arXiv subjects
Publications and source records attributed to J. H. Kuehn.
The cross-section for the production of $t\bar{t}$ pairs in $e^+e^-$-annihilation via a virtual photon is determined up to order $α_s^2$. For the cm-energies of interest the effect of the top mass may not be considered as a small parameter and the full mass dependence must be included.
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 be produced.
Second order ${\cal O}(α_s^2)$ corrections to the heavy quark production cross-section due to massless quarks and coloured scalars are calculated for all energies above threshold. Based on the method introduced in this letter also the gauge non-invariant second order corrections due to the pure gluonic selfenergy insertion and a certain class of ${\cal{O}}(α_s^3)$ and ${\cal{O}}(α_s^4)$ corrections are determined. For the special choice of the gauge parameter, $ξ=4$, the leading threshold and high energy behaviour of the pure second order gluonic corrections to the cross-section are governed by the gluonic self energy insertion.
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 quark polarization.
The real and imaginary part of the vacuum polarisation function $Π(q^2)$ induced by a massive quark is calculated in perturbative QCD up to order $α_s^2$. The method is described and the results are presented. This extends the calculation by Källén and Sabry from two to three loops.
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.
It is demonstrated that the analyzing power of multi-meson final states in semileptonic $τ$ decays with respect to the $τ$ spin is equal and maximal for all decay modes.
The vacuum polarization functions $Π(q^2)$ 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 $Π(q^2)$ 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.
The three-loop QCD corrections to the $ρ$ 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 $\drho=\frac{3G_F M_t^2}{8\sqrt{2}π^2}(1- 2.8599 \api - 14.594 (\api)^2 )$.
We point out that an apparent discrepancy between the values of $α_s(M_z)$ as determined from low versus high energy experiments can be explained if an electrically neutral coloured fermion exists which slows down the running of the strong coupling constant $α_s$.