103 research outputs found
Polarized top decay into polarized W: t(up)->W(up) + b at O(alpha_s)
We consider the decay of a polarized top quark into a polarized W-boson plus
a bottom quark, followed by the decay of the W-boson into a pair of leptons or
quarks. The polar angle distribution of the top spin relative to the W-momentum
and the polar angle distribution of the lepton (or quark) in the W-rest frame
is governed by three polarized and three unpolarized rate functions which are
related to the double density matrix elements of the decay . We
obtain analytical expressions for the radiative corrections to
the three polarized and three unpolarized rate functions. We also provide a
comprehensive discussion of the dependence of the longitudinal, transverse and
normal polarization of the top quarks produced at -colliders on beam
polarization parameters.Comment: 13 pages, 3 postscript figures added, file standard.sty included,
material on normal polarization added, some misprints correcte
Spin-Momentum Correlations in Inclusive Semileptonic Decays of Polarized Baryons
We consider spin-momentum correlations between the spin of the bottom baryon
and the momenta of its decay products in its inclusive
semileptonic decay. We define several polar and azimuthal spin-momentum
correlation measures in different event coordinate systems. The values of the
spin-momentum correlation measures are calculated up to
using the standard OPE und HQET methods. Some of the measures turn out to be
sufficiently large to make them good candidates for a determination of the
polarization of the in e.g. decays.Comment: 13 pages, 2 figures, more numerical results added; version published
as Phys. Rev. D60, 014021 (1999
On the evaluation of a certain class of Feynman diagrams in x-space: Sunrise-type topologies at any loop order
We review recently developed new powerful techniques to compute a class of
Feynman diagrams at any loop order, known as sunrise-type diagrams. These
sunrise-type topologies have many important applications in many different
fields of physics and we believe it to be timely to discuss their evaluation
from a unified point of view. The method is based on the analysis of the
diagrams directly in configuration space which, in the case of the sunrise-type
diagrams and diagrams related to them, leads to enormous simplifications as
compared to the traditional evaluation of loops in momentum space. We present
explicit formulae for their analytical evaluation for arbitrary mass
configurations and arbitrary dimensions at any loop order. We discuss several
limiting cases of their kinematical regimes which are e.g. relevant for
applications in HQET and NRQCD.Comment: 100 pages, 16 eps-figures include
Towards Order alpha_s^4 Accuracy in tau-decays
Recently computed terms of orders O(\alpha_s^4 n_f^2) in the perturbative
series for the tau decay rate, and similar (new) strange quark mass
corrections, are used to discuss the validity of various optimization schemes.
The results are then employed to arrive at improved predictions for the
complete terms order O(\alpha_s^4) and O(\alpha_s^5) in the massless limit as
well as for terms due to the strange quark mass. Phenomenological implications
are presented.Comment: 14 pages, REVTeX4. The complete paper is also available via the www
at http://www-ttp.physik.uni-karlsruhe.de/Preprints
Hadronic top-quark pair production in association with a hard jet at next-to-leading order QCD: Phenomenological studies for the Tevatron and the LHC
We report on the calculation of the next-to-leading order QCD corrections to
the production of top--antitop-quark pairs in association with a hard jet at
the Tevatron and at the LHC. Results for integrated and differential cross
sections are presented. We find a significant reduction of the scale
dependence. In most cases the corrections are below 20% indicating that the
perturbative expansion is well under control. Moreover, the forward--backward
charge asymmetry of the top-quark, which is analyzed at the Tevatron, is
studied at next-to-leading order. We find large corrections suggesting that the
definition of the observable has to be refined.Comment: 37 pages, version to be publishe
PYTHIA 6.4 Physics and Manual
The PYTHIA program can be used to generate high-energy-physics `events', i.e.
sets of outgoing particles produced in the interactions between two incoming
particles. The objective is to provide as accurate as possible a representation
of event properties in a wide range of reactions, within and beyond the
Standard Model, with emphasis on those where strong interactions play a role,
directly or indirectly, and therefore multihadronic final states are produced.
The physics is then not understood well enough to give an exact description;
instead the program has to be based on a combination of analytical results and
various QCD-based models. This physics input is summarized here, for areas such
as hard subprocesses, initial- and final-state parton showers, underlying
events and beam remnants, fragmentation and decays, and much more. Furthermore,
extensive information is provided on all program elements: subroutines and
functions, switches and parameters, and particle and process data. This should
allow the user to tailor the generation task to the topics of interest.Comment: 576 pages, no figures, uses JHEP3.cls. The code and further
information may be found on the PYTHIA web page:
http://www.thep.lu.se/~torbjorn/Pythia.html Changes in version 2: Mistakenly
deleted section heading for "Physics Processes" reinserted, affecting section
numbering. Minor updates to take into account referee comments and new colour
reconnection option
Further Experimental Studies of Two-Body Radiative \Upsilon Decays
Continuing our studies of radiative Upsilon(1S) decays, we report on a search
for Upsilon to gamma eta and Upsilon to gamma f_{J}(2220) in 61.3 pb^{-1} of
e^{+}e^{-} data taken with the CLEO II detector at the Cornell Electron Storage
Ring. For the gamma eta search the three decays of the eta meson to
pi^{+}pi^{-}pi^{0}, pi^{0}pi^{0}pi^{0}, and gamma gamma were investigated. We
found no candidate events in the two (3\pi)^{0} modes and no significant excess
over expected backgrounds in the gamma gamma mode to set a limit on the
branching fraction of B(Upsilon to gamma eta) < 2.1 x 10^{-5} at 90% C.L. The
three charged two-body final states h h-bar (h = pi^{+}, K^{+}, p) were
investigated for f_{J}(2220) production, with one, one, and two events found,
respectively. Limits at 90% C.L. of B(\Upsilon to gamma f_{J}) x B(f_{J} to h
h-bar) ~ 1.5 x 10^{-5} have been set for each of these modes. We compare our
results to measurements of other radiative Upsilon decays, to measurements of
radiative J/psi decays, and to theoretical predictions.Comment: 19 pages postscript, also available through
http://w4.lns.cornell.edu/public/CLNS, submitted to Physical Review
A measurement of the tau mass and the first CPT test with tau leptons
We measure the mass of the tau lepton to be 1775.1+-1.6(stat)+-1.0(syst.) MeV
using tau pairs from Z0 decays. To test CPT invariance we compare the masses of
the positively and negatively charged tau leptons. The relative mass difference
is found to be smaller than 3.0 10^-3 at the 90% confidence level.Comment: 10 pages, 4 figures, Submitted to Phys. Letts.
K pi vector form factor, dispersive constraints and tau -> nu_tau K pi decays
Recent experimental data for the differential decay distribution of the decay
by the Belle collaboration are described by a
theoretical model which is composed of the contributing vector and scalar form
factors and . Both form factors are constructed
such that they fulfil constraints posed by analyticity and unitarity. A good
description of the experimental measurement is achieved by incorporating two
vector resonances and working with a three-times subtracted dispersion relation
in order to suppress higher-energy contributions. The resonance parameters of
the charged meson, defined as the pole of in the
complex -plane, can be extracted, with the result MeV and MeV. Finally, employing the
three-subtracted dispersion relation allows to determine the slope and
curvature parameters and
of the vector form factor
directly from the data.Comment: 16 pages, 1 figure, version to appear in Eur. Phys. J.
Heavy baryon properties with NLO accuracy in perturbative QCD
We present an analysis of the static properties of heavy baryons at
next-to-leading order in the perurbative expansion of QCD. We obtain analytical
next-to-leading order three-loop results for the two-point correlators of
baryonic currents with one finite mass quark field for a variety of quantum
numbers of the baryonic currents. We consider both the massless limit and the
HQET limit of the correlator as special cases of the general finite mass
formula and find agreement with previous results. We present closed form
expressions for the moments of the spectral density. We determine the residues
of physical baryon states using sum rule techniques.Comment: 43 pages in LaTeX, including 3 figure
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