arXiv subjects

R. Harlander

Publications and source records attributed to R. Harlander.

30 records · Page 2Linked to original sources

arXiv1999

Automatic Computation of Feynman Diagrams

Quantum corrections significantly influence the quantities observed in modern particle physics. The corresponding theoretical computations are usually quite lengthy which makes their automation mandatory. This review reports on the current status of automatic calculation of Feynman diagrams in particle physics. The most important theoretical techniques are introduced and their usefulness is demonstrated with the help of simple examples. A survey over frequently used programs and packages is provided, discussing their abilities and fields of applications. Subsequently, some powerful packages which have already been applied to important physical problems are described in more detail. The review closes with the discussion of a few typical applications for the automated computation of Feynman diagrams, addressing current physical questions like properties of the $Z$ and Higgs boson, four-loop corrections to renormalization group functions and two-loop electroweak corrections.

hep-ph↗

arXiv1998

Complete Corrections of O(αα_s) to the Decay of the Z Boson into Bottom Quarks

For the vertex corrections to the partial decay rate $Γ(Z \to b\bar{b})$ involving the top quark only the leading terms of order $αα_s$ in the $1/M_t$ expansion are known. In this work we compute the missing next-to-leading corrections. Thus at $O(αα_s)$ the complete corrections to the decay of the Z boson into bottom quarks are at hand.

hep-ph↗

arXiv1998

Singlet Polarization Functions at O(α_s^2)

We consider the three-loop singlet diagrams induced by axial-vector, scalar and pseudo-scalar currents. Expansions for small and large external momentum $q$ are presented. They are used in combination with conformal mapping and Padé approximations in order to arrive at results for the polarization functions valid for all $q^2$. Results are presented for the imaginary parts which are directly related to physical quantities like the production of top quarks or the decay of scalar or pseudo-scalar Higgs bosons.

hep-ph↗

arXiv1997

O(α_s^2) Corrections to Top Quark Production at $e^+e^-$ Colliders

In this article we evaluate mass corrections up to $O((m^2/q^2)^6)$ to the three-loop polarization function induced by an axial-vector current. Special emphasis is put on the evaluation of the singlet diagram which is absent in the vector case. As a physical application $O(α_s^2)$ corrections to the production of top quarks at future $e^+e^-$ colliders is considered. It is demonstrated that for center of mass energies $\sqrt{s} >~ 500$ GeV the inclusion of the first seven terms into the cross section leads to a reliable description.

hep-ph↗

arXiv1997

Higgs Decay to Top Quarks at O(α_s^2)

Three-loop corrections to the scalar and pseudo-scalar current correlator are calculated. By applying the large momentum expansion mass terms up to order (m^2/q^2)^4 are evaluated analytically. As an application O(α_s^2) corrections to the decay of a scalar and pseudo-scalar Higgs boson into top quarks are considered. It is shown that for a Higgs mass not far above the $t\bar{t}$ threshold these higher order mass corrections are necessary to get reliable results.

hep-ph↗

arXiv1997

Mass Corrections to the Vector Current Correlator

Three-loop QCD corrections to the vector current correlator are considered. The large momentum procedure is applied in order to evaluate mass corrections up to order $(m^2/q^2)^6$. The inclusion of the first seven terms to the ratio $R=σ(e^+e^- \to hadrons)/σ(e^+e^- \to μ^+μ^-)$ leads to reliable predictions from the high energy region down to relatively close to threshold.

hep-ph↗

arXiv1996

Automatic Computation Of Three-Loop Two-Point Functions In Large Momentum Expansion

We discuss the calculation of two-point three-loop functions with an arbitrary number of massive propagators and one large external momentum. The relevant subdiagrams are generated automatically. The resulting massless two-point integrals and massive tadpoles are transformed on-line to FORM-expressions ready to be used by existing FORM packages which calculate them analytically. As an example we compute the quartic mass corrections to the photon polarization function.

hep-ph↗

arXiv1996

Top Quark Polarization in Polarized $e^+e^-$ 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 be produced.

hep-ph↗

arXiv1995

Polarized Top Quarks

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.

hep-ph↗

arXiv1995

Higgs effects in top quark pair production

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.

hep-ph↗

arXiv1994

Polarization in Top Quark Pair Production near Threshold

The polarization dependent momentum distributions of top quarks and their decay products are calculated for $t\bar t$ production at future $e^+e^-$ colliders with polarized beams. The Green function formalism is applied to this reaction near energy 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 at large momentum transfers and Richardson ansatz at intermediate and small ones. It is demonstrated that for the longitudinally polarized electron beam an optimally polarized sample of top quarks can be produced.

hep-ph↗

arXiv1994

Polarization in Top Quark Production and Decay near Threshold

Theoretical results are presented for top quarks produced in annihilation of polarized electrons on positrons. Polarization studies for $t\bar t$ pairs near threshold are free from hadronization ambiguities. This is due to the short lifetime of the top quark. Semileptonic decays are discussed as well as their applications in studying polarization dependent processes involving top quarks. The Green function formalism is applied to $t\bar t$ production at future $e^+e^-$ colliders with polarized beams. Lippmann--Schwinger equation is solved numerically for the QCD chromostatic potential given by the two-loop formula at large momentum transfers and Richardson ansatz at intermediate and small ones. The polarization dependent momentum distributions of top quarks and their decay products are calculated.

hep-ph↗