16,794 research outputs found
1/N_c Expansion of the Heavy Baryon Isgur-Wise Functions
The 1/N_c expansion of the heavy baryon Isgur-Wise functions is discussed.
Because of the contracted SU(2N_f) light quark spin-flavor symmetry, the
universality relations among the Isgur-Wise functions of \Lambda_b to \Lambda_c
and \Sigma_b^{(*)} to \Sigma_c^{(*)} are valid up to the order of 1/N_c^2.Comment: 7 pages, latex, no figures, to appear in Phys. Rev.
A Comparison of Semi-Analytic and Smoothed Particle Hydrodynamics Galaxy Formation
We compare the statistical properties of galaxies found in two different
models of hierarchical galaxy formation: the semi-analytic model of Cole et al.
and the smoothed particle hydrodynamics (SPH) simulations of Pearce et al.
Using a `stripped-down' version of the semi-analytic model which mimics the
resolution of the SPH simulations and excludes physical processes not included
in them, we find that the two models produce an ensemble of galaxies with
remarkably similar properties, although there are some differences in the gas
cooling rates and in the number of galaxies that populate halos of different
mass. The full semi-analytic model, which has effectively no resolution limit
and includes a treatment of star formation and supernovae feedback, produces
somewhat different (but readily understandable) results. Agreement is
particularly good for the present-day global fractions of hot gas, cold dense
(i.e. galactic) gas and uncollapsed gas, for which the SPH and stripped-down
semi-analytic calculations differ by at most 25%. In the most massive halos,
the stripped-down semi-analytic model predicts, on the whole, up to 50% less
gas in galaxies than is seen in the SPH simulations. The two techniques
apportion this cold gas somewhat differently amongst galaxies in a given halo.
This difference can be tracked down to the greater cooling rate in massive
halos in the SPH simulation compared to the semi-analytic model. (abridged)Comment: 19 pages, 13 figures, to appear in MNRAS. Significantly extended to
explore galaxy progenitor distributions and behaviour of models at high
redshift
Baryon magnetic moments and sigma terms in lattice-regularized chiral perturbation theory
An SU(3) chiral Lagrangian for the lightest decuplet of baryons is
constructed on a discrete lattice of spacetime points, and is added to an
existing lattice Lagrangian for the lightest octets of mesons and baryons. A
nonzero lattice spacing renders all loop integrations finite, and the continuum
limit of any physical observable is identical to the result obtained from
dimensional regularization. Chiral symmetry and gauge invariance are preserved
even at nonzero lattice spacing. Specific calculations discussed here include
the non-renormalization of a conserved vector current, the magnetic moments of
octet baryons, and the pi N and KN sigma terms that relate to the nucleon's
strangeness content. The quantitative difference between physics at a nonzero
lattice spacing and physics in the continuum limit is easily computed, and it
represents an expectation for the size of discretization errors in
corresponding lattice QCD simulations.Comment: 19 pages, 5 figures, one paragraph added to introduction, to appear
in Phys Rev
Study of perturbed periodic systems of differential equations - The Stroboscopic method
Stroboscopic method for solving perturbed periodic systems of differential equation
Half-life of the electron-capture decay of 97Ru: Precision measurement shows no temperature dependence
We have measured the half-life of the electron-capture (ec) decay of 97Ru in
a metallic environment, both at low temperature (19K), and also at room
temperature. We find the half-lives at both temperatures to be the same within
0.1%. This demonstrates that a recent claim that the ec decay half-life for 7Be
changes by $0.9% +/- 0.2% under similar circumstances certainly cannot be
generalized to other ec decays. Our results for the half-life of 97Ru,
2.8370(14)d at room temperature and 2.8382(14)d at 19K, are consistent with,
but much more precise than, previous room-temperature measurements. In
addition, we have also measured the half-lives of the beta-emitters 103Ru and
105Rh at both temperatures, and found them also to be unchanged.Comment: 6 pages, 6 figure
An eccentric companion at the edge of the brown dwarf desert orbiting the 2.4 Msun giant star HIP67537
We report the discovery of a substellar companion around the giant star
HIP67537. Based on precision radial velocity measurements from CHIRON and FEROS
high-resolution spectroscopic data, we derived the following orbital elements
for HIP67537: msin = 11.1 M,
= 4.9 AU and = 0.59. Considering
random inclination angles, this object has 65% probability to be
above the theoretical deuterium-burning limit, thus it is one of the few known
objects in the planet to brown-dwarf transition region. In addition, we
analyzed the Hipparcos astrometric data of this star, from which we derived a
minimum inclination angle for the companion of 2 deg. This value
corresponds to an upper mass limit of 0.3 M, therefore the
probability that HIP67537 is stellar in nature is 7%. The large
mass of the host star and the high orbital eccentricity makes HIP67537 a
very interesting and rare substellar object. This is the second candidate
companion in the brown dwarf desert detected in the sample of intermediate-mass
stars targeted by the EXPRESS radial velocity program, which corresponds to a
detection fraction of = 1.6%. This value is larger than the
fraction observed in solar-type stars, providing new observational evidence of
an enhanced formation efficiency of massive substellar companions in massive
disks. Finally, we speculate about different formation channels for this
object.Comment: Accepted for publication to A&
Baryon masses at second order in large- chiral perturbation theory
We consider flavor breaking in the the octet and decuplet baryon masses at
second order in large- chiral perturbation theory, where is the number
of QCD colors. We assume that , where is the number of light quark
flavors, and are the parameters controlling
flavor breaking in chiral perturbation theory. We consistently include
non-analytic contributions to the baryon masses at orders , , and . The corrections are small for
the relations that follow from symmetry alone, but the corrections to
the large- relations are large and have the wrong sign. Chiral
power-counting and large- consistency allow a 2-loop contribution at order
, and a non-trivial explicit calculation is required to show
that this contribution vanishes. At second order in the expansion, there are
eight relations that are non-trivial consequences of the expansion, all
of which are well satisfied within the experimental errors. The average
deviation at this order is 7 \MeV for the \De I = 0 mass differences and
0.35 \MeV for the \De I \ne 0 mass differences, consistent with the
expectation that the error is of order .Comment: 19 pages, 2 uuencoded ps figs, uses revte
Recoil Order Chiral Corrections to Baryon Octet Axial Currents
We calculate chiral corrections to the octet axial currents through using baryon chiral perturbation theory (BCPT). The relativistic BCPT
framework allows one to sum an infinite series of recoil corrections at a given
order in the chiral expansion. We also include SU(3)-breaking operators
occuring at not previously considered. We determine the
corresponding low-energy constants (LEC's) from hyperon semileptonic decay data
using a variety of infrared regularization schemes. We find that the chiral
expansion of the axial currents does not display the proper convergence
behavior, regardless of which scheme is chosen. We explore the implications of
our analysis for determinations of the strange quark contribution to the
nucleon spin, .Comment: RevTex, 19 pages + 2 PS figure
Spin-Flavor Structure of Large N Baryons
The spin-flavor structure of large N baryons is described in the 1/N
expansion of QCD using quark operators. The complete set of quark operator
identities is obtained, and used to derive an operator reduction rule which
simplifies the 1/N expansion. The operator reduction rule is applied to the
axial currents, masses, magnetic moments and hyperon non-leptonic decay
amplitudes in the limit, to first order in breaking, and
without assuming symmetry. The connection between the Skyrme and quark
representations is discussed. An explicit formula is given for the quark model
operators in terms of the Skyrme model operators to all orders in for
the two flavor case.Comment: 36 pages, 2 eps figures, uses revte
The Origin of a Repose Angle: Kinetics of Rearrangements for Granular Materials
A microstructural theory of dense granular materials is presented, based on
two main ideas. Firstly, that macroscopic shear results form activated local
rearrangements at a mesoscopic scale. Secondly, that the update frequency of
microscopic processes is determined by granular temperature. In a shear cell,
the resulting constitutive equations account for Bagnold's scaling and for the
existence of a Coulomb criterion of yield. In the case of a granular flow down
an inclined plane, they account for the rheology observed in recent experiments
and for the temperature and velocity profiles measured numerically.Comment: submitted to PR
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