57 research outputs found
Momentum Distributions in
We apply the Green function formalism for production and decay
near threshold in a study of the effects due to the momentum dependent width
for such a system. We point out that these effects are likely to be much
smaller than expected from the reduction of the available phase space. The
Lippmann--Schwinger equation for the QCD chromostatic potential is solved
numerically for partial wave. We compare the results on the total cross
section, top quark intrinsic momentum distributions and on the energy spectra
of bosons from top quark decays with those obtained for the constant width.Comment: 12 pages (without figures) (11 (sub)figures available on request),
Karlsruhe preprint TTP93-11, hep-ph/yymmnn
Comment on the Average Momentum of Top Quarks in the Threshold Region
The behavior of the momentum distribution of top quarks in the threshold
region is investigated. The qualitative behavior, in particular the dependence
of the average momentum on the strong coupling constant can be understood from
analytical calculations for the Coulomb potential. Ambiguities in the relation
between the excitation curve and the top mass are addressed.Comment: Univ. of Karlsruhe preprint TTP93-31, LaTeX, 6 pages including two
figures (uses epsf) appended as uuencoded postscript fil
Polarization in Top Quark Production and Decay near Threshold
Theoretical results are presented for top quarks produced in annihilation of
polarized electrons on positrons. Polarization studies for pairs near
threshold are free from hadronization ambiguities. This is due to the short
lifetime of the top quark. Semileptonic decays are discussed as well as their
applications in studying polarization dependent processes involving top quarks.
The Green function formalism is applied to production at future
colliders with polarized beams. Lippmann--Schwinger equation is solved
numerically for the QCD chromostatic potential given by the two-loop formula at
large momentum transfers and Richardson ansatz at intermediate and small ones.
The polarization dependent momentum distributions of top quarks and their decay
products are calculated.Comment: 6 pages, LaTex (uses epsf.sty, 2 figures appended as uuencoded ps
files). The complete paper, including figures, is also available via
anonymous ftp at ttpux2.physik.uni-karlsruhe.de (129.13.102.139) as
/ttp94-24/ttp94-24.ps, or via www at
http://ttpux2.physik.uni-karlsruhe.de/preprints.html
Final-State Interactions in e^+e^- -> t\bar{t} -> b l^+ \nu \bar{b} W^- Near Top Quark Threshold
We calculate final-state interaction corrections to the energy-angular
distribution of l^+ in semi-leptonic top quark decay, where the parent top
quark is produced via e^+e^- -> t\bar{t} near threshold. These are the
corrections due to gluon exchange between t and \bar{b} (\bar{t} and b) and
between b and \bar{b}. Combining with previously known other corrections, we
explicitly write down the l^+ energy-angular distribution including the full
O(alpha_s)=O(beta) corrections near t\bar{t} threshold. Numerical analyses of
the final-state interaction corrections are given. We find that they deform the
l^+ distribution typically at the 10% level. We also find that all qualitative
features of the numerical results can be understood from intuitive pictures.
The mechanisms of various effects of the final-state interactions are
elucidated. Finally we define an observable which is proper to the decay
process of the top quark (dependent only on d\Gamma_{t -> b l^+ \nu}/ dE_l
d\Omega_l of a free polarized top quark) near t\bar{t} threshold. Such a
quantity will be useful in extracting the decay property of the top quark using
the highly polarized top quark samples.Comment: Section 6 is enlarged substantially and a paragraph is added to
Section 8 correspondingly. Reference list is also improved. (27 pages
including figures, LaTeX
Gluon interference effects in t-tbar production at the Next Linear Collider
We study the effects of gluon interference in the production and
semi-leptonic decay of a t-tbar pair above threshold at the Next Linear
Collider (NLC). We calculate all matrix elements to next-to-leading order and
use the resulting expressions for the development of a Monte Carlo event
generator. Our results show effects at the level of 10% in differential
cross-sections. We thus extend previous results obtained by analytical means in
the soft-gluon limit.Comment: 20 pages, incl. 9 figures, LaTeX (revtex, eepic, axodraw
Electroweak and finite width corrections to top quark decays into transverse and longitudinal -bosons
We calculate the electroweak and finite width corrections to the decay of an
unpolarized top quark into a bottom quark and a -gauge boson where the
helicities of the are specified as longitudinal, transverse-plus and
transverse-minus. Together with the corrections these
corrections may become relevant for the determination of the mass of the top
quark through angular decay measurements.Comment: 4 pages, 7 postscript figures adde
The Perturbative QCD Potential and the ttbar Threshold
We include the full second-order corrections to the static QCD potential in
the analysis of the ttbar threshold cross section. There is an unexpectedly
large difference between the QCD potential improved by the
renormalization-group equation in momentum space and the potential improved by
the renormalization-group equation in coordinate space. This difference remains
even at a fairly short distance 1/r \simeq 100 GeV and its origin can be
understood within perturbative QCD. We scrutinize the theoretical uncertainties
of the QCD potential in relation to the ttbar threshold cross section. In
particular there exists a theoretical uncertainty which limits our present
theoretical accuracy of the ttbar threshold cross section at the peak to be not
better than 6% within perturbative QCD.Comment: 16 pages, LaTeX. Improved version of hep-ph/9801419 for submission to
a journa
Semileptonic b --> u decays: lepton invariant mass spectrum
We compute O(alpha_s^2) QCD corrections to the lepton invariant mass spectrum
in the decay b --> u l nu_l, relevant for the determination of the CKM matrix
element |V_{ub}|. Our method can also be used to evaluate moments of the lepton
energy distribution with an O(alpha_s^2) accuracy. The abelian part of our
result gives the neutrino invariant mass spectrum in the muon decay and, upon
integration, the O(alpha^2) correction to the muon lifetime.Comment: 5 pages, revte
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