnection using Myrinet is
discussed. We find a price/performance ratio comparable with state-of-the-art
SIMD-systems for lattice QCD.Comment: LATTICE99(machines), 3 page
SSOR Preconditioning of Improved Actions
We generalize local lexicographic SSOR preconditioning for the
Sheikholeslami-Wohlert improved Wilson fermion action and the truncated perfect
free fermion action. In our test implementation we achieve performance gains as
known from SSOR preconditioning of the standard Wilson fermion action.Comment: 3 pages, Latex, 3 figures, Talk presented at Lattice'9
A Parallel SSOR Preconditioner for Lattice QCD
A parallelizable SSOR preconditioning scheme for Krylov subspace iterative
solvers in lattice QCD applications involving Wilson fermions is presented. In
actual Hybrid Monte Carlo and quark propagator calculations it helps to reduce
the number of iterations by a factor of 2 compared to conventional odd-even
preconditioning. This corresponds to a gain in cpu-time of 30\% - 70\% over
odd-even preconditioning.Comment: Talk presented at LATTICE96(algorithms), 3 pages, LaTeX file, 3
epsf-files include
Photoionization Broadening of the 1S-2S Transition in a Beam of Atomic Hydrogen
We consider the excitation dynamics of the two-photon \sts transition in a
beam of atomic hydrogen by 243 nm laser radiation. Specifically, we study the
impact of ionization damping on the transition line shape, caused by the
possibility of ionization of the 2S level by the same laser field. Using a
Monte-Carlo simulation, we calculate the line shape of the \sts transition for
the experimental geometry used in the two latest absolute frequency
measurements (M. Niering {\it et al.}, PRL 84, 5496 (2000) and M. Fischer {\it
et al.}, PRL 92, 230802 (2004)). The calculated line shift and line width are
in excellent agreement with the experimentally observed values. From this
comparison we can verify the values of the dynamic Stark shift coefficient for
the \sts transition for the first time on a level of 15%. We show that the
ionization modifies the velocity distribution of the metastable atoms, the line
shape of the \sts transition, and has an influence on the derivation of its
absolute frequency.Comment: 10 pages, 5 figure
Reanalysis of the FEROS observations of HIP 11952
Aims. We reanalyze FEROS observations of the star HIP 11952 to reassess the
existence of the proposed planetary system. Methods. The radial velocity of the
spectra were measured by cross-correlating the observed spectrum with a
synthetic template. We also analyzed a large dataset of FEROS and HARPS
archival data of the calibrator HD 10700 spanning over more than five years. We
compared the barycentric velocities computed by the FEROS and HARPS pipelines.
Results. The barycentric correction of the FEROS-DRS pipeline was found to be
inaccurate and to introduce an artificial one-year period with a semi-amplitude
of 62 m/s. Thus the reanalysis of the FEROS data does not support the existence
of planets around HIP 11952.Comment: 7 pages, 8 figures, 1 tabl
Exactly solvable model of a quantum spin glass
A mean field spherical model with random couplings between pairs, quartets,
and possibly higher multiplets of spins is considered. It has the same critical
behavior as the Sherrington-Kirkpatrick model. It thus exhibits replica
symmetry breaking. The order parameter function is solved exactly in the whole
low temperature phase. The zero field cooled susceptibility remains finite at
low . Next a quantum version of the system is considered. Whereas the
magnetic properties are not altered qualitatively, the thermodynamics is now
regular at small temperatures.Comment: 4 pages postscript. Revised version, to appear in Phys. Rev. Let
Discovery of a stellar companion to the nearby solar-analogue HD 104304
Sun-like stars are promising candidates to host exoplanets and are often
included in exoplanet surveys by radial velocity (RV) and direct imaging. In
this paper we report on the detection of a stellar companion to the nearby
solar-analogue star HD 104304, which previously was considered to host a
planetary mass or brown dwarf companion. We searched for close stellar and
substellar companions around extrasolar planet host stars with high angular
resolution imaging to characterize planet formation environments. The detection
of the stellar companion was achieved by high angular resolution measurements,
using the "Lucky Imaging" technique at the ESO NTT 3.5m with the AstraLux Sur
instrument. We combined the results with VLT/NACO archive data, where the
companion could also be detected. The results were compared to precise RV
measurements of HD 104304, obtained at the Lick and Keck observatories from
2001-2010.
We confirmed common proper motion of the binary system. A spectral type of
M4V of the companion and a mass of 0.21 M_Sun was derived. Due to comparison of
the data with RV measurements of the unconfirmed planet candidate listed in the
Extrasolar Planets Encyclopaedia, we suggest that the discovered companion is
the origin of the RV trend and that the inclination of the orbit of
approximately 35 degrees explains the relatively small RV signal.Comment: 4 pages, 4 PNG figures, use aa.cls, accepted for publication in
Astronomy & Astrophysic
Structural properties in Sr0.61a0.39Nb2O6 in the temperature range 10 K to 500 K investigated by high-resolution neutron powder diffraction and specific heat measurements
We report high-resolution neutron powder diffraction on Sr0.61Ba0.39Nb2O6,
SBN61, in the temperature range 15-500 K. The results indicate that the
low-temperature anomalies (T<100K) observed in the dielectric dispersion are
due to small changes in the incommensurate modulation of the NbO6-octahedra, as
no structural phase transition of the average structure was observed. This
interpretation is supported by specific heat measurements, which show no latent
heat, but a glass-like behavior at low temperatures. Furthermore we find that
the structural changes connected with the ferroelectric phase transition at Tc
approx. 350K start already at 200K, explaining the anisotropic thermal
expansion in the temperature range 200-300K observed in a recent x-ray
diffraction study.Comment: Accepted by PRB (2006
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