8,218 research outputs found
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 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 to other modes,
branching fractions of -flavored baryons, and fragmentation fractions of
charmed and bottom quarks. Here we advocate a lattice QCD calculation of the
form factors in (the case
is simpler as the mass of the lepton can be neglected). Such a calculation
would yield an absolute prediction for the rate for . When combined with the lifetime, it could provide
a calibration for an improved set of 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
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