3,233 research outputs found
A QED Shower Including the Next-to-leading Logarithm Correction in e+e- Annihilation
We develop an event generator, NLL-QEDPS, based on the QED shower including
the next-to-leading logarithm correction in the e^+e^- annihilation. The shower
model is the Monte Carlo technique to solve the renormalization group equation
so that they can calculate contributions of alpha^m log^n(S/m_e^2) for any m
and n systematically. Here alpha is the QED coupling, m_e is the mass of
electron and S is the square of the total energy in the e^+e^- system. While
the previous QEDPS is limited to the leading logarithm approximation which
includes only contributions of (alpha log(S/m_e^2))^n, the model developed here
contains terms of alpha(alpha log(S/m_e^2))^n, the the next-to-leading
logarithm correction.
The shower model is formulated for the initial radiation in the e^+e^-
annihilation. The generator based on it gives us events with q^2, which is a
virtual mass squared of the virtual photon and/or Z-boson, in accuracy of
0.04%, except for small q^2/S.Comment: 35 pages, 1 figure(eps-file
Test of QEDPS: A Monte Carlo for the hard photon distributions in e+ e- annihilation proecss
The validity of a photon shower generator QEDPS has been examined in detail.
This is formulated based on the leading-logarithmic renormalization equation
for the electron structure function and it provides a photon shower along the
initial e+-. The main interest in the present work is to test the reliability
of the generator to describe a process accompanying hard photons which are
detected. For this purpose, by taking the HZ production as the basic reaction,
the total cross section and some distributions of the hard photons are compared
between two cases that these photons come from either those generated by QEDPS
or the hard process e+e- -> H Z gamma gamma. The comparison performed for the
single and the double hard photon has shown a satisfactory agreement which
demonstrated that the model is self-consistent.Comment: 22 pages, 4 Postscript figures, LaTeX, uses epsf.te
Four-quark final state in W-pair production: Case of signal and background
We discuss theoretical predictions for W-pair production and decay at LEP2
and higher energies in a form suitable for comparison with raw data. We present
a practical framework for calculating uncertainties of predictions given by the
KORALW and grc4f Monte Carlo programs. As an example we use observables in the
decay channel: the total four-quark (four-jet) cross section
and two-quark/jet invariant-mass distribution and cross section, in the case
when the other two may escape detection. Effects of QED bremsstrahlung,
effective couplings, running W and Z widths, Coulomb interaction and the
complete tree level set of diagrams are discussed. We also revisit the question
of technical precision of the new version 1.21 of the KORALW Monte Carlo code
as well as of version 1.2(26) of the grc4f one.
Finally we find predictions of the two programs to have an overall physical
uncertainty of 2%.
As a side result we show, on the example of an invariant mass
distribution, the strong interplay of spin correlations and detector cut-offs
in the case of four-fermion final states.Comment: 26 pages, LaTe
QED Radiative Corrections to the Non-annihilation Processes Using the Structure Function and the Parton Shower
Inclusion of the QED higher order radiative corrections in the two-photon
process, e+e- -> e+e- mu+mu-, is examined by means of the structure function
and the parton shower. Results are compared with the exact
calculations and give a good agreement. These two methods should be universally
applicable to any other non-annihilation processes like the single-W
productions in the e+e- collisions. In this case, however, the energy scale for
the evolution by the renormalization-group equation should be chosen properly
depending on the dominant diagrams for the given process. A method to find the
most suitable energy scale is proposed.Comment: 17 pages, LaTeX, 5 figure
Probing New Physics using top quark polarization in the e+e- -> t \bar{t} process at future Linear Colliders
We investigate the sensitivity to new physics of the process e+e- -> t bar{t}
when the top polarization is analyzed using leptonic final states e+e- -> t
\bar{t} -> l+l- b \bar{b} nu_l \bar{nu}_l. We first show that the kinematical
reconstruction of the complete kinematics is experimentally tractable for this
process. Then we apply the matrix element method to study the sensitivity to
the Vt\bar{t} coupling (V being a vector gauge boson), at the tree level and in
the narrow width approximation. Assuming the ILC baseline configuration,
sqrt{S}=500 GeV, and a luminosity of 500 fb^{-1}, we conclude that this optimal
analysis allows to determine simultaneously the ten form factors that
parameterize the Vt\bar{t} coupling, below the percent level. We also discuss
the effects of the next leading order (NLO) electroweak corrections using the
GRACE program with polarized beams. It is found that the NLO corrections to
different beam polarization lead to significantly different patterns of
contributions.Comment: 14 pages, 4 figures, Proceedings for the TYL-FJPPL workshops on "Top
Physics at ILC
QED Radiative Correction for the Single-W Production using a Parton Shower Method
A parton shower method for the photonic radiative correction is applied to
the single W-boson production processes. The energy scale for the evolution of
the parton shower is determined so that the correct soft-photon emission is
reproduced. Photon spectra radiated from the partons are compared with those
from the exact matrix elements, and show a good agreement. Possible errors due
to a inappropriate energy-scale selection or due to the ambiguity of energy
scale determination are also discussed, particularly for the measurements on
triple gauge-couplings.Comment: 17 pages, 6 Postscript figure
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