17 research outputs found
Modified perturbation theory approach for tt-bar production and decay
The modified perturbation theory (MPT), based on direct expansion of
probabilities instead of amplitudes, allows one to avoid divergences in the
phase-space integrals resulting from production and decay of unstable
particles. In the present paper the range of applicability of MPT is determined
numerically in the case of the process . It is shown that with the complete expansion in powers of
the coupling constant (without Dyson resummation) MPT operates best at the
energies located near the maximum of the cross-section and slightly above the
maximum. In this region the MPT expansion within the next-to-leading order
considerably exceeds in accuracy well-known DPA approach.Comment: LaTeX, 11 pages, 3 eps figure
OMS-bar scheme of UV renormalization in the presence of unstable fundamental particles
A generalization of the on-mass-shell scheme of UV renormalization (the
OMS-bar scheme) to the case of presence of unstable fundamental particles (like
W and Z bosons) is proposed. Its basic ingredients are as follows: (i) the
renormalized mass coincides with a real part of the position of the complex
pole of the corresponding propagator, (ii) the imaginary part of the on-shell
self-energy coincides with the imaginary part of the complex pole position. The
latter property implies the gauge-invariance of the imaginary part of the
on-shell self-energy in the OMS-bar scheme and its connection with the lifetime
of an unstable particle. Starting with the three-loops this connection becomes
nontrivial.Comment: 8 pages, LaTeX, no figures. 3 references are added, 2 misprints are
corrected, minor changes in the tex
Elimination of Threshold Singularities in the Relation Between On-Shell and Pole Widths
In a previous communication by two of us, Phys. Rev. Lett. 81, 1373 (1998),
the gauge-dependent deviations of the on-shell mass and total decay width from
their gauge-independent pole counterparts were investigated at leading order
for the Higgs boson of the Standard Model. In the case of the widths, the
deviation was found to diverge at unphysical thresholds, m_H = 2 root{xi_V} m_V
(V = W,Z), in the R_xi gauge. In this Brief Report, we demonstrate that these
unphysical threshold singularities are properly eliminated if a recently
proposed definition of wave-function renormalization for unstable particles is
invoked.Comment: 8 pages (Latex), 1 figure (Postscript
MHV Vertices and Scattering Amplitudes in Gauge Theory
The generic googly amplitudes in gauge theory are computed by using the
Cachazo-Svrcek-Witten approach to perturbative calculation in gauge theory and
the results are in agreement with the previously well-known ones. Within this
approach we also discuss the parity transformation, charge conjugation and the
dual Ward identity. We also extend this calculation to include fermions and the
googly amplitudes with a single quark-anti-quark pair are obtained correctly
from fermionic MHV vertices. At the end we briefly discuss the possible
extension of this approach to gravity.Comment: Latex file, 38 pages, 15 figures; v2, minor changes, references
added; v2, minor changes, 2 references adde
MHV Vertices and Fermionic Scattering Amplitudes in Gauge Theory with Quarks and Gluinos
The Cachazo-Svrcek-Witten approach to perturbative gauge theory is extended
to gauge theories with quarks and gluinos. All googly amplitudes with
quark-antiquark pairs and gluinos are computed and shown to agree with the
previously known results. The computations of the non-MHV or non-googly
amplitudes are also briefly discussed, in particular the purely fermionic
amplitude with 3 quark-antiquark pairs.Comment: 41 pages, 21 figures; v2, minor changes, references added;v3, 2
important additions, references adde