e+e-</EM> Annihilation near Threshold > Top quark polarization in $e^+e^-$ annihilation into $t\bar t$ is calculated for linearly polarized beams. The Green function formalism is applied to this reaction near threshold. The Lippmann–Schwinger equations for the $S$-wave and $P$-wave Green functions are solved numerically for the QCD chromostatic potential given by the two-loop formula for large momentum transfer and Richardson's ansatz for intermediate and small momenta. $S$-$P$–wave interference contributes to all components of the top quark polarization vector. Rescattering of the decay products is considered. The mean values $\langle n \ell \rangle$ of the charged lepton four-momentum projections on appropriately chosen directions $n$ in semileptonic top decays are proposed as experimentally observable quantities sensitive to top quark polarization. The results for $\langle n \ell \rangle$ are obtained including $S$-$P$–wave interference and rescattering of the decay products. It is demonstrated that for the longitudinally polarized electron beam a highly polarized sample of top quarks can

Recent calculations are presented of top quark polarization in $t\bar t$ pair production close to threshold. S–P-wave interference gives contributions to all components of the top quark polarization vector. Rescattering of the decay products is considered. Moments of the fourmomentum of the charged lepton in semileptonic top decays are calculated and shown to be very sensitive to the top

Recent results on production and decays of polarized top quarks are reviewed. Top quark pair production in $e^+e^-$ annihilation is considered near energy threshold. For longitudinally polarized electrons the produced top quarks and antiquarks are highly polarized. Dynamical effects originating from strong interactions and Higgs boson exchange in the $t-\bar t$ system can be calculated using the Green function method. Energy-angular distributions of leptons in semileptonic decays are sensitive to the polarization of the decaying top quark and to the Lorentz structure of the weak charged current.

We consider the scalar form factor in $\tau \to K\pi \nu_\tau$ decays. It receives contributions both from the scalar resonance $K_0^*(1430)$ and from the scalar projection of off-shell vector resonances. We construct a model for the hadronic current which includes the vector resonances $K^*(892)$ and $K^*(1410)$ and the scalar resonance $K_0^*(1430)$. The parameters of the model are fixed by matching to the $O(p^4)$ predictions of chiral perturbation theory. Suitable angular correlations of the $K\pi$ system allow for a model independent separation of the vector and scalar form factor. Numerical results for the relevant structure functions are presented.

Two-jet cross sections in deep inelastic scattering at HERA are calculated in next-to-leading order. The QCD corrections are implemented in a new $ep\rightarrow n$ jets event generator, MEPJET, which allows to analyze arbitrary jet definition schemes and general cuts in terms of parton 4-momenta. First results are presented for the JADE, the cone and the $k_T$ schemes. For the $W$-scheme, disagreement with previous results and large radiative corrections and recombination scheme ambiguties are traced to a common origin.

The real and imaginary part of the vacuum polarization function $\Pi(q^2)$ induced by a massive quark is calculated in perturbative QCD up to order $\alpha_s^2$. The method is described and the results are presented. This extends the calculation by K\“all\'en and Sabry from two to three loops.

We review current issues in exclusive semileptonic tau decays. We present the formalism of structure functions, and then discuss predictions for final states with kaons, for decays into four pions and for radiative corrections to the decay into a single pion.

Recent analytic calculations on the rate for the production of a pair of massive fermions in $e^+ e^-$ annihilation plus real or virtual radiation of a pair of massless fermions are discussed. The contributions for real and virtual radiation are displayed separately. The asymptotic behaviour close to threshold is given in a compact form and an application to the angular distribution of massive quarks close to threshold is presented.

We construct an effective Lagrangian for the hadronic decays of a heavy excited $s$-wave-spin-one quarkonium $\Psi'$ into a lower $s$-wave-spin-one state $\Psi$. We show that reasonable fits to the measured invariant mass spectra in the charmonium and bottomonium systems can be obtained within this framework. The mass dependence of the various terms in the Lagrangian is discussed on the basis of a quark model.

The electromagnetic corrections to the masses of the pseudoscalar mesons $\pi$ and $K$ are considered. We calculate in chiral perturbation theory the contributions which arise from resonances within a photon loop at order $O(e^2 m_q)$. Within this approach we find rather moderate deviations to Dashen's

Predictions for the angular distribution of massive quarks and leptons are presented, including QCD and QED corrections. Recent results for the fermionic part of the two-loop corrections to the electromagnetic form factors are combined with the BLM scale fixing prescription. Two distinctly different scales arise as arguments of $\alpha_s(\mu^2)$ near threshold: the relative momentum of the quarks governing the soft gluon exchange responsible for the Coulomb potential, and a large momentum scale approximately equal to twice the quark mass for the corrections induced by transverse gluons. Numerical predictions for charmed, bottom, and top quarks are given. One obtains a direct determination of $\alpha_{\mbox{\it\scriptsize V}}(Q^2)$, the coupling in the heavy quark potential, which can be compared with lattice gauge theory predictions. The corresponding QED results for $\tau$ pair production allow for a measurement of the magnetic moment of the $\tau$ and could be tested at a future $\tau$-charm factory.

Top quark production in $p\bar p$ and $e^+e^-$ collisions is enhanced by the exchange of a Higgs boson. The enhancement factors are calculated in the threshold region using the Greens function method.

Baryons with a single heavy quark are being studied experimentally at present. Baryons with two units of heavy flavor will be abundantly produced not only at future colliders, but also at existing facilities. In this paper we study the production via heavy quark fragmentation of baryons containing two heavy quarks at the Tevatron, the LHC, HERA, and the NLC. The production rate is woefully small at HERA and at the NLC, but significant at $pp$ and $p\bar{p}$ machines. We present distributions in various kinematical variables in addition to the integrated cross sections at hadron colliders.

It is demonstrated that the analyzing power of multi-meson final states in semileptonic $\tau$ decays with respect to the $\tau$ spin is equal and maximal for all decay modes.

QCD corrections to electroweak observables are reviewed. Recent results on contributions from the top-bottom doublet of ${\cal O}(\as^2)$ to $\drho$, $\Delta r$ and $\Delta\kappa$ are presented. It is demonstrated that the first three terms in the expansion in $M_Z^2/M_t^2$ provide an excellent approximation to the exact result. Calculational techniques are briefly discussed.

Secondary radiation of hadrons from a tau pair produced in electron positron collisions may constitute an important obstacle for precision measurements of the production cross section and of branching ratios. The rate for real and virtual radiation is calculated and various distributions are presented. For Z decays a comprehensive analysis is performed which incorporates real and virtual radiation of leptons. The corresponding results are also given for primary electron and muon pairs. Compact analytical formulae are presented for entirely leptonic configurations. Measurements of $Z$ partial decay rates which eliminate all hadron and lepton radiation are about 0.3\% to 0.4\% lower than totally inclusive measurements, a consequence of the ${\cal O}(\alpha^2)$ negative virtual corrections which are enhanced by the third power of a large logarithm.

Possibilites for measuring the $J^{PC}$ quantum numbers of the Higgs particle through its interactions with gauge bosons and with fermions are discussed. Observables which indicate CP violation in these couplings are also identified.

The vacuum polarization functions $\Pi(q^2)$ of charged and neutral gauge bosons which arise from top and bottom quark loops lead to important shifts in relations between electroweak parameters which can be measured with ever-increasing precision. The large mass of the top quark allows approximation of these functions through the first two terms of an expansion in $M_Z^2/M_t^2$. The first three terms of the Taylor series of $\Pi(q^2)$ are evaluated analytically up to order $\as^2$. The first two are required to derive the approximation, the third can be used to demonstrate the smallness of the neglected terms. The paper improves earlier results based on the leading term $\propto G_F M_t^2 \as^2$. Results for the subleading contributions to $\dr$ and the effective mixing angle $\sineff$ are presented.

Recent theoretical results on the production and decay of top quarks are presented. The implications of the new experimental results from the TEVATRON are briefly discussed. Predictions for the top quark decay rate and distributions are described, including the influence of QCD and electroweak radiative corrections. Top production at an $e^+e^-$ collider is discussed with emphasis towards the threshold region. The polarization of top quarks in the threshold region is calculated with techniques based on Green's functions for $S$ and $P$ waves.

The rate for the production of a pair of massive fermions in $e^+ e^-$ annihilation plus real or virtual radiation of a pair of massless fermions is calculated analytically. The contributions for real and virtual radiation are displayed separately. The asymptotic behaviour close to threshold and for high energies is given in a compact form. These approximations provide arguments for the appropriate choice of the scale in the ${\cal O}(\alpha)$ result, such that no large logarithms remain in the final answer.

To obtain the most accurate predictions for the Higgs masses in the minimal supersymmetric model (MSSM), one should compute the full set of one-loop radiative corrections, resum the large logarithms to all orders, and add the dominant two-loop effects. A complete computation following this procedure yields a complex set of formulae which must be analyzed numerically. We discuss a very simple approximation scheme which includes the most important terms from each of the three components mentioned above. We estimate that the Higgs masses computed using our scheme lie within 2 GeV of their theoretically predicted values over a very large fraction of MSSM

In order to calculate the $\rho^0 -\omega$ mixing we extend the chiral couplings of the low-lying vector mesons in chiral perturbation theory to a lagrangian that contains two vector fields. We determine the $p^2$ dependence of the two-point function and recover an earlier result for the on-shell expression.

Recent progress in the theoretical description of inclusive heavy flavour decays is reviewed. After an outline of the theoretical methods applications to

QCD corrections to all the allowed decays of superheavy groundstate quarkonia into electroweak gauge and Higgs bosons are presented. For quick estimates, approximations that reproduce the exact results within less than at worst two percent are also given.

We summarize recent results on QCD corrections to various observables in semileptonic $b$ quark decays. For massless leptons in the final state we present effects of such corrections on triple differential distribution of leptons which are important in studies of polarized $b$ quark decays. Analogous formulas for distributions of neutrinos are applicable in decays of polarized $c$ quarks. In the case of decays with a $\tau$ lepton in the final state mass effect of $\tau$ has to be included. In this case we concentrate on corrections

The three-loop QCD corrections to the $\rho$ parameter from top and bottom quark loops are calculated. The result differs from the one recently calculated by Avdeev et al. As function of the pole mass the numerical value is given by $\drho=\frac{3G_F M_t^2}{8\sqrt{2}\pi^2}(1- 2.8599\, \api - 14.594\, (\api)^2 )$.

The spectra of baryons containing two heavy quarks test the form of the $QQ$ potential through the spin-averaged masses and hyperfine splittings. The mass splittings in these spectra are calculated in a nonrelativistic potential model and the effects of varying the potential studied. The simple description in terms of light quark