5,690 research outputs found
Determining the Quark Mixing Matrix From CP-Violating Asymmetries
If the Standard Model explanation of CP violation is correct, then
measurements of CP-violating asymmetries in meson decays can in principle
determine the entire quark mixing matrix.Comment: 8 pages (plain TeX), 1 figure (postscript file appended), DAPNIA/SPP
94-06, NSF-PT-94-2, UdeM-LPN-TH-94-18
The bar PANDA focussing-lightguide disc DIRC
bar PANDA will be a fixed target experiment internal to the HESR antiproton storage ring at the future FAIR complex. The ANDA detector requires excellent particle-identification capabilities in order to achieve its scientific potential. Cherenkov counters employing the DIRC principle were chosen as PID detectors for the Target Spectrometer. The proposed Focussing-Lightguide Disc DIRC will cover the forward part of the Target Spectrometer acceptance in the angular range between 5° and 22°. Its design includes a novel approach to mitigate dispersion effects in the solid radiator of a DIRC counter using optical elements. The dispersion correction will enable the Focussing-Lightguide Disc DIRC to provide pion-kaon identification for momenta well above 3.5 GeV/c
Flavor symmetry and decays of charmed mesons to pairs of light pseudoscalars
New data on the decays of the charmed particles , , and to
pairs of light pseudoscalar mesons allow the testing of flavor symmetry and
the extraction of key amplitudes. Information on relative strong phases is
obtained. One sees evidence for the expected interference between
Cabibbo-favored and doubly-Cabibbo-suppressed decays in the differing patterns
of and decays.Comment: 12 pages, 5 figures, to be submitted to Phys. Rev.
Superconductivity in the New Platinum Germanides MPt4Ge12 (M = Rare-earth and Alkaline-earth Metals) with Filled Skutterudite Structure
New germanium-platinum compounds with the filled-skutterudite crystal
structure were synthesized. The structure and composition were investigated by
X-ray diffraction and microprobe analysis. Magnetic susceptibility, specific
heat, and electrical resistivity measurements evidence superconductivity in
LaPt4Ge12 and PrPt4Ge12 below 8.3K. The parameters of the normal and
superconducting states were established. Strong coupling and a crystal electric
field singlet groundstate is found for the Pr compound. Electronic structure
calculations show a large density of states at the Fermi level. Similar
behavior with lower T_c was observed for SrPt4Ge12 and BaPt4Ge12.Comment: RevTeX, 4 figures, submitted to Physical Review Letters July 12, 200
Antiferromagnetic fluctuations in the normal state of LiFeAs
We present a detailed study of 75As NMR Knight shift and spin-lattice
relaxation rate in the normal state of stoichiometric polycrystalline LiFeAs.
Our analysis of the Korringa relation suggests that LiFeAs exhibits strong
antiferromagnetic fluctuations, if transferred hyperfine coupling is a dominant
interaction between 75As nuclei and Fe electronic spins, whereas for an on-site
hyperfine coupling scenario, these are weaker, but still present to account for
our experimental observations. Density-functional calculations of electric
field gradient correctly reproduce the experimental values for both 75As and
7Li sites.Comment: 5 pages, 3 figures, thoroughly revised version with refined
experimental data, accepted for publication as a Rapid Communication in
Physical Review B
Interplay of atomic displacements in the quantum magnet (CuCl)LaNb2O7
We report on the crystal structure of the quantum magnet (CuCl)LaNb2O7 that
was controversially described with respect to its structural organization and
magnetic behavior. Using high-resolution synchrotron powder x-ray diffraction,
electron diffraction, transmission electron microscopy, and band structure
calculations, we solve the room-temperature structure of this compound
[alpha-(CuCl)LaNb2O7] and find two high-temperature polymorphs. The
gamma-(CuCl)LaNb2O7 phase, stable above 640K, is tetragonal with a(sub) = 3.889
A, c(sub) = 11.738 A, and the space group P4/mmm. In the gamma-(CuCl)LaNb2O7
structure, the Cu and Cl atoms are randomly displaced from the special
positions along the {100} directions. The beta-phase [a(sub) x 2a(sub) x
c(sub), space group Pbmm] and the alpha-phase [2a(sub) x 2a(sub) x c(sub),
space group Pbam] are stable between 640 K and 500 K and below 500 K,
respectively. The structural changes at 500 K and 640 K are identified as
order-disorder phase transitions. The displacement of the Cl atoms is frozen
upon the gamma --> beta transformation, while a cooperative tilting of the NbO6
octahedra in the alpha-phase further eliminates the disorder of the Cu atoms.
The low-temperature alpha-(CuCl)LaNb2O7 structure thus combines the two types
of the atomic displacements that interfere due to the bonding between the Cu
atoms and the apical oxygens of the NbO6 octahedra. The precise structural
information resolves the controversy between the previous computation-based
models and provides the long-sought input for understanding the magnetic
properties of (CuCl)LaNb2O7.Comment: 12 pages, 10 figures, 5 tables; crystallographic information (cif
files) include
Angular distributions in states near threshold
A resonance X(3872), first observed in the decays , has been seen
to decay to . The mass spectrum peaks near
its kinematic upper limit, prompting speculation that the dipion system may be
in a . The decay also has been observed.
The reaction has been studied.
Consequently, angular distributions in decays of
states near threshold are of interest, and results are presented.Comment: 10 pages, no figures. To be submitted to Phys. Rev.
Comparative investigation of the coupled-tetrahedra quantum spin systems Cu2Te2O5X2, X=Cl, Br and Cu4Te5O12Cl4
We present a comparative study of the coupled-tetrahedra quantum spin systems
Cu2Te2O5X2, X=Cl, Br (Cu-2252(X)) and the newly synthesized Cu4Te5O12Cl4
(Cu-45124(Cl)) based on ab initio Density Functional Theory calculations. The
magnetic behavior of Cu-45124(Cl) with a phase transition to an ordered state
at a lower critical temperature T=13.6K than in Cu-2252(Cl) (T=18K) can
be well understood in terms of the modified interaction paths. We identify the
relevant structural changes between the two systems and discuss the
hypothetical behavior of the not yet synthesized Cu-45124(Br) with an ab initio
relaxed structure using Car-Parrinello Molecular Dynamics.Comment: 2 pages, 1 figure; submitted to Proceedings of M2S-HTSC VIII, Dresden
200
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