572 research outputs found
Top quark precision physics at the International Linear Collider
Top quark production in the process at a future
linear electron positron collider with polarized beams is a powerful tool to
determine the scale of new physics. Studies at the \ttbar threshold will
allow for precise determination of the top quark mass in a well defined
theoretical framework. At higher energies vector, axial vector and tensorial CP
conserving couplings can be separately determined for the photon and the
component in the electro-weak production process. The sensitivity to new
physics would be dramatically improved w.r.t. to what expected from LHC for
electroweak couplings.Comment: White paper for Snowmass CSS 201
Search for Anomalous Couplings in Top Decay at Hadron Colliders
We present a quantitative study on sensitivities to the top-decay anomalous
couplings, taking into account realistic experimental conditions expected at
Tevatron and LHC. A double angular distribution of W and charged lepton in the
top decay is analyzed, using ttbar events in the lepton+jets channel. In order
to improve sensitivities to the anomalous couplings, we apply two techniques:
(1) We use a likelihood fitting method for full kinematical reconstruction of
each top event. (2) We develop a new effective spin reconstruction method for
leptonically-decayed top quarks; this method does not require spin information
of the antitop side. For simplicity, we neglect couplings of right-handed
bottom quark as well as CP violating couplings. The 95% C.L. estimated bound on
a ratio of anomalous couplings reads -0.81 < f_2/f_1 < -0.70,
-0.12<f_2/f_1<0.14 using 1000 reconstructed top events at Tevatron, while
-0.74<f_2/f_1<-0.72, -0.01<f_2/f_1<0.01 is expected with 100k reconstructed top
events at LHC, where only statistical errors are taken into account. A two-fold
ambiguity in the allowed range remains when the number of events exceeds a few
hundred.Comment: 21 pages, 15 figure
Probe of CP Violation in e+e- -> ttbar Near Threshold
We study how to probe the anomalous CP-violating couplings of top quark with
photon, Z and gluon in the ttbar threshold region at future e+e- colliders.
These couplings contribute to the difference of the t and tbar polarization
vectors delta P and to the CP-odd spin correlation tensor delta Q_ij. We find
that typical sizes of delta P and delta Q_ij are 5-20% times the couplings
(d_tgamma, d_tZ, d_tg) in the threshold region. Experimentally delta P can be
measured efficiently using the CP-odd combination of the l+/l- momenta or of
the l+/l- directions. We have similar sensitivities to both the real and
imaginary parts of the couplings independently using the two components of
delta P. Taking advantage of different dependences of delta P on the e+/e-
polarizations and on the c.m. energy, we will be able to disentangle the
effects of the three couplings d_tgamma, d_tZ, d_tg in the ttbar threshold
region. We give rough estimates of sensitivities to the anomalous couplings
expected at future e+e- colliders. The sensitivities to d_tgamma and d_tZ are
comparable to those attainable in the open-top region at e+e- colliders. The
sensitivity to d_tg is worse than that expected at a hadron collider but
exceeds the sensitivity in the open-top region at e+e- colliders.Comment: 25 pages, 9 figure
Final-State Interactions in e^+e^- -> t\bar{t} -> b l^+ \nu \bar{b} W^- Near Top Quark Threshold
We calculate final-state interaction corrections to the energy-angular
distribution of l^+ in semi-leptonic top quark decay, where the parent top
quark is produced via e^+e^- -> t\bar{t} near threshold. These are the
corrections due to gluon exchange between t and \bar{b} (\bar{t} and b) and
between b and \bar{b}. Combining with previously known other corrections, we
explicitly write down the l^+ energy-angular distribution including the full
O(alpha_s)=O(beta) corrections near t\bar{t} threshold. Numerical analyses of
the final-state interaction corrections are given. We find that they deform the
l^+ distribution typically at the 10% level. We also find that all qualitative
features of the numerical results can be understood from intuitive pictures.
The mechanisms of various effects of the final-state interactions are
elucidated. Finally we define an observable which is proper to the decay
process of the top quark (dependent only on d\Gamma_{t -> b l^+ \nu}/ dE_l
d\Omega_l of a free polarized top quark) near t\bar{t} threshold. Such a
quantity will be useful in extracting the decay property of the top quark using
the highly polarized top quark samples.Comment: Section 6 is enlarged substantially and a paragraph is added to
Section 8 correspondingly. Reference list is also improved. (27 pages
including figures, LaTeX
QCD Correction to Neutralino Annihilation Process and Dark Matter Density in Supersymmetric Models
We calculate QCD correction to the neutralino annihilation cross section into
quark anti-quark final state and discuss its implications to the calculation of
neutralino relic density. We see that the QCD correction enhances the
pair-annihilation cross section by O(10 %) when final-state quarks are
non-relativistic. Consequently, when the lightest neutralinos dominantly
annihilate into a t\bar{t} pair, the relic density of the lightest neutralino
is significantly affected by the QCD correction, in particular when the
lightest-neutralino mass is close to the top-quark mass.Comment: 19 pages, 5 figure
Singe Top Production at LEP 200
We present exact tree level cross sections for the single top production
process at LEP~200. The
results reproduce roughly those obtained earlier by using the equivalent real
photon approximation and we confirm the observation that detecting a top
heavier than half the c.m.~energy is not feasible at LEP~200. The calculation
has been performed by a new automatic Feynman amplitude generator MadGraph
which produces HELAS code for the helicity amplitudes.Comment: 7 pages, 4 postscript figure
Neutrino Masses and Bimaximal Mixing
Solar and atmospheric neutrino anomalies are described by bimaximal mixing of
three neutrinos. Neutrino oscillations in appearance nu_e nu_mu,nu_tau and
in disappearance nu_e long baseline and atmospheric experiments are sensitive
to deviations from the ideal bimaximal mixing. It is suggested that these
deviations may be dominated by a rotation in the electron--muon plane of the
generation space. Simple seesaw models are described supporting this idea. The
rotation angle is estimated from Fritzsch's relation. Predictions are presented
for oscillations in long baseline experiments, for solar neutrinos, and for the
rates of neutrinoless double beta decays.Comment: Latex, 13 p., no figure
Chemo-Sensitive Running Droplet
Chemical control of the spontaneous motion of a reactive oil droplet moving
on a glass substrate under an aqueous phase is reported. Experimental results
show that the self-motion of an oil droplet is confined on an acid-treated
glass surface. The transient behavior of oil-droplet motion is also observed
with a high-speed video camera. A mathematical model that incorporates the
effect of the glass surface charge is built based on the experimental
observation of oil-droplet motion. A numerical simulation of this mathematical
model reproduced the essential features concerning confinement within a certain
chemical territory of oil-droplet motion, and also its transient behavior. Our
results may shed light on physical aspects of reactive spreading and a
chemotaxis in living things.Comment: 17 pages, 10 figure
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