30,142 research outputs found
Charmless decays B->pipi, piK and KK in broken SU(3)symmetry
Charmless B decay modes and aresystematically
investigated with and without flavor SU(3) symmetry. Independent analyses on
and modes both favor a large ratio between color-suppressed
tree () and tree ( diagram, which suggests that they are more likely to
originate from long distance effects. The sizes of QCD penguin diagrams
extracted individually from , and modes are found to
follow a pattern of SU(3) breaking in agreement with the naive factorization
estimates. Global fits to these modes are done under various scenarios of
SU(3)relations. The results show good determinations of weak phase in
consistency with the Standard Model (SM), but a large electro-weak penguin
(P_{\tmop{EW}}) relative to with a large relative strong phase are
favored, which requires an big enhancement of color suppressed electro-weak
penguin (P_{\tmop{EW}}^C) compatible in size but destructively interfering
with P_{\tmop{EW}} within the SM, or implies new physics. Possibility of
sizable contributions from nonfactorizable diagrams such as -exchange (),
annihilation() and penguin-annihilation diagrams() are investigated.
The implications to the branching ratios and CP violations in modes are
discussed.Comment: 27 pages, 9 figures, reference added, to appear in Phy.Rev.
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Collapse Mechanism of Single-Layer Cylindrical Latticed Shell under Severe Earthquake
In this paper, the results of finite element analyses of a single-layer cylindrical latticed shell under severe earthquake are presented. A 3D Finite Element model using fiber beam elements is used to investigate the collapse mechanism of this type of shell. The failure criteria of structural members are simulated based on the theory of damage accumulation. Severe earthquakes with peak ground acceleration (PGA) values of 0.5 g are applied to the shell. The stress and deformation of the shell are studied in detail. A three-stage collapse mechanism “double-diagonal -members-failure-belt” of this type of structure is discovered. Based on the analysis results, measures to mitigate the collapse of this type of structure are recommended
Kinetics of spin coherence of electrons in -type InAs quantum wells under intense terahertz laser fields
Spin kinetics in -type InAs quantum wells under intense terahertz laser
fields is investigated by developing fully microscopic kinetic spin Bloch
equations via the Floquet-Markov theory and the nonequilibrium Green's function
approach, with all the relevant scattering, such as the electron-impurity,
electron-phonon, and electron-electron Coulomb scattering explicitly included.
We find that a {\em finite} steady-state terahertz spin polarization induced by
the terahertz laser field, first predicted by Cheng and Wu [Appl. Phys. Lett.
{\bf 86}, 032107 (2005)] in the absence of dissipation, exists even in the
presence of all the scattering. We further discuss the effects of the terahertz
laser fields on the spin relaxation and the steady-state spin polarization. It
is found that the terahertz laser fields can {\em strongly} affect the spin
relaxation via hot-electron effect and the terahertz-field-induced effective
magnetic field in the presence of spin-orbit coupling. The two effects compete
with each other, giving rise to {\em non-monotonic} dependence of the spin
relaxation time as well as the amplitude of the steady state spin polarization
on the terahertz field strength and frequency. The terahertz field dependences
of these quantities are investigated for various impurity densities, lattice
temperatures, and strengths of the spin-orbit coupling. Finally, the importance
of the electron-electron Coulomb scattering on spin kinetics is also addressed.Comment: 17 pages, 16 figures, Phys. Rev. B 78, 2008, in pres
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Electron quantum interference in epitaxial antiferromagnetic NiO thin films
The electron reflectivity from NiO thin films grown on Ag(001) has been systematically studied as a function of film thickness and electron energy. A strong electron quantum interference effect was observed from the NiO film, which is used to derive the unoccupied band dispersion above the Fermi surface along the Γ-X direction using the phase accumulation model. The experimental bands agree well with first-principles calculations. A weaker electron quantum interference effect was also observed from the CoO film
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