1,443 research outputs found
Single Top Quark Production as a Probe for Anomalous Moments at Hadron Colliders
Single production of top quarks at hadron colliders via fusion is
examined as a probe of possible anomalous chromomagnetic and/or chromoelectric
moment type couplings between the top and gluons. We find that this channel is
far less sensitive to the existence of anomalous couplings of this kind than is
the usual production of top pairs by or fusion. This result is
found to hold at both the Tevatron as well as the LHC although somewhat greater
sensitivity for anomalous couplings in this channel is found at the higher
energy machine.Comment: New discussion and 10 new figures added. uuencoded postscript fil
Unitarization of Gluon Exchange Amplitudes and Rapidity Gaps at the Tevatron
Rapidity gaps between two hard jets at the Tevatron have been interpreted as
being due to the exchange of two gluons which are in an overall color-singlet
state. We show that this simple picture involves unitarity violating
amplitudes. Unitarizing the gluon exchange amplitude leads to qualitatively
different predictions for the fraction of -channel color singlet exchange
events in forward , or scattering, which better fit Tevatron
data.Comment: 21 pages, Revtex, 7 postscript figures included via epsf.sty.
Compressed postscript file of complete paper also available at
http://pheno.physics.wisc.edu/pub/preprints/1998/madph-98-1024.ps.Z or at
ftp://pheno.physics.wisc.edu/pub/preprints/1998/madph-98-1024.ps.
Window on Higgs Boson: Fourth Generation Decays Revisited
Direct and indirect searches of the Higgs boson suggest that 113 GeV
170 GeV is likely. With the LEP era over and the
Tevatron Run II search via arduous, we revisit a case where
or jets could arise via strong pair
production. In contrast to 10 years ago, the tight electroweak constraint on
-- (hence --) splitting reduces FCNC
, rates, making naturally competitive.
Such a "cocktail solution" is precisely the mix that could evade the CDF search
for , and the may well be lurking below the top. In
light of the Higgs program, this two-in-one strategy should be pursued.Comment: 4 pages, RevTex, 4 eps figures, One more figure, version to be
published in Phys. Rev.
Eliminating the Hadronic Uncertainty
The Standard Model Lagrangian requires the values of the fermion masses, the
Higgs mass and three other experimentally well-measured quantities as input in
order to become predictive. These are typically taken to be ,
and . Using the first of these, however, introduces a hadronic
contribution that leads to a significant error. If a quantity could be found
that was measured at high energy with sufficient precision then it could be
used to replace as input. The level of precision required for this to
happen is given for a number of precisely-measured observables. The boson
mass must be measured with an error of \,MeV, to \,MeV
and polarization asymmetry, , to that would seem to be the
most promising candidate. The r\^ole of renormalized parameters in perturbative
calculations is reviewed and the value for the electromagnetic coupling
constant in the renormalization scheme that is consistent
with all experimental data is obtained to be .Comment: 8 pages LaTeX2
The Sources of b-Quarks at the Tevatron and their Correlations
The leading-log order QCD hard scattering Monte-Carlo models of HERWIG,
ISAJET, and PYTHIA are used to study the sources of b-quarks at the Tevatron.
The reactions responsible for producing b and bbar quarks are separated into
three categories; flavor creation, flavor excitation, and
parton-shower/fragmentation. Flavor creation corresponds to the production of a
b-bbar pair by gluon fusion or by annihilation of light quarks, while flavor
excitation corresponds to a b or bbar quark being knocked out of the
initial-state by a gluon or a light quark or antiquark. The third source occurs
when a b-bbar pair is produced within a parton shower or during the
fragmentation process of a gluon or a light quark or antiquark (includes gluon
splitting). The QCD Monte-Carlo models indicate that all three sources of
b-quarks are important at the Tevatron and when combined they qualitatively
describe the inclusive cross-section data. Correlations between the b and bbar
quark are very different for the three sources and can be used to isolate the
individual contributions.Comment: RevTex4, 14 pages, 20 figures, submitted to Phys. Rev.
Can We Observe Weak Anomalous Couplings of Heavy Quarks Through Three Jet Events?
The rates and corresponding jet distributions for the decay
and the process may be sensitive to anomalous dipole-like
couplings of heavy quarks to the photon and . In the -quark case, after
updating our previous analysis on the constraints imposed by current
experiments on anomalous couplings, we show that the variation of
these couplings within their presently allowed ranges leads to rather minor
modifications to the Standard Model expectations for
observables. In the -quark case, significant deviations from the Standard
Model predictions for production at the Next Linear Collider are
possible.Comment: 26 pages with 9 embedded figures; gzipped, uuencoded postscript file.
To obtain a copy of this paper send e-mail to [email protected]
Direct Measurement of the Top Quark Charge at Hadron Colliders
We consider photon radiation in tbar-t events at the upgraded Fermilab
Tevatron and the CERN Large Hadron Collider (LHC) as a tool to measure the
electric charge of the top quark. We analyze the contributions of tbar-t-gamma
production and radiative top quark decays to p-p, pbar-p -> gamma l^+/- nu
bbar-b jj, assuming that both b-quarks are tagged. With 20~fb^{-1} at the
Tevatron, the possibility that the ``top quark'' discovered in Run I is
actually an exotic charge -4/3 quark can be ruled out at the 95% confidence
level. At the LHC, it will be possible to determine the charge of the top quark
with an accuracy of about 10%.Comment: Revtex, 24 pages, 2 tables, 9 figure
Single-photon signals at LEP in supersymmetric models with a light gravitino
We study the single-photon signals expected at LEP in models with a very
light gravitino. The dominant process is neutralino-gravitino production (e+e-
-> chi+ G) with subsequent neutralino decay via chi->gamma+G, giving a
gamma+E_miss signal. We first calculate the cross section at arbitrary
center-of-mass energies and provide new analytic expressions for the
differential cross section valid for general neutralino compositions. We then
consider the constraints on the gravitino mass from LEP 1 and LEP161
single-photon searches, and possible such searches at the Tevatron. We show
that it is possible to evade the stringent LEP 1 limits and still obtain an
observable rate at LEP 2, in particular in the region of parameter space that
may explain the CDF e+e+gamma+gamma+E_T,miss event. As diphoton events from
neutralino pair-production would not be kinematically accessible in this
scenario, the observation of whichever photonic signal will discriminate among
the various light-gravitino scenarios in the literature. We also perform a
Monte Carlo simulation of the expected energy and angular distributions of the
emitted photon, and of the missing invariant mass expected in the events.
Finally we specialize the results to the case of a recently proposed
one-parameter no-scale supergravity model.Comment: 31 pages, LaTeX, 14 figures (included
New Supersymmetric Contributions to
We calculate the electroweak-like one-loop supersymmetric contributions to
the rare and flavor-violating decay of the top quark into a charm quark and a
gauge boson: , with . We consider loops of both charginos
and down-like squarks (where we identify and correct an error in the
literature) and neutralinos and up-like squarks (which have not been calculated
before). We also account for left-right and generational squark mixing. Our
numerical results indicate that supersymmetric contributions to can be
upto 5 orders of magnitude larger than their Standard Model counterparts.
However, they still fall short of the sensitivity expected at the
next-generation top-quark factories.Comment: 13 pages, LaTex, 1 figure included. Final version to appear in
Physical Review D. Chargino contribution dealt with in greater detail. Minor
revisions in tex
Precision SUSY Measurements at LHC
If supersymmetry exists at the electroweak scale, then it should be
discovered at the LHC. Determining masses, of supersymmetric particles however,
is more difficult. In this paper, methods are discussed to determine
combinations of masses and of branching ratios precisely from experimentally
observable distributions. In many cases such measurements alone can greatly
constrain the particular supersymmetric model and determine its parameters with
an accuracy of a few percent. Most of the results shown correspond to one year
of running at LHC at ``low luminosity'.Comment: 52 pages, Latex with 42 postscript figures. Postscript version also
at http://www-physics.lbl.gov/www/theorygroup/papers/39412.p
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