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

On the top-antitop invariant mass spectrum at the LHC from a Higgs boson signal perspective

We investigate the effect of one-loop corrections of ${\cal O}(\alpha_S^2\alpha_W)$ on the $t\bar t$ invariant mass spectrum at the Large Hadron Collider (LHC) in presence of both resonant and non-resonant Higgs boson effects. We show that corrections of ${\cal O}(\alpha_S^2\alpha_W)$ involving a non-resonant Higgs boson are comparable to or even larger than those involving interference with the s-channel resonant Higgs boson amplitude and that both of these are subleading with respect to all other (non-Higgs) diagrams through that order. We also compute the contribution through ${\cal O}(\alpha_S^2\alpha_W^2)$ of resonant Higgs boson production (i.e. Higgs production via $gg$ fusion) as well as the pure QCD ones of ${\cal O}(\alpha_S^3)$. Altogether, we show that the well known peak-dip structure of the $M_{t\bar t}$ spectrum emerging from interference effects between the $t,u$-channel $gg$-induced Leading Order (LO) QCD diagrams and the one due to a Higgs boson in s-channel via $gg$-fusion is drastically swamped by the remainder of the terms of ${\cal O}(\alpha_S^2\alpha_W)$ discussed above.Comment: 6 pages, 8 figure

Determining the CP parity of Higgs bosons at the LHC in the τ\tau to 1-prong decay channels

We propose a method for determining the CP nature of a neutral Higgs boson or spin-zero resonance $\phi$ at the CERN Large Hadron Collider (LHC) in its $\phi\to\tau^{-}\tau^{+}$ decay channel. The method can be applied to any 1-prong $\tau$-decay mode, which comprise the majority of the $\tau$-lepton decays. The proposed observables allow to discriminate between pure scalar and pseudoscalar Higgs-boson states and/or between a CP-conserving and CP-violating Higgs sector. We show for the decays $\tau\to\pi\nu_{\tau}$ that the method maintains its discriminating power when measurement uncertainties are taken into account. The method will be applicable also at a future linear $e^{+}e^{-}$ collider.Comment: Latex, 15 pages, 6 figure

Spin-Spin Correlations of Top Quark Pairs at Hadron Colliders

Top quark pairs are produced with strongly correlated spins in the partonic reactions $q\bar{q}\to t\bar{t}$ and $gg\to t\bar{t}$. A complete description of these effects in terms of the spin density matrix of the $t\bar{t}$ system in leading order QCD is given. We further discuss the prospects to observe the spin-spin correlations at $p\bar{p}$ and $pp$ colliders by measuring suitable angular correlations among the $t$ and $\bar{t}$ decay products.Comment: 16 pages, LaTeX with 4 postscript figures, epsf macro include

Tau EDM at Low Energies

Low energy tau pair production, at B factories and on top of the $\Upsilon$ resonances, allows for a detailed investigation on the CP violation at the electromagnetic tau pair production vertex. High statistic available at low energies offers the opportunity for an independent analysis of CP-violation in the $\tau$ lepton physics. We show that stringent and independent bounds on the $\tau$ electric dipole moment, competitive with the high energy measurements, can be established in low energies experiments.Comment: Talk at the Seventh International Workshop on Tau Lepton Physics (TAU02), Santa Cruz, Ca, USA, Sept 2002, 5 pages, LaTeX, 1 eps figur

Transverse Polarization of Top Quark Pairs at the Tevatron and the Large Hadron Collider

We investigate the prospects to observe effects of transverse polarization of top quarks in $pp,p\bar{p}\to t\bar{t}X$. QCD absorptive parts generate a polarization of top quarks and antiquarks transverse to the production plane in the partonic processes $q\bar{q}\to t\bar{t}$ and $gg\to t\bar{t}$, which reaches values of a few percent. These perturbative QCD effects are decreased at the hadronic level. A measurement through momentum correlations among the $t$ and $\bar{t}$ decay products will be difficult both at an upgraded Tevatron and at the LHC.Comment: 14 pages, LaTeX; 8 uuencoded, tar-compressed figures added separatel

Chiral-invariant CP-violating Effective Interactions in Z Decays to three Jets

Tests of CP violation by appropriate momentum correlations in $Z\to 3$ jets and in particular in $Z\to b\bar bX$ probe CP-violating effective couplings -- that manifest themselves as form factors -- which conserve the quark chirality and quark flavour. By giving two examples we show that such couplings can be induced at one-loop order in extensions of the Standard Model with CP violation beyond the Kobayashi-Maskawa phase. In one of the models we compute the chirality-conserving part of the CP-violating $Zb{\bar b}$-gluon amplitude for massless $b$ quarks, determine the resulting effective dimension $d=6$ couplings in the local limit, and discuss the possible size of the effects. Finally we show that in models with excited quarks the chiral-invariant CP-violating effective interactions could be quite large if appropriate couplings are of a size characteristic of a strong interactionComment: 14 pages, LaTeX with 4 postscript figures, epsf macro include

CP-violating asymmetries in top-quark production and decay in e+e−e^+ e^- annihilation within the MSSM

We obtain analytic formulae for the cross section of the sequential processes of $e^+ e^- \to t \bar t$ and $t \to b l \nu$ in the laboratory frame where the dependence on triple product correlations of the type (\hat(q}_1 x \hat{q}_2 . \hat{q}_3), induced by CP violation both in the production and the decay are explicitely shown. Different observables sensitive to CP violation are defined and calculated in the Minimal Supersymmetric Standard Model (MSSM). The observables sensitive to CP violation are of the order of $10^-3$. The dependence on the masses of the supersymmetric particles is also shown.Comment: 17 pages of LateX plus five uuencoded Postscript figures, LateX file and PS-figures are also available via anonymous ftp at ftp://info.oeaw.ac.at/pub/hephy-pub/62

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 $t(\uparrow)$ -> 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 Beyond the Standard Model and Tau Pair Production in e+e−e^+ e^- Collisions

We show that the CP-violating dipole form factors of the tau lepton can be of the order of $\alpha/\pi$ in units of the length scale set by the inverse $Z$ boson mass. We propose a few observables which are sensitive to these form factors at LEP2 and higher e^+e^- collision energies.Comment: 11 pages LaTeX + 2 figure