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

Cross ratios between Dalitz plot amplitudes in three-body D0D^0 decays

A recent study of $D^0 \to \pi^0 K^+ K^-$ and $D^0 \to K_S \pi^+\pi^-$ describes a flavor-symmetric approach to calculate relative amplitudes and phases, for characteristic interferences between $D$ decays to a light pseudoscalar $P$ and a light vector $V$, on Dalitz plots for $D \to PPP$ decays. The flavor-symmetric approach used an earlier fit to $D \to P V$ decay rates and was found to agree fairly well with experiments for $D^0 \to \pi^0 \pi^+ \pi^-$ but not as well for $D^0 \to \pi^0 K^+ K^-$ and $D^0 \to K_S \pi^+\pi^-$. The present work extends this investigation to include $D^0 \to K^- \pi^+ \pi^0$. We use an SU(3) flavor symmetry relationship between ratios of Cabibbo-favored (CF) $D \to P V$ amplitudes in $D^0 \to K^- \pi^+ \pi^0$ and ratios of singly- Cabibbo-suppressed (SCS) $D \to P V$ amplitudes in $D^0 \to \pi^0 K^+ K^-$ and $D^0 \to \pi^0 \pi^+ \pi^-$. 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 $D^0 \to K^- \pi^+ \pi^0$ is emphasized.Comment: 9 pages, one figure. Submitted to Phys. Rev. D (Brief Reports). Additional comparisons with previous result

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

Microscopic magnetic modeling for the SS=1/2 alternating chain compounds Na3_3Cu2_2SbO6_6 and Na2_2Cu2_2TeO6_6

The spin-1/2 alternating Heisenberg chain system Na$_3$Cu$_2$SbO$_6$ features two relevant exchange couplings: $J_{1a}$ within the structural Cu$_2$O$_6$ dimers and $J_{1b}$ between the dimers. Motivated by the controversially discussed nature of $J_{1a}$, we perform extensive density-functional-theory (DFT) calculations, including DFT+$U$ and hybrid functionals. Fits to the experimental magnetic susceptibility using high-temperature series expansions and quantum Monte Carlo simulations yield the optimal parameters $J_{1a}$ = $-$217 K and $J_{1b}$ = 174 K with the alternation ratio $\alpha = J_{1a}/J_{1b} \simeq$ $-$1.25. For the closely related system Na$_2$Cu$_2$TeO$_6$, DFT yields substantially enhanced $J_{1b}$, but weaker $J_{1a}$. The comparative analysis renders the buckling of the chains as the key parameter altering the magnetic coupling regime. Numerical simulation of the dispersion relations of the alternating chain model clarify why both antiferromagnetic and ferrromagnetic $J_{1a}$ can reproduce the experimental magnetic susceptibility data.Comment: published version: 11 pages, 8 figures, 5 tables + Supplemental materia

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

Isospin Considerations in Correlations of Pions and BB mesons

The correlations between a $B$ meson and a pion produced nearby in phase space should respect isospin reflection symmetry $I_3 \to -I_3$. Thus, one generally expects similar $\pi^+ B^0$ and $\pi^- B^+$ correlations (non-exotic channels), and similar $\pi^- B^0$ and $\pi^+ B^+$ correlations (exotic channels). Exceptions include (a) fragmentation processes involving exchange of quarks with the producing system, (b) misidentification of charged kaons as charged pions, and (c) effects of decay products of the associated $\overline{B}$. All of these can affect the apparent signal for correlations of charged $B$ mesons with charged hadrons. The identification of the flavor of neutral $B$ mesons through the decay $B^0 \to K^{*0} J/\psi$ requires good particle identification in order that the decay $K^{*0} \to K^+ \pi^-$ not be mistaken for $\overline{K}^{*0} \to K^- \pi^+$, in which case the correlations of neutral $B$ mesons with hadrons can be underestimated.Comment: LaTeX EPSF file; 8 uuencoded figures to be submitted separatel

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