13,064 research outputs found
Computation of and with Lattice QCD
We pursue a new method, based on lattice QCD, for determining the quantities
, , and of heavy-quark effective theory.
We combine Monte Carlo data for the meson mass spectrum with perturbative
calculations of the short-distance behavior, to extract and
from a formula from HQET. Taking into account uncertainties from
fitting the mass dependence and from taking the continuum limit, we find
and in the quenched approximation.Comment: 7 pp, 4 figs (in v2 Fig. 4 now shows Ref. 13, as advertised); in v3
error in BLM scale is correcte
Charmonium mass splittings at the physical point
We present results from an ongoing study of mass splittings of the lowest
lying states in the charmonium system. We use clover valence charm quarks in
the Fermilab interpretation, an improved staggered (asqtad) action for sea
quarks, and the one-loop, tadpole-improved gauge action for gluons. This study
includes five lattice spacings, 0.15, 0.12, 0.09, 0.06, and 0.045 fm, with two
sets of degenerate up- and down-quark masses for most spacings. We use an
enlarged set of interpolation operators and a variational analysis that permits
study of various low-lying excited states. The masses of the sea quarks and
charm valence quark are adjusted to their physical values. This large set of
gauge configurations allows us to extrapolate results to the continuum physical
point and test the methodology.Comment: 7 pp, 6 figs, Lattice 201
Low lying charmonium states at the physical point
We present results for the mass splittings of low-lying charmonium states
from a calculation with Wilson clover valence quarks with the Fermilab
interpretation on an asqtad sea. We use five lattice spacings and two values of
the light sea quark mass to extrapolate our results to the physical point.
Sources of systematic uncertainty in our calculation are discussed and we
compare our results for the 1S hyperfine splitting, the 1P-1S splitting and the
P-wave spin orbit and tensor splittings to experiment.Comment: For the Fermilab Lattice and MILC Collaborations; 7 pages, 6 figures;
Contribution to the 32nd International Symposium on Lattice Field Theory,
23-28 June, 2014, Columbia University New York, N
Modulation of galactic protons in the heliosphere during the unusual solar minimum of 2006 to 2009
The last solar minimum activity period, and the consequent minimum modulation
conditions for cosmic rays, was unusual. The highest levels of galactic protons
were recorded at Earth in late 2009 in contrast to expectations. Proton spectra
observed for 2006 to 2009 from the PAMELA cosmic ray detector on-board the
Resurs-DK1 satellite are presented together with the solutions of a
comprehensive numerical model for the solar modulation of cosmic rays. The
model is used to determine what mechanisms were mainly responsible for the
modulation of protons during this period, and why the observed spectrum for
2009 was the highest ever recorded. From mid-2006 until December 2009 we find
that the spectra became significantly softer because increasingly more low
energy protons had reached Earth. To simulate this effect, the rigidity
dependence of the diffusion coefficients had to decrease significantly below ~3
GeV. The modulation minimum period of 2009 can thus be described as relatively
more "diffusion dominated" than previous solar minima. However, we illustrate
that drifts still had played a significant role but that the observable
modulation effects were not as well correlated with the waviness of the
heliospheric current sheet as before. Protons still experienced global gradient
and curvature drifts as the heliospheric magnetic field had decreased
significantly until the end of 2009, in contrast to the moderate decreases
observed during previous minimum periods. We conclude that all modulation
processes contributed to the observed increases in the proton spectra for this
period, exhibiting an intriguing interplay of these major mechanisms
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