Experimental Limits on Weak Annihilation Contributions to b → ulv Decays

We present the first experimental limits on high-q^2 contributions to charmless semileptonic B decays of the form expected from the weak annihilation (WA) decay mechanism. Such contributions could bias determinations of |V_(ub)| from inclusive measurements of B→X_ulν. Using a wide range of models based on available theoretical input we set a limit of Γ_(WA)/Γ_(b→u) <7.4% (90% confidence level) on the WA fraction, and assess the impact on previous inclusive determinations of |V_(ub)|

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

Modified kagome physics in the natural spin-1/2 kagome lattice systems - kapellasite Cu3Zn(OH)6Cl2 and haydeeite Cu3Mg(OH)6Cl2

The recently discovered natural minerals Cu3Zn(OH)6Cl2 and Cu3Mg(OH)6Cl2 are spin 1/2 systems with an ideal kagome geometry. Based on electronic structure calculations, we develop a realistic model which includes couplings across the kagome hexagons beyond the original kagome model that are intrinsic in real kagome materials. Exact diagonalization studies for the derived model reveal a strong impact of these couplings on the magnetic ground state. Our predictions could be compared to and supplied with neutron scattering, thermodynamic and NMR data.Comment: 5 pages, 5 figures, 1 tabl

The Effect of Ru substitution for Ni on the superconductivity in MgCNi3-xRux

The superconductor MgCNi3 has been chemically doped by partial substitution of Ru for Ni in the solid solution MgCNi3-xRux for 0<x<0.5. Magnetic and specific heat measurements show that the Sommerfeld parameter (gamma_exp) and TC decrease immediately on Ru substitution, but that a TC above 2K is maintained even for a relatively large decrease in gamma_exp. Ferromagnetism is not observed to develop through Ru substitution, and the normal state magnetic susceptibility is suppressed.Comment: 18 pages, 13 figure

Prospects for improved Λc\Lambda_c branching fractions

The experimental uncertainty on the branching fraction \b(\Lambda_c \to p K^- \pi^+) = (5.0 \pm 1.3)% has not decreased since 1998, despite a much larger data sample. Uncertainty in this quantity dominates that in many other quantities, including branching fractions of $\Lambda_c$ to other modes, branching fractions of $b$-flavored baryons, and fragmentation fractions of charmed and bottom quarks. Here we advocate a lattice QCD calculation of the form factors in $\Lambda_c \to \Lambda \ell^+ \nu_\ell$ (the case $\ell = e^+$ is simpler as the mass of the lepton can be neglected). Such a calculation would yield an absolute prediction for the rate for $\Lambda_c \to \Lambda \ell^+ \nu_\ell$. When combined with the $\Lambda_c$ lifetime, it could provide a calibration for an improved set of $\Lambda_c$ branching fractions as long as the accuracy exceeds about 25%.Comment: 8 pages, 2 figures, version to be published in Phys.\ Rev.\

Coupled frustrated quantum spin-1/2 chains with orbital order in volborthite Cu3V2O7(OH)2(H2O)2

We present a microscopic magnetic model for the spin-liquid candidate volborthite Cu3V2O7(OH)2(H2O)2. The essentials of this DFT-based model are (i) the orbital ordering of Cu(1) 3d 3z2-r2 and Cu(2) 3d 3x2-y2, (ii) three relevant couplings J_ic, J_1 and J_2, (iii) the ferromagnetic nature of J_1 and (iv) frustration governed by the next-nearest-neighbor exchange interaction J_2. Our model implies magnetism of frustrated coupled chains in contrast to the previously proposed anisotropic kagome model. Exact diagonalization studies reveal agreement with experiments.Comment: 5 pages, 4 figures + supplementar

Properties of the Charmed P-wave Mesons

Two broad charmed mesons, the D_0^* and D_1', have recently been observed. We examine the quark model predictions for the D_0^* and D_1' properties and discuss experimental measurements that can shed light on them. We find that these states are well described as the broad, j=1/2 non-strange charmed P-wave mesons. Understanding the D_0^* and D_1' states can provide important insights into the D_{sJ}^*(2317), D_{sJ}(2460) states whose unexpected properties have led to renewed interest in hadron spectroscopy.Comment: 7 pages. Some additional discussion and reference

Electroweak Constraints from Atomic Parity Violation and Neutrino Scattering

Precision electroweak physics can provide fertile ground for uncovering new physics beyond the Standard Model (SM). One area in which new physics can appear is in so-called "oblique corrections", i.e., next-to-leading order expansions of bosonic propagators corresponding to vacuum polarization. One may parametrize their effects in terms of quantities $S$ and $T$ that discriminate between conservation and non-conservation of isospin. This provides a means of comparing the relative contributions of precision electroweak experiments to constraints on new physics. Given the prevalence of strongly $T$-sensitive experiments, there is an acute need for further constraints on $S$, such as provided by atomic parity-violating experiments on heavy atoms. We evaluate constraints on $S$ arising from recently improved calculations in the Cs atom. We show that the top quark mass $m_t$ provides stringent constraints on $S$ within the context of the Standard Model. We also consider the potential contributions of next-generation neutrino scattering experiments to improved $(S,T)$ constraints.Comment: 10 pages, 4 figures, final corrected version to be published in Physical Review