10,373 research outputs found
Bethe--Salpeter equation in QCD
We extend to regular QCD the derivation of a confining
Bethe--Salpeter equation previously given for the simplest model of scalar QCD
in which quarks are treated as spinless particles. We start from the same
assumptions on the Wilson loop integral already adopted in the derivation of a
semirelativistic heavy quark potential. We show that, by standard
approximations, an effective meson squared mass operator can be obtained from
our BS kernel and that, from this, by expansion the
corresponding Wilson loop potential can be reobtained, spin--dependent and
velocity--dependent terms included. We also show that, on the contrary,
neglecting spin--dependent terms, relativistic flux tube model is reproduced.Comment: 23 pages, revte
From scalar to string confinement
We outline a connection between scalar quark confinement, a
phenomenologically successful concept heretofore lacking fundamental
justification, and QCD. Although scalar confinement does not follow from QCD,
there is an interesting and close relationship between them. We develop a
simple model intermediate between scalar confinement and the QCD string for
illustrative purposes. Finally, we find the bound state masses of scalar,
time-component vector, and string confinement analytically through
semi-classical quantization.Comment: ReVTeX, 9 pages, 5 figure
Semi-leptonic B decays into higher charmed resonances
We apply HQET to semi-leptonic meson decays into a variety of excited
charm states. Using three realistic meson models with fermionic light degrees
of freedom, we examine the extent that the sum of exclusive single charmed
states account for the inclusive semi-leptonic decay rate. The consistency
of form factors with the Bjorken and Voloshin sum rules is also investigated.Comment: Latex, 27 pages. A few references and errors corrected, to appear in
Phys. Rev.
The Impact of NLO-Corrections on the Determination of the $\bar{u},\bar{d} Content of Nucleons from Drell-Yan Production
The interpretation of Drell-Yan production in terms of the antiquark
densities depends on NLO corrections. Besides the NLO corrections to the
familiar annihilation , there is a
substantial contribution from the QCD Compton subprocesses and . The beam and target
dependence of the two classes of corrections is different. We discuss the
impact of this difference on the determination of the
asymmetry in the proton from the comparison of the and Drell-Yan
production.Comment: 4 pages, 1 eps-figure. To be published in Proceedings of DIS'9
Switching Dynamics in Reaction Networks Induced by Molecular Discreteness
To study the fluctuations and dynamics in chemical reaction processes,
stochastic differential equations based on the rate equation involving chemical
concentrations are often adopted. When the number of molecules is very small,
however, the discreteness in the number of molecules cannot be neglected since
the number of molecules must be an integer. This discreteness can be important
in biochemical reactions, where the total number of molecules is not
significantly larger than the number of chemical species. To elucidate the
effects of such discreteness, we study autocatalytic reaction systems
comprising several chemical species through stochastic particle simulations.
The generation of novel states is observed; it is caused by the extinction of
some molecular species due to the discreteness in their number. We demonstrate
that the reaction dynamics are switched by a single molecule, which leads to
the reconstruction of the acting network structure. We also show the strong
dependence of the chemical concentrations on the system size, which is caused
by transitions to discreteness-induced novel states.Comment: 11 pages, 5 figure
Reduction of the QCD string to a time component vector potential
We demonstrate the equivalence of the relativistic flux tube model of mesons
to a simple potential model in the regime of large radial excitation. We make
no restriction on the quark masses; either quark may have a zero or finite
mass. Our primary result shows that for fixed angular momentum and large radial
excitation, the flux tube/QCD string meson with a short-range Coulomb
interaction is described by a spinless Salpeter equation with a time component
vector potential V(r) = ar - k/r.Comment: RevTeX4, 10 pages, 3 eps figure
Semileptonic form factors - a model-independent approach
We demonstrate that the B->D(*) l nu form factors can be accurately predicted
given the slope parameter rho^2 of the Isgur-Wise function. Only weak
assumptions, consistent with lattice results, on the wavefunction for the light
degrees of freedom are required to establish this result. We observe that the
QCD and 1/m_Q corrections can be systematically represented by an effective
Isgur-Wise function of shifted slope. This greatly simplifies the analysis of
semileptonic B decay. We also investigate what the available semileptonic data
can tell us about lattice QCD and Heavy Quark Effective Theory. A rigorous
identity relating the form factor slope difference rho_D^2-rho_A1^2 to a
combination of form factor intercepts is found. The identity provides a means
of checking theoretically evaluated intercepts with experiment.Comment: 18 pages, Revtex, 4 postscript figures, uses epsfig.st
Scanning tunneling microscopy and spectroscopy of sodium-chloride overlayers on the stepped Cu(311) surface: Experimental and theoretical study
The physical properties of ultrathin NaCl overlayers on the stepped Cu(311)
surface have been characterized using scanning tunneling microscopy (STM) and
spectroscopy, and density functional calculations. Simulations of STM images
and differential conductance spectrum were based on the Tersoff-Hamann
approximation for tunneling with corrections for the modified tunneling barrier
at larger voltages and calculated Kohn-Sham states. Characteristic features
observed in the STM images can be directly related to calculated electronic and
geometric properties of the overlayers. The measured apparent barrier heights
for the mono-, bi-, and trilayers of NaCl and the corresponding
adsorption-induced changes in the work function, as obtained from the distance
dependence of the tunneling current, are well reproduced by and understood from
the calculated results. The measurements revealed a large reduction of the
tunneling conductance in a wide voltage region, resembling a band gap. However,
the simulated spectrum showed that only the onset at positive sample voltages
may be viewed as a valence band edge, whereas the onset at negative voltages is
caused by the drastic effect of the electric field from the tip on the
tunneling barrier
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