2,075 research outputs found
A Perturbative Determination of Mass Dependent Improvement Coefficients in a Relativistic Heavy Quark Action
We present the results for a perturbative determination of mass dependent
improvement coefficients , , and in a relativistic heavy
quark action, which we have designed to control errors by extending the
on-shell improvement program to the case of ,
where is the heavy quark mass. The parameters and are
determined from the quark propagator and and are from the on-shell
quark-quark scattering amplitude. We show that all the parameters, together
with the quark wave function and the mass renormalization factors, are
determined free from infrared divergences once their tree level values are
properly tuned. Results of these parameters are shown as a function of
for various improved gauge actions.Comment: 41 pages, 6 figures as eps-fil
The core-cusp problem in cold dark matter halos and supernova feedback: Effects of Mass Loss
The core-cusp problem remains as one of the unsolved discrepancies between
observations and theories predicted by the standard paradigm of cold dark
matter (CDM) cosmology. To solve this problem, we perform N-body simulations to
study the nonlinear response of CDM halos to the variance of the gravitational
potential induced by gas removal from galaxy centers. In this study, we focus
on the timescale of the gas ejection, which is strongly correlated with stellar
activities, and demonstrate that it is one of the key factors in determining
the dynamical response of CDM halos. The results of simulations show that the
power-low index of the mass-density profile of the dark matter halo correlated
with the timescale of the mass loss, and it is flatter when the mass loss
occurs over a short time than when it occurs over a long time. However, it is
still larger than typical observational values; in other words, the central
cusp remains for any mass loss model in the simulations. Moreover, for the slow
mass-loss case, the final density profile of the dark matter halo recovers the
universal density profiles predicted by the CDM cosmology. Therefore, mass loss
driven by stellar feedback may not be an effective mechanism to flatten the
central cusp.Comment: 4 pages, 3 figures, 1 table, accepted for publication in
Astrophysical Journal Letter
Two Loop Computation of the Schroedinger Functional in Lattice QCD
We compute the Schroedinger functional (SF) for the case of lattice QCD with
Wilson fermions (with and without SW improvement) at two-loop order in lattice
perturbation theory. This allows us to extract the three-loop beta-function in
the SF-scheme. These results are required to compute the running coupling, the
Lambda-parameter and quark masses by finite size techniques with negligible
systematic errors. In addition our results enable the implementation of
two-loop O(a) improvement in SF-simulations.Comment: 30 pages, numerical changes in eqs.(A.9), (A.10), (5.6), (5.9),
(5.12) due to the revision of hep-lat/9801007v2 (Christou et al.
The M81 Group Dwarf Irregular Galaxy DDO 165. II. Connecting Recent Star Formation with ISM Structures and Kinematics
We compare the stellar populations and complex neutral gas dynamics of the
M81 group dIrr galaxy DDO 165 using data from the HST and the VLA. Paper I
identified two kinematically distinct HI components, multiple localized high
velocity gas features, and eight HI holes and shells (the largest of which
spans ~2.2x1.1 kpc). Using the spatial and temporal information from the
stellar populations in DDO 165, we compare the patterns of star formation over
the past 500 Myr with the HI dynamics. We extract localized star formation
histories within 6 of the 8 HI holes identified in Paper I, as well as 23 other
regions that sample a range of stellar densities and neutral gas properties.
From population synthesis modeling, we derive the energy outputs (from stellar
winds and supernovae) of the stellar populations within these regions over the
last 100 Myr, and compare with refined estimates of the energies required to
create the HI holes. In all cases, we find that "feedback" is energetically
capable of creating the observed structures in the ISM. Numerous regions with
significant energy inputs from feedback lack coherent HI structures but show
prominent localized high velocity gas features; this feedback signature is a
natural product of temporally and spatially distributed star formation. In DDO
165, the extended period of heightened star formation activity (lasting more
than 1 Gyr) is energetically capable of creating the observed holes and high
velocity gas features in the neutral ISM.Comment: The Astrophysical Journal, in press. Full-resolution version
available on request from the first autho
BB Potentials in Quenched Lattice QCD
The potentials between two B-mesons are computed in the heavy-quark limit
using quenched lattice QCD at . Non-zero central
potentials are clearly evident in all four spin-isospin channels, (I,s_l) =
(0,0) , (0,1) , (1,0) , (1,1), where s_l is the total spin of the light degrees
of freedom. At short distance, we find repulsion in the channels and
attraction in the I=s_l channels. Linear combinations of these potentials that
have well-defined spin and isospin in the t-channel are found, in three of the
four cases, to have substantially smaller uncertainties than the potentials
defined with the s-channel (I,s_l), and allow quenching artifacts from single
hairpin exchange to be isolated. The BB*\pi coupling extracted from the
long-distance behavior of the finite-volume t-channel potential is found to be
consistent with quenched calculations of the matrix element of the isovector
axial-current. The tensor potentials in both of the s_l = 1 channels are found
to be consistent with zero within calculational uncertainties.Comment: 30 page
Universality and scaling behavior of RG gauge actions
We study universality and scaling properties of RG gauge actions (Iwasaki and
DBW2). In the first part we consider the critical temperature T_{c} and compute
the reference energy scale r_{0} for critical couplings \beta_{c} corresponding
to N_{t}=3,4,6,8. The universality of T_{c}r_{0} between Iwasaki and Wilson
action is confirmed and the scaling behavior of the Iwasaki action is found to
be better than the one for the Wilson action. The results for the DBW2 action
show larger lattice artefacts. A continuum value T_{c}r_{0}=0.7498(50) is
extracted. We compute also the glueball masses for the states 0^{++} and
2^{++}, investigate the scaling of m_{0^{++}}r_{0} and m_{2^{++}}r_{0} and
point out practical problems which are due to the violation of positivity
present in the RG actions.Comment: 36 page
The Lattice Parameter in Domain Wall QCD
We evaluate the ratio of the scale parameter in domain wall QCD to
the one in the continuum theory at one loop level incorporating the effect of
massless quarks. We show that the Pauli-Villars regulator is required to
subtract the unphysical massive fermion modes which emerge in the fermion loop
contributions to the gluon self energy. Detailed results are presented as a
function of the domain wall height .Comment: 16 pages, 1 figure as eps-file, some references adde
The ACS Nearby Galaxy Survey Treasury VII. The NGC 4214 Starburst and the Effects of Star Formation History on Dwarf Morphology
We present deep Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2)
optical observations obtained as part of the ACS Nearby Galaxy Survey Treasury
(ANGST) as well as early release Wide Field Camera 3 (WFC3) ultra-violet and
infrared observations of the nearby dwarf starbursting galaxy NGC 4214. Our
data provide a detailed example of how covering such a broad range in
wavelength provides a powerful tool for constraining the physical properties of
stellar populations. The deepest data reach the ancient red clump at M_F814W
-0.2. All of the optical data reach the main sequence turnoff for stars younger
than ~300 Myr, and the blue He burning sequence for stars younger than 500 Myr.
The full CMD-fitting analysis shows that all three fields in our data set are
consistent with ~75% of the stellar mass being older than 8 Gyr, in spite of
showing a wide range in star formation rates at the present day. Thus, our
results suggest that the scale length of NGC 4214 has remained relatively
constant for many Gyr. As previously noted by others, we also find the galaxy
has recently ramped up production, consistent with its bright UV luminosity and
its population of UV-bright massive stars. In the central field we find UV
point sources with F336W magnitudes as bright as -9.9. These are as bright as
stars with masses of at least 52-56 M_sun and ages near 4 Myr in stellar
evolution models. Assuming a standard IMF, our CMD is well-fitted by an
increase in star formation rate beginning 100 Myr ago. The stellar populations
of this late-type dwarf are compared with those of NGC 404, an early-type dwarf
that is also the most massive galaxy in its local environment. The late-type
dwarf appears to have a similar high fraction of ancient stars, suggesting that
these dominant galaxies may form at early epochs even if they have low total
mass and very different present-day morphologies.Comment: 17 pages, 10 figures, accepted for publication in Ap
Diquark Masses from Lattice QCD
We present first results for diquark correlation functions calculated in
Landau gauge on the lattice. Masses have been extracted from the long distance
behaviour of these correlation functions. We find that the ordering of diquark
masses with spin 0 and 1 states in colour anti-triplet and sextet channels is
in accordance with instanton motivated interaction models. Although we find
evidence for an attractive interaction in colour anti-triplet states with a
splitting between spin 0 and spin 1 diquarks that can account for the mass
splitting between the nucleon and the delta, there is no evidence for a deeply
bound diquark state.Comment: 12 pages, LaTeX2e uses revtex, 3 EPS-figures, small corrections,
references update
On the Origin of the Supergiant HI Shell and Putative Companion in NGC 6822
We present new Hubble Space Telescope Advanced Camera for Surveys imaging of
six positions spanning 5.8 kpc of the HI major axis of the Local Group dIrr NGC
6822, including both the putative companion galaxy and the large HI hole. The
resulting deep color magnitude diagrams show that NGC 6822 has formed >50% of
its stars in the last ~5 Gyr. The star formation histories of all six positions
are similar over the most recent 500 Myr, including low-level star formation
throughout this interval and a weak increase in star formation rate during the
most recent 50 Myr. Stellar feedback can create the giant HI hole, assuming
that the lifetime of the structure is longer than 500 Myr; such long-lived
structures have now been observed in multiple systems and may be the norm in
galaxies with solid-body rotation. The old stellar populations (red giants and
red clump stars) of the putative companion are consistent with those of the
extended halo of NGC 6822; this argues against the interpretation of this
structure as a bona fide interacting companion galaxy and against its being
linked to the formation of the HI hole via an interaction. Since there is no
evidence in the stellar population of a companion galaxy, the most likely
explanation of the extended HI structure in NGC 6822 is a warped disk inclined
to the line of sight.Comment: The Astrophysical Journal, in press. Full-resolution version
available on request from the first autho
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