137 research outputs found
Operator Analysis of L=1 Baryon Masses in Large N_c QCD
We consider in detail the mass operator analysis for the nonstrange L=1
excited baryons in large N_c QCD. We present a straightforward procedure for
constructing the large N_c baryon wavefunctions, and provide complete analytic
expressions for the matrix elements of all the independent isosinglet mass
operators. We discuss the relationship between the old-fashioned operator
analyses based on nonrelativistic SU(6) symmetry and the modern large N_c
approach, which has a firmer theoretical foundation. We then suggest a possible
dynamical interpretation for the subset of operators preferred strongly by the
data.Comment: 36 pages LaTe
Nonexotic Neutral Gauge Bosons
We study theoretical and experimental constraints on electroweak theories
including a new color-singlet and electrically-neutral gauge boson. We first
note that the electric charges of the observed fermions imply that any such Z'
boson may be described by a gauge theory in which the Abelian gauge groups are
the usual hypercharge along with another U(1) component in a kinetic-diagonal
basis. Assuming that the observed quarks and leptons have
generation-independent U(1) charges, and that no new fermions couple to the
standard model gauge bosons, we find that their U(1) charges form a
two-parameter family consistent with anomaly cancellation and viable fermion
masses, provided there are at least three right-handed neutrinos. We then
derive bounds on the Z' mass and couplings imposed by direct production and
Z-pole measurements. For generic charge assignments and a gauge coupling of
electromagnetic strength, the strongest lower bound on the Z' mass comes from
Z-pole measurements, and is of order 1 TeV. If the new U(1) charges are
proportional to B-L, however, there is no tree-level mixing between the Z and
Z', and the best bounds come from the absence of direct production at LEPII and
the Tevatron. If the U(1) gauge coupling is one or two orders of magnitude
below the electromagnetic one, these bounds are satisfied for most values of
the Z' mass.Comment: 26 pages, 2 figures. A comparison with the LEP bounds on sneutrino
resonances is include
Partially fluidized shear granular flows: Continuum theory and MD simulations
The continuum theory of partially fluidized shear granular flows is tested
and calibrated using two dimensional soft particle molecular dynamics
simulations. The theory is based on the relaxational dynamics of the order
parameter that describes the transition between static and flowing regimes of
granular material. We define the order parameter as a fraction of static
contacts among all contacts between particles. We also propose and verify by
direct simulations the constitutive relation based on the splitting of the
shear stress tensor into a``fluid part'' proportional to the strain rate
tensor, and a remaining ``solid part''. The ratio of these two parts is a
function of the order parameter. The rheology of the fluid component agrees
well with the kinetic theory of granular fluids even in the dense regime. Based
on the hysteretic bifurcation diagram for a thin shear granular layer obtained
in simulations, we construct the ``free energy'' for the order parameter. The
theory calibrated using numerical experiments with the thin granular layer is
applied to the surface-driven stationary two dimensional granular flows in a
thick granular layer under gravity.Comment: 20 pages, 19 figures, submitted to Phys. Rev.
Half-metallicity and Slater-Pauling behavior in the ferromagnetic Heusler alloys
Introductory chapter for the book "Halfmetallic Alloys - Fundamentals and
Applications" to be published in the series Springer Lecture Notes on Physics,
P. H. Dederichs and I. Galanakis (eds). It contains a review of the theoretical
work on the half-metallic Heusler alloys.Comment: Introductory chapter for the book "Halfmetallic Alloys - Fundamentals
and Applications" to be published in the series Springer Lecture Notes on
Physics, P. H. Dederichs and I. Galanakis (eds
Nonleptonic Weak Decays of Bottom Baryons
Cabibbo-allowed two-body hadronic weak decays of bottom baryons are analyzed.
Contrary to the charmed baryon sector, many channels of bottom baryon decays
proceed only through the external or internal W-emission diagrams. Moreover,
W-exchange is likely to be suppressed in the bottom baryon sector.
Consequently, the factorization approach suffices to describe most of the
Cabibbo-allowed bottom baryon decays. We use the nonrelativistic quark model to
evaluate heavy-to-heavy and heavy-to-light baryon form factors at zero recoil.
When applied to the heavy quark limit, the quark model results do satisfy all
the constraints imposed by heavy quark symmetry. The decay rates and up-down
asymmetries for bottom baryons decaying into and
are calculated. It is found that the up-down asymmetry is negative except for
decay and for decay modes with in the final
state. The prediction for
is consistent with the recent CDF measurement. We also present
estimates for decays and compare with various model
calculations.Comment: 24 pages, to appear in Phys. Rev. Uncertainties with form factor q^2
dependence are discusse
A Phenomenological Analysis of Heavy Hadron Lifetimes
A phenomenological analysis of lifetimes of bottom and charmed hadrons within
the framework of the heavy quark expansion is performed. The baryon matrix
element is evaluated using the bag model and the nonrelativistic quark model.
We find that bottom-baryon lifetimes follow the pattern
.
However, neither the lifetime ratio nor the
absolute decay rates of the baryon and mesons can be explained.
One way of solving both difficulties is to allow the presence of linear
corrections by scaling the inclusive nonleptonic width with the fifth power of
the hadron mass rather than the heavy quark mass . The hierarchy
of bottom baryon lifetimes is dramatically modified to
: The
longest-lived among bottom baryons in the OPE prescription now
becomes shortest-lived. The replacement of by in nonleptonic
widths is natural and justified in the PQCD-based factorization approach
formulated in terms of hadron-level kinematics. For inclusive charmed baryon
decays, we argue that since the heavy quark expansion does not converge, local
duality cannot be tested in this case. We show that while the ansatz of
substituting the heavy quark mass by the hadron mass provides a much better
description of the charmed-baryon lifetime {\it ratios}, it appears unnatural
and unpredictive for describing the {\it absolute} inclusive decay rates of
charmed baryons, contrary to the bottom case.Comment: 35 pages, to appear in Phys. Rev. The CDF result on the lifetime
ratio of Lambda_b and B_d is discusse
Origin and Properties of the Gap in the Half-Ferromagnetic Heusler Alloys
We study the origin of the gap and the role of chemical composition in the
half-ferromagnetic Heusler alloys using the full-potential screened KKR method.
In the paramagnetic phase the C1_b compounds, like NiMnSb, present a gap.
Systems with 18 valence electrons, Z_t, per unit cell, like CoTiSb, are
semiconductors, but when Z_t > 18 antibonding states are also populated, thus
the paramagnetic phase becomes unstable and the half-ferromagnetic one is
stabilized. The minority occupied bands accommodate a total of nine electrons
and the total magnetic moment per unit cell in mu_B is just the difference
between Z_t and . While the substitution of the transition metal
atoms may preserve the half-ferromagnetic character, substituting the atom
results in a practically rigid shift of the bands and the loss of
half-metallicity. Finally we show that expanding or contracting the lattice
parameter by 2% preserves the minority-spin gap.Comment: 11 pages, 7 figures New figures, revised tex
Molecular epidemiology of coagulase-negative Staphylococcus species isolated at different lactation stages from dairy cattle in the United States
Background Coagulase negative Staphylococcus (CNS) species are currently the most prevalent intra-mammary pathogens causing subclinical mastitis and occasional clinical mastitis or persistent infection in lactating dairy cattle. More than 10 CNS species have been identified, but they are generally managed as one group on most dairies in the United States. However, improved management decisions and treatment outcomes may be achieved with better understanding of the prevalent species, pathogenicity and strain diversity within and across dairies. Methodology A total of 604 CNS isolates were cultured from milk samples collected during a dry-cow treatment clinical trial conducted on 6 dairy herds in 4 states in the US. All the study cows were randomized to receive 1 of the 3 different intra-mammary antimicrobial infusions (Quatermaster, Spectramast DC or ToMorrow Dry Cow) at dry-off. Milk samples were collected at dry-off, calving (0–6 days in milk, DIM), post-calving (7–13 DIM) and at mastitis events within the first 100 DIM. The CNS isolates were identified to species level by partial sequencing of the rpoβ gene, and genetic relatedness within species was investigated by phylogenetic analysis of the pulse-field gel electrophoresis profiles of the isolates. Results The major CNS species identified were S. chromogenes (48.3%), S. haemolyticus (17.9%), S. simulans and S. epidermidis (each at 6.5%). Other CNS species identified at lower frequencies included S. hominis, S. auricularis, S. sciuri, S. spp KS-SP, S. capitis, S. cohnii, S. warneri, S. pasteuri, S. xylosus, S. hyicus, S. equorum, S. microti, S. rostri, S. gallinarum, S. saprophyticus and S. succinus. Phylogenetic analyses of the major species types demonstrated an association between genetic relatedness and epidemiological distributions of S. chromogenes, S. simulans, S. haemolyticus and S. auricularis. Additionally, identical strains of S. chromogenes and S. simulans were isolated from the same udder quarter of several cows at consecutive sample stages. The rest of the minor species had no deducible genetic-epidemiological link. Discussion The observed association between genetic and epidemiological distributions indicated animal-adapted nature of four CNS species, suggesting possible host-adapted and environmental transmission of these species. Multi-stage isolation of the same udder quarter strain was evidence for chronic intra-mammary infection. Conclusion The different CNS species and strains circulating on US dairy herds were genetically diverse. Four species identified were likely udder-adapted pathogens, 2 of which caused persistent infection. Our findings are important in guiding the design of effective mastitis control strategies
The evolution of language: a comparative review
For many years the evolution of language has been seen as a disreputable topic, mired in fanciful "just so stories" about language origins. However, in the last decade a new synthesis of modern linguistics, cognitive neuroscience and neo-Darwinian evolutionary theory has begun to make important contributions to our understanding of the biology and evolution of language. I review some of this recent progress, focusing on the value of the comparative method, which uses data from animal species to draw inferences about language evolution. Discussing speech first, I show how data concerning a wide variety of species, from monkeys to birds, can increase our understanding of the anatomical and neural mechanisms underlying human spoken language, and how bird and whale song provide insights into the ultimate evolutionary function of language. I discuss the ‘‘descended larynx’ ’ of humans, a peculiar adaptation for speech that has received much attention in the past, which despite earlier claims is not uniquely human. Then I will turn to the neural mechanisms underlying spoken language, pointing out the difficulties animals apparently experience in perceiving hierarchical structure in sounds, and stressing the importance of vocal imitation in the evolution of a spoken language. Turning to ultimate function, I suggest that communication among kin (especially between parents and offspring) played a crucial but neglected role in driving language evolution. Finally, I briefly discuss phylogeny, discussing hypotheses that offer plausible routes to human language from a non-linguistic chimp-like ancestor. I conclude that comparative data from living animals will be key to developing a richer, more interdisciplinary understanding of our most distinctively human trait: language
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