389 research outputs found
Optimal-observable Analysis of Possible Non-standard Top-quark Couplings in pp -> t t-bar X -> l^+ X'
Possible non-standard top-quark interactions with the gluon and the W boson
induced by SU(3) X SU(2) X U(1) gauge-invariant dimension-6 effective operators
are studied for hadron-collider experiments. Current limits on top-gluon
couplings are presented by using the latest experimental data of t t-bar
productions at the Tevatron and the Large Hadron Collider (LHC). The
optimal-observable procedure is applied to the charged-lepton distributions in
p p -> t t-bar X -> l^+ X' (l = e or mu) at the LHC in order to estimate the
expected statistical uncertainties in measurements of those non-standard
top-gluon and top-W couplings that contribute to this process in the leading
order.Comment: 16 pages, 1 figure, Discussion of stabilities in estimating
statistical errors and related one reference added. Final version (to appear
in Phys. Lett. B
Anomalous Chromomagnetic Moments of Quarks and Large Transverse Energy Jets
We consider the jet cross sections for gluons coupling to quarks with an
anomalous chromomagnetic moment. We then apply this to the deviation and bounds
from QCD found in the CDF and D0 Fermilab data, respectively, to find a range
of possible values for the anomalous moments. The quadratic and quartic terms
in the anomalous moments can fit to the rise of a deviation with transverse
energy. Since previous analyses have been done on the top quark total cross
section, here we assume the same moment on all quarks except the top and find
the range TeV for the
CDF data. Assuming the anomalous moment is present only on a charm or bottom
quark which is pair produced results in a range TeV. The magnitudes here are compared with anomalous magnetic moments
that could account for and found to be in the same general range, as well
as not inconsistent with LEP and SLD bounds on .Comment: REVTeX, 11 pages, 2 postscript figure
Graviton collider effects in one and more large extra dimensions
Astrophysical bounds severely limit the possibility of observing collider
signals of gravity with less than 3 flat extra dimensions. However, small
distortions of the compactified space can lift the masses of the lightest
graviton excitations, evading astrophysical bounds without affecting collider
signals of quantum gravity. Following this procedure we reconsider theories
with one large extra dimension. A slight space warping gives a model which is
safe in the infrared against astrophysical and observational bounds, and which
has the ultraviolet properties of gravity with a single flat extra dimension.
We extend collider studies to the case of one extra dimension, pointing out its
peculiarities. Finally, for a generic number of extra dimensions, we compare
different channels in LHC searches for quantum gravity, introducing an
ultraviolet cutoff as an additional parameter besides the Planck mass.Comment: 24 pages, 10 figures. v2: erratum added at the end of the pape
Probing extra dimensions using Nambu-Goldstone bosons
We investigate a possibility that our four-dimensional world is a brane-like
object embedded in a higher dimensional spacetime. In such a situation, the
transverse coordinates of the brane become the Nambu-Goldstone bosons which
appear as a result of spontaneous breaking of the translation symmetry. We
determine the form of the effective action of the system, finding the explicit
form of the vierbein induced on the brane in terms of the Nambu-Goldstone boson
variables and the bulk vielbein. As was pointed out in the previous paper, the
Kaluza-Klein mode couplings are suppressed by the effect of the brane
fluctuation and the suppression is stronger if the brane tension is smaller.
However, we here show that the brane tension cannot be arbitrarily small since
the inverse of the brane tension gives the coupling constant of the
Nambu-Goldstone bosons. A rather stringent bound is obtained for the brane
tension and the fundamental (`string') scale from the consideration of the
cooling process of the supernova.Comment: 35 pages, 5 eps figure
New physics in ttbar spin correlations at the Tevatron
We show that the angular distributions of leptons or jets due to ttbar spin
correlations allow a determination of the top chromomagnetic moment kappa with
an accuracy of order 0.1. The method is quite insensitive to background, event
reconstruction, and other experimental uncertainties. The total event number is
important, so we suggest the inclusion of all-hadronic events.Comment: 13 pages, 3 figures, two references added and cosmetic change
QCD-corrected spin analysing power of jets in decays of polarized top quarks
We present results for the differential distributions of jets from
non-leptonic decays of polarized top quarks within the Standard Model,
including QCD radiative corrections. Our work extends existing results which
are only available for semileptonic top quark decays at the parton level. For
-> b-jet + 2 light jets we compute in particular the
QCD-corrected top-spin analysing power of the b-quark jet and the least
energetic light jet. The dependence of the results on the choice of the jet
recombination scheme is found to be small. In addition we compute the spin
analysing power of the thrust axis. Our results constitute a so far missing
ingredient to analyse top quark production and subsequent non-leptonic decay at
next-to-leading order in alpha_s, keeping the full information on the top quark
polarization.Comment: 11 page
CP Violation in Top Physics
CP violation in top physics is reviewed. The Standard Model has negligible
effects, consequently CP violation searches involving the top quark may
constitute the best way to look for physics beyond the Standard Model.
Non-standard sources of CP violation due to an extended Higgs sector with and
without natural flavor conservation and supersymmetric theories are discussed.
Experimental feasibility of detecting CP violation effects in top quark
production and decays in high energy e+ e-, gamma-gamma, mu+ mu-, pp and p-bar
p colliders are surveyed. Searches for the electric, electro-weak and the
chromo-electric dipole moments of the top quark in e+ e- -> t-bar t and in p p
-> t-bar t X are descibed. In addition, other mechanisms that appear promising
for experiments, e.g., tree-level CP violation in e+ e- -> t-bar t h, t-bar t
Z, t-bar t nu_e-bar nu_e and in the top decay t -> b tau nu_tau and CP
violation driven by s-channel Higgs exchanges in p p, gamma gamma, mu+ mu- ->
t-bar t etc., are also discussed.Comment: 253 pages, 70 figures, A 2-up version of this postscript file may be
obtained at http://thy.phy.bnl.gov/~soni/topreview.htm
Search for anomalous top-gluon couplings at LHC revisited
Through top-quark pair productions at LHC, we study possible effects of
nonstandard top-gluon couplings yielded by SU(3)xSU(2)xU(1) invariant
dimension-6 effective operators. We calculate the total cross section and also
some distributions for p p -> t tbar X as functions of two anomalous-coupling
parameters, i.e., the chromoelectric and chromomagnetic moments of the top,
which are constrained by the total cross section sigma(p pbar -> t tbar X)
measured at Tevatron. We find that LHC might give us some chances to observe
sizable effects induced by those new couplings.Comment: One comment and related two refs. added. Final version (to appear in
Eur.Phys.J. C
Dijet Production at Hadron Colliders in Theories with Large Extra Dimensions
We consider the production of high invariant mass jet pairs at hadron
colliders as a test for TeV scale gravitational effects. We find that this
signal can probe effective Planck masses of about 10 TeV at the LHC with center
of mass energy of 14 TeV and 1.5 TeV at the Tevatron with center of mass energy
of 2 TeV. These results are compared to analogous scattering processes at
leptonic colliders.Comment: 15 pages with 3 figure
Higgs production in association with top quark pair at e+e- colliders in theories of higher dimensional gravity
The models of large extra compact dimensions, as suggested by Arkani-Hamed,
Dimopoulos and Dvali, predict exciting phenomenological consequences with
gravitational interactions becoming strong at the TeV scale. Such theories can
be tested at the existing and future colliders. In this paper, we study the
contribution of virtual Kaluza-Klein excitations in the process at future linear collider (NLC). We find that the virtual exchange
KK gravitons can modify the cross-section
significantly from its Standard Model value and will allow the effective string
scale to be probed up to 7.9 TeV.Comment: 10 pages, Latex, 4 postscript figure
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