50 research outputs found
Constraining a CP-violating WWV coupling from the W^+W^- threshold cross section at LEP2
The most general form of the and interaction contains a
-violating term which has the same threshold behaviour as the Standard
Model \ee \to \ww cross section. We calculate the cross section as a function
of the corresponding anomalous coupling, and estimate the bounds which can be
obtained from a measurement of the threshold cross section at LEP2. We show how
the effect of the coupling is most pronounced in the angular distributions of
the final-state fermions.Comment: 7 pages, LaTeX file, 5 ps figure
Flavour Mixing, Gauge Invariance and Wave-function Renormalisation
We clarify some aspects of the LSZ formalism and wave function
renormalisation for unstable particles in the presence of electroweak
interactions when mixing and CP violation are considered. We also analyse the
renormalisation of the CKM mixing matrix which is closely related to wave
function renormalisation. We critically review earlier attempts to define a set
of "on-shell" wave function renormalisation constants. With the aid of an
extensive use of the Nielsen identities complemented by explicit calculations
we corroborate that the counter term for the CKM mixing matrix must be
explicitly gauge independent and demonstrate that the commonly used
prescription for the wave function renormalisation constants leads to gauge
parameter dependent amplitudes, even if the CKM counter term is gauge invariant
as required. We show that a proper LSZ-compliant prescription leads to gauge
independent amplitudes. The resulting wave function renormalisation constants
necessarily possess absorptive parts, but we verify that they comply with the
expected requirements concerning CP and CPT. The results obtained using this
prescription are different (even at the level of the modulus squared of the
amplitude) from the ones neglecting the absorptive parts in the case of top
decay. The difference is numerically relevant.Comment: 19 pages, plain latex, one ps figur
Flavon exchange effects in models with abelian flavor symmetry
In models with abelian flavor symmetry the small mixing angles and mass
ratios of quarks and leptons are typically given by powers of small parameters
characterizing the spontaneous breaking of flavor symmetry by "flavon" fields.
If the scale of the breaking of flavor symmetry is near the weak scale, flavon
exchange can lead to interesting flavor-violating and CP violating effects.
These are studied. It is found that d_e, mu -> e + gamma, and mu-e conversion
on nuclei can be near present limits. For significant range of parameters mu-e
conversion can be the most sensitive way to look for such effects.Comment: 19 pages, 5 Postscript figures, LATE
The Effects of a New Vector Boson on the Top Quark Cross Section at the Tevatron
It has recently been shown that a new neutral vector boson , with a mass
of order 1~TeV and weak couplings to quarks only, could explain both the
anomalous values of and and the apparent excess of large jet
events measured by the CDF collaboration. We calculate the effects of
exchange on the production cross section at Tevatron
collider energies, including next-to-leading-order QCD corrections. We find a
significant enhancement of the cross section and study the resulting
invariant mass distribution, which could provide a decisive test of the
model.Comment: 7 pages, LATEX, 4 figures include
CP Violation in
We consider CP violating effects in the decays where both the resonance, , and
resonance, , can contribute. The interference
between the and resonances can lead to enhanced CP-violating
asymmetries whose magnitudes depend crucially on the decay
constant, . We make an estimate of with a
simplified chiral Lagrangian coupled to a massive pseudoscalar field, and we
compare the estimates from the non-relativistic quark model and from the QCD
sum rule with the estimate from the `mock' meson model. We then estimate
quantitatively the size of CP-violating effects in a multi-Higgs-doublet model
and scalar-leptoquark models. We find that, while CP-violating effects in the
scalar-leptoquark models may require more than leptons,
CP-violating effects from the multi-Higgs-doublet model can be seen at the
level with about leptons using the chiral Lagrangian
estimate of GeV.Comment: Latex, 30 pages, 2 figures (not included). Three compressed
postscript files of the paper available at
ftp://ftp.kek.jp/kek/preprints/TH/TH-419/kekth419.ps.gz, Tau1.ps.gz,
Tau2.ps.g
Rare Charm Decays in the Standard Model and Beyond
We perform a comprehensive study of a number of rare charm decays,
incorporating the first evaluation of the QCD corrections to the short distance
contributions, as well as examining the long range effects. For processes
mediated by the transitions, we show that sensitivity to
short distance physics exists in kinematic regions away from the vector meson
resonances that dominate the total rate. In particular, we find that
and are sensitive to non-universal
soft-breaking effects in the Minimal Supersymmetric Standard Model with
R-parity conservation. We separately study the sensitivity of these modes to
R-parity violating effects and derive new bounds on R-parity violating
couplings. We also obtain predictions for these decays within extensions of the
Standard Model, including extensions of the Higgs, gauge and fermion sectors,
as well as models of dynamical electroweak symmetry breaking.Comment: 45 pages, typos fixed, discussions adde
Testing the Nature of Kaluza-Klein Excitations at Future Lepton Colliders
With one extra dimension, current high precision electroweak data constrain
the masses of the first Kaluza-Klein excitations of the Standard Model gauge
fields to lie above TeV. States with masses not much larger than
this should be observable at the LHC. However, even for first excitation masses
close to this lower bound, the second set of excitations will be too heavy to
be produced thus eliminating the possibility of realizing the cleanest
signature for KK scenarios. Previous studies of heavy and production
in this mass range at the LHC have demonstrated that very little information
can be obtained about their couplings to the conventional fermions given the
limited available statistics and imply that the LHC cannot distinguish an
ordinary from the degenerate pair of the first KK excitations of the
and . In this paper we discuss the capability of lepton colliders
with center of mass energies significantly below the excitation mass to resolve
this ambiguity. In addition, we examine how direct measurements obtained on and
near the top of the first excitation peak at lepton colliders can confirm these
results. For more than one extra dimension we demonstrate that it is likely
that the first KK excitation is too massive to be produced at the LHC.Comment: 38 pages, 10 Figs, LaTex, comments adde
Search for Single Top Production at LEP
Single top production in e+e- annihilations is searched for in data collected
by the L3 detector at centre-of-mass energies from 189 to 209 GeV,
corresponding to a total integrated luminosity of 634 pb-1. Investigating
hadronic and semileptonic top decays, no evidence of single top production at
LEP is obtained and upper limits on the single top cross section as a function
of the centre-of-mass energy are derived. Limits on possible anomalous
couplings, as well as on the scale of contact interactions responsible for
single top production are determined
Production at the Fermilab Tevatron Collider: Gauge Invariance and Radiation Amplitude Zero
The electroweak process is calculated
at tree level, including finite W width effects. In order to obtain a gauge
invariant amplitude, the imaginary parts of triangle graphs and
box diagrams have to be included, in addition to resumming the
imaginary contributions to the W polarization. We demonstrate the existence of
a radiation amplitude zero in , and discuss how it may be observed in correlations of
the and lepton rapidities at the Fermilab Tevatron.Comment: 37 pages,12 figures, postscript file also available via anonymous ftp
at ftp://ucdhep.ucdavis.edu/han/vvv/Wgmgm.p
A Phenomenological Study of the Process at High Energy Colliders and Measurement of the and Couplings
We perform a detailed study of the process including all contributions. The contributions other
than from real gauge boson production leads to a rich phenomenology. We explore
the use of the process as a means of precision measurement of the and
vertices. We concentrate on LEP II energies, GeV,
and energies appropriate to the proposed Next Linear Collider (NLC) high energy
collider with center of mass energies and 1~TeV. At 200
GeV, the process offers, at best, a consistency check of other processes being
considered at LEP200. At 500~GeV, the parameters ,
, , and can be measured to about or better at 95\% C.L. while at 1 TeV, they can be measured to about . At the high luminosities anticipated at high energy linear colliders
precision measurements are likely to be limited by systematic rather than
statistical errors.Comment: 33 pages, OCIP/C 93-18, UQAM-PHE-930