4,161 research outputs found
Top Quark Pair Production close to Threshold: Top Mass, Width and Momentum Distribution
The complete NNLO QCD corrections to the total cross section in the kinematic region close to the top-antitop
threshold are calculated by solving the corresponding Schroedinger equations
exactly in momentum space in a consistent momentum cutoff regularization
scheme. The corrections coming from the same NNLO QCD effects to the top quark
three-momentum distribution are determined. We discuss
the origin of the large NNLO corrections to the peak position and the
normalization of the total cross section observed in previous works and propose
a new top mass definition, the 1S mass M_1S, which stabilizes the peak in the
total cross section. If the influence of beamstrahlung and initial state
radiation on the mass determination is small, a theoretical uncertainty on the
1S top mass measurement of 200 MeV from the total cross section at the linear
collider seems possible. We discuss how well the 1S mass can be related to the
mass. We propose a consistent way to implement the top quark width
at NNLO by including electroweak effects into the NRQCD matching coefficients,
which then can become complex.Comment: 53 pages, latex; minor changes, a number of typos correcte
SU(3)_LxU(1)_N Model for Right-Handed Neutrino Neutral Currents
A model based on the \mbox{SU(3)}_L\otimes \mbox{U(1)}_N gauge group, in
which neutrinos have right-handed neutral currents is considered. We argue that
in order to have a result consistent with low-energy one, the right-handed
neutrino component must be treated as correction instead of an equivalent spin
state.Comment: 6 pages, Latex, no figures, Accepted for publication in Phys. Rev.
Applications of QCD
Talk given at XIXth International Symposium on Lepton and Photon Interactions
at High Energies (LP 99), Stanford, California, 9-14 August 1999.Comment: latex, 26 page
The b quark low-scale running mass from Upsilon sum rules
The b quark low-scale running mass m_kin is determined from an analysis of
the Upsilon sum rules in the next-to-next-to-leading order (NNLO). It is
demonstrated that using this mass one can significantly improve the convergence
of the perturbation series for the spectral density moments. We obtain m_kin(1
GeV) = 4.56 \pm 0.06 GeV. Using this result we derive the value of the MS-bar
mass m: m(m) = 4.20 \pm 0.1 GeV. Contrary to the low-scale running mass, the
pole mass of the b quark cannot be reliably determined from the sum rules. As a
byproduct of our study we find the NNLO analytical expression for the cross
section e+e- --> Q\bar Q of the quark antiquark pair production in the
threshold region, as well as the energy levels and the wave functions at the
origin for the ^1S_3 bound states of Q\bar Q.Comment: 22 pages, Late
Exclusive Hadronic D Decays to eta' and eta
Hadronic decay modes and
are studied in the generalized
factorization approach. Form factors for transitions
are carefully evaluated by taking into account the wave function normalization
of the eta and eta'. The predicted branching ratios are generally in agreement
with experiment except for and
; the calculated decay rates for the first two decay modes
are too small by an order of magnitude. We show that the weak decays and followed by resonance-induced final-state
interactions (FSI), which are amenable technically, are able to enhance the
branching ratios of and dramatically
without affecting the agreement between theory and experiment for and . We argue that it is difficult to understand
the observed large decay rates of and
simultaneously; FSI, W-annihilation and the production of excess eta' from
gluons are not helpful in this regard. The large discrepancy between the
factorization hypothesis and experiment for the ratio of
and remains as an enigma.Comment: 15 pages, 1 figure, to appear in Phys. Rev. D. Form factors for D to
eta and eta' transitions are slightly change
Relativistic dynamical polarizability of hydrogen-like atoms
Using the operator representation of the Dirac Coulomb Green function the
analytical method in perturbation theory is employed in obtaining solutions of
the Dirac equation for a hydrogen-like atom in a time-dependent electric field.
The relativistic dynamical polarizability of hydrogen-like atoms is calculated
and analysed.Comment: 15 pages, 3 figures (not included, but hard copies are available upon
request
Preparation and Foliar Application of Oligochitosan - Nanosilica on the Enhancement of Soybean Seed Yield
Oligochitosan with weight average molecu-lar weight (Mw) of 5000 g/mol was prepared by gamma Co-60 radiation degradation of 4% chitosan solution containing 0.5% H2O2 at 21 kGy. Nanosilica with size of 10 – 30 nm was synthesized by calcination of acid treated rice husk at 700o C for 2 h. The mixture of 2% oligo-chitosan-2% nanosilica was prepared by dispersion of nanosilica in oligochitosan solution. Oligochitosan, nanosilica and their mixture were characterized by gel permeation chromatography (GPC), transmission electr-on microscopy (TEM), X-ray diffraction (XRD), energy dispersive x-ray spectroscopy (EDX), Ultraviolet-visible spectroscopy (UV-Vis), and Furrier transform infrared spectroscopy (FT-IR). Effect of foliar application of oli-gochitosan and oligochitosan-nanosilica on soybean seed yield was conducted in experimental field. Results indi-cated that soybean seed yield increased 10.5 and 17.0% for oligochitosan and oligochitosan-nanosilica, respect-tively for the control. Radiation degraded oligo-chitosan and its mixture with nanosilica can be potentially used for cultivation of soybean with enhanced seed yield
Low-energy gluon contributions to the vacuum polarization of heavy quarks
We calculate a correction to the electromagnetic current induced by a heavy
quark loop. The contribution of this correction to the vacuum polarization
function appears at the O(alpha_s^3) order of perturbation theory and has a
qualitatively new feature -- its absorptive part starts at zero energy in
contrast to other contributions where the absorptive parts start at the
two-particle threshold. Our result imposes a constraint on the order n of the
moments used in the heavy-quark sum rules, n<4.Comment: 8 pages in LaTeX, 1 PostScript figure included in the tex
Virtual and Soft Pair Corrections to Polarized Muon Decay Spectrum
Radiative corrections to the muon decay spectrum due to soft and virtual
electron--positron pairs are calculated.Comment: 10pp, 2 PS figs, details of calculations are adde
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