8,534 research outputs found
New Bound on gamma from B^+- -> pi K Decays
A bound on the angle gamma of the unitarity triangle is derived using
experimental information on the CP-averaged branching ratios for the rare
decays B^+- -> pi^+- K^0 and B^+- -> pi^0 K^+-. The theoretical description is
cleaner than the Fleischer-Mannel analysis of the decays B^+- -> pi^+- K^0 and
B^0 -> pi^-+ K^+- in that the two decay rates differ only in a single isospin
amplitude, which has a simple structure in the SU(3) limit. As a consequence,
electroweak penguin contributions and strong rescattering effects can be taken
into account in a model-independent way. The resulting bound excludes values of
cos(gamma) around 0.6 and is thus largely complementary to indirect constraints
derived from a global analysis of the unitarity triangle.Comment: minor corrections, version to appear in Physics Letters
Using sentence combining in technical writing classes
Sentence combining exercises are advanced as a way to teach technical writing style without reliance upon abstractions, from which students do not learn. Such exercises: (1) give students regular writing practice; (2) teach the logic of sentence structure, sentence editing, and punctuation; (3) paragraph development and organization; and (4) rhetorical stance. Typical sentence, paragraph, and discourse level sentence combining exercises are described
Cross ratios between Dalitz plot amplitudes in three-body decays
A recent study of and
describes a flavor-symmetric approach to calculate relative amplitudes and
phases, for characteristic interferences between decays to a light
pseudoscalar and a light vector , on Dalitz plots for
decays. The flavor-symmetric approach used an earlier fit to decay
rates and was found to agree fairly well with experiments for but not as well for and . The present work extends this investigation to include . We use an SU(3) flavor symmetry relationship between ratios
of Cabibbo-favored (CF) amplitudes in and
ratios of singly- Cabibbo-suppressed (SCS) amplitudes in and . We observe that experimental
values for Dalitz plot cross ratios obey this relationship up to discrepancies
noted previously. The need for an updated Dalitz plot analysis of is emphasized.Comment: 9 pages, one figure. Submitted to Phys. Rev. D (Brief Reports).
Additional comparisons with previous result
Large quantum fluctuations in the strongly coupled spin-1/2 chains of green dioptase: a hidden message from birds and trees
The green mineral dioptase Cu6Si6O18(H2O)6 has been known since centuries and
plays an important role in esoteric doctrines. In particular, the green
dioptase is supposed to grant the skill to speak with trees and to understand
the language of birds. Armed with natural samples of dioptase, we were able to
unravel the magnetic nature of the mineral (presumably with hidden support from
birds and trees) and show that strong quantum fluctuations can be realized in
an essentially framework-type spin lattice of coupled chains, thus neither
frustration nor low-dimensionality are prerequisites. We present a microscopic
magnetic model for the green dioptase. Based on full-potential DFT
calculations, we find two relevant couplings in this system: an
antiferromagnetic coupling J_c, forming spiral chains along the hexagonal c
axis, and an inter-chain ferromagnetic coupling J_d within structural Cu2O6
dimers. To refine the J_c and J_d values and to confirm the proposed spin
model, we perform quantum Monte-Carlo simulations for the dioptase spin
lattice. The derived magnetic susceptibility, the magnetic ground state, and
the sublattice magnetization are in remarkably good agreement with the
experimental data. The refined model parameters are J_c = 78 K and J_d = -37 K
with J_d/J_c ~ -0.5. Despite the apparent three-dimensional features of the
spin lattice and the lack of frustration, strong quantum fluctuations in the
system are evidenced by a broad maximum in the magnetic susceptibility, a
reduced value of the Neel temperature T_N ~ 15 K >> J_c, and a low value of the
sublattice magnetization m = 0.55 Bohr magneton. All these features should be
ascribed to the low coordination number of 3 that outbalances the
three-dimensional nature of the spin lattice.Comment: Dedicated to Stefan-Ludwig Drechsler on the occasion of his 60th
birthday (9 pages, 6 figures
On the electronic structure of CaCuO2 and SrCuO2
Recent electronic structure calculations for the prototypical lowdimensional
cuprate compounds CaCuO2 ans SrCuO2 performed by Wu et. al. (J. Phys.: Condens.
Matter v. 11 p.4637 (1999))are critically reconsidered, applying high precision
full-potential bandstructure methods. It is shown that the bandstructure
calculations presented by the authors contain several important
inconsistencies, which make their main conclusions highly questionable.Comment: 4 pages, 3 figures, submitted to J. Phys. Condens. Matte
Magnetism of CuX2 frustrated chains (X = F, Cl, Br): the role of covalency
Periodic and cluster density-functional theory (DFT) calculations, including
DFT+U and hybrid functionals, are applied to study magnetostructural
correlations in spin-1/2 frustrated chain compounds CuX2: CuCl2, CuBr2, and a
fictitious chain structure of CuF2. The nearest-neighbor and second-neighbor
exchange integrals, J1 and J2, are evaluated as a function of the Cu-X-Cu
bridging angle, theta, in the physically relevant range 80-110deg. In the ionic
CuF2, J1 is ferromagnetic for theta smaller 100deg. For larger angles, the
antiferromagnetic superexchange contribution becomes dominant, in accord with
the Goodenough-Kanamori-Anderson rules. However, both CuCl2 and CuBr2 feature
ferromagnetic J1 in the whole angular range studied. This surprising behavior
is ascribed to the increased covalency in the Cl and Br compounds, which
amplifies the contribution from Hund's exchange on the ligand atoms and renders
J1 ferromagnetic. At the same time, the larger spatial extent of X orbitals
enhances the antiferromagnetic J2, which is realized via the long-range
Cu-X-X-Cu paths. Both, periodic and cluster approaches supply a consistent
description of the magnetic behavior which is in good agreement with the
experimental data for CuCl2 and CuBr2. Thus, owing to their simplicity, cluster
calculations have excellent potential to study magnetic correlations in more
involved spin lattices and facilitate application of quantum-chemical methods
- …