Mixed QCD and weak corrections to top quark pair production at hadron colliders

The order $\alpha_s^2 \alpha$ mixed QCD and weak corrections to top quark pair production by quark antiquark annihilation are computed, keeping the full dependence on the $t$ and $\bar t$ spins. We determine the contributions to the cross section and to single and double top spin asymmetries at the parton level. These results are necessary ingredients for precise standard model predictions of top quark observables, in particular of top spin-induced parity-violating angular correlations and asymmetries at hadron colliders.Comment: 12 pages, 9 figure

Effects of Higgs sector CP violation in top-quark pair production at the LHC

A striking manifestation of CP violation in the electroweak symmetry breaking sector would be the existence of neutral Higgs boson(s) with undefined CP parity. We analyse signatures of such a boson, with a mass of about 300 GeV or larger, produced in high energy proton-proton collisions at LHC energies in its top-quark antitop-quark decay channel. The large irreducible $t\bar t$ background is taken into account. We propose, both for the dilepton and the lepton + jets decay channels of $t\bar t$, several correlations and asymmetries with which (Higgs sector) CP violation can be traced. We show that for judiciously chosen cuts on the $t\bar t$ invariant mass these CP observables yield, for an LHC integrated luminosity of 100 $\rm{fb}^{-1}$, statistically significant signals for a range of Higgs boson masses and Yukawa couplings.Comment: 36 pages, 25 Postscript figure

CP Violation and the Width Z→bbˉZ\rightarrow b\bar{b}

We discuss the effect of CP-violating $Zb\bar{b}$, $Zb\bar{b}G$ and $Zb\bar{b}\gamma$ couplings on the width $\Gamma(Z\rightarrow b\bar bX)$. The presence of such couplings leads in a natural way to an increase of this width relative to the prediction of the standard model. Various strategies of a direct search for such CP-violating couplings by using CP-odd observables are outlined. The number of $Z$ bosons required to obtain significant information on the couplings in this way is well within the reach of present LEP experiments.Comment: 18 pages, LaTeX, no figure

Weak dipole moment of τ\tau in e+e−e^+e^- collisions with longitudinally polarized electrons

It is pointed out that certain CP-odd momentum correlations in the production and subsequent decay of tau pairs in $e^+e^-$ collisions get enhanced when the $e^-$ is longitudinally polarized. Analytic expressions for these correlations are obtained for the single-pion decay mode of $\tau$ when $\tau^+\tau^-$ have a ``weak" dipole form factor (WDFF) coupling to $Z$ . For $e^+e^-$ collisions at the $Z$ peak, a sensitivity of about 1-5$\times 10^{-17}$\mbox{$e$ cm} for the $\tau$ WDFF can be reached using a {\em single} $\tau^+\tau^-$ decay channel, with $10^6\, Z$'s likely to be available at the SLC at Stanford with $e^-$ polarization of 62\%-75\%.Comment: 9 pages, Latex, PRL-TH-93/17 (Revised

Mixed QCD and Weak Corrections to ttˉt\bar{t} Production by Quark-Antiquark Annihilation

We report on our computation of the mixed QCD and weak corrections to $q {\bar q} \to t\bar{t}$ including $t, \bar t$ spin effects, in particular on single top spin asymmetries.Comment: Talk given at "International Conference on QCD and Hadronic Physics", Beijing, China, 16-20 Jun 200

Measurement of the τ\tau electric dipole moment using longitudinal polarization of e+e−e^+e^- beams

Certain CP-odd momentum correlations in the production and subsequent decay of $\tau$ pairs in $e^+ e^-$ collisions are enhanced significantly when the $e^+$ and $e^-$ beams are longitudinally polarized. These may be used to probe the real and imaginary parts of $d_\tau^\gamma$, the electric dipole moment of the $\tau$. Closed-form expressions for these ``vector correlations'' and the standard deviation of the operators defining them due to standard model interactions are presented for the two-body final states of $\tau$ decays. If 42\% average polarization of each beam is achieved, as proposed for the tau-charm factories, with equal integrated luminosities for each sign of polarization and a total yield of $2\cdot 10^7$ $\tau^+ \tau^-$ pairs, it is possible to attain sensitivities for $\vert\delta {\rm Re} d_{\tau}^{\gamma}\vert$ of $8\cdot 10^{-19}$, $1\cdot 10^{-19}$, $1\cdot 10^{-19}$ $e$ cm respectively and for $\vert\delta {\rm Im} d_{\tau}^{\gamma}\vert$ of $4\cdot 10^{-14}$, $6\cdot 10^{-15}$, $5\cdot 10^{-16}$ $e$ cm respectively at the three operating center-of-mass energies of 3.67, 4.25 and 10.58 GeV. These bounds emerge when the effects of a posible weak dipole form factor $d_\tau^Z$ are negligible as is the case when it is of the same order of magnitude as $d_\tau^\gamma$. Furthermore, in such a polarization experiment where different polarizations are possible, a model-independent disentangling of their individual effects is possible, and a technique to achieve this is described. A strong longitudinal polarization physics programme at the tau-charm factory appears warranted.Comment: 30 pages, latex, no figure

Angular Correlations in Top Quark Pair Production and Decay at Hadron Colliders

We show how to observe sizable angular correlations between the decay products of the top quark and those of the anti-top quark in top quark pair production and decay at hadron colliders. These correlations result from the large asymmetry in the rate for producing like-spin versus unlike-spin top quark pairs provided the appropriate spin axes are used. The effects of new physics at production or decay on these correlations are briefly discussed.Comment: 34 pages, revtex, including 12 uuencoded postscript figure

Anomalous gauge-boson couplings and the Higgs-boson mass

We study anomalous gauge-boson couplings induced by a locally SU(2) x U(1) invariant effective Lagrangian containing ten operators of dimension six built from the boson fields of the Standard Model (SM) before spontaneous symmetry breaking (SSB). After SSB some operators lead to new three- and four-gauge-boson interactions, some contribute to the diagonal and off-diagonal kinetic terms of the gauge bosons and to the mass terms of the W and Z bosons. This requires a renormalisation of the gauge-boson fields, which, in turn, modifies the charged- and neutral-current interactions, although none of the additional operators contain fermion fields. Bounds on the anomalous couplings from electroweak precision measurements at LEP and SLD are correlated with the Higgs-boson mass m_H. Rather moderate values of anomalous couplings allow m_H up to 500 GeV. At a future linear collider the triple-gauge-boson couplings gammaWW and ZWW can be measured in the reaction e+e- --> WW. We compare three approaches to anomalous gauge-boson couplings: the form-factor approach, the addition of anomalous coupling terms to the SM Lagrangian after and, as outlined above, before SSB. The translation of the bounds on the couplings from one approach to another is not straightforward. We show that it can be done for the process e+e- --> WW by defining new effective ZWW couplings.Comment: 50 pages, 4 figures; version to appear in EPJ

CP Violation in the Top-Quark Pair Production at a Next Linear Collider

We provide a detailed, model-independent, study for CP violation effects due to the T-odd top-quark electric dipole moment (EDM) and weak dipole moment (WDM) in the top-quark pair production via $e^+e^-$ and two-photon annihilation at a next $e^+e^-$ linear collider (NLC). There are two methods in detecting CP violation effects in these processes. One method makes use of measurements of various spin correlations in the final decay products of the produced top-quark pair, while the other is to measure various CP-odd polarization asymmetry effects of the initial states. In the $e^+e^-$ case only the first method can be used, and in the $\gamma\gamma$ case both methods can be employed. We provide a complete classification of angular correlations of the $t$ and $\bar{t}$ decay products under CP and CP\tilde{T} which greatly faciliate CP tests in the $e^+e^-$ mode. Concentrating on the second method with the Compton back-scattered high-energetic laser light off the electron or positron beam in the two-photon mode, we construct two CP-odd and CP\tilde{T}-even initial polarization configurations and apply them to investigating CP-violating effects due to the top-quark EDM. With a typical set of experimental parameters at the NLC, we compare the 1-\sigma sensitivities to the top-quark EDM and WDM in the $e^+e^-$ mode and the two-photon mode. Some model expectation values of the T-odd parameters are compared with the results.Comment: 45 pages(LaTeX), 10 eps figures, uses epsfig.st

CP Violation in radiative Z Decays

We propose to test the CP symmetry in the reactions Z -> mu+ mu- gamma and Z -> tau+ tau- gamma. The experimental analysis of angular correlations allows to determine a set of effective couplings: the electric and weak dipole moments of the muon and the tau lepton, and in particular chirality conserving 4-particle couplings, all of which can be induced by CP--violation in renormalizable theories of electroweak interactions beyond the Standard Model. We update an indirect bound on the weak dipole moment of the muon.Comment: 19 pages, Latex, 4 Figures in PostScript forma