Top Quark Production at Hadron Colliders: an Overview

The study of the properties of the top quark is one of the main goals of the Large Hadron Collider (LHC) physics program. The experimental precision expected at the LHC requires the calculation of several top-quark related observables beyond leading order in the strong coupling constant. In this work we briefly review the status of the theoretical predictions for the top-quark production processes at hadron colliders. Special attention is devoted to recent progress in the calculation of next-to-next-to-leading-order corrections to the top-quark pair production cross section.Comment: Submitted to the proceedings of "The 2009 Europhysics Conference on High Energy Physics", July 16 - 22, 2009, Krakow, Poland. References adde

Two-Loop QED Heavy-Flavor Contribution to Bhabha Scattering

We briefly review the status of the calculation of next-to-next-to-leading order corrections to large angle Bhabha scattering in pure QED. In particular, we focus on the analytic calculation of the two-loop virtual corrections involving a heavy-flavor fermion loop, which was recently completed. We conclude by assessing the numerical impact of these corrections on the Bhabha scattering cross section at colliders operating at a center of mass energy of 1 GeV and at the future ILC.Comment: 6 pages, 2 figures. Contribution to the Proceedings of the 9th DESY workshop on "Loops and Legs in Quantum Field Theory", Sondershausen, April 2008. References adde

On the Generalized Harmonic Polylogarithms of One Complex Variable

We describe how to compute numerically in the complex plain a set of Generalized Harmonic Polylogarithms (GHPLs) with square roots in the weights, using the C++/GiNaC numerical routines of Vollinga and Weinzierl. As an example, we provide the numerical values of the NLO electroweak light-fermion corrections to the Higgs boson production in gluon fusion in the case of complex W and Z masses.Comment: 25 pages, 1 figur

Two-Loop QCD Corrections to the Heavy Quark Form Factors: Axial Vector Contributions

We consider the Z Q Qbar vertex to second order in the QCD coupling for an on-shell massive quark-antiquark pair and for arbitrary momentum transfer of the Z boson. We present closed analytic expressions for the two parity-violating form factors of that vertex at the two-loop level in QCD, excluding the contributions from triangle diagrams. These form factors are expressed in terms of 1-dimensional harmonic polylogarithms of maximum weight 4.Comment: 57 pages, 5 figures. All the results in Section 6 of the paper are available in an electronic form in the file formulas.in

Two-loop QCD corrections to the vector form factors for the heavy-quark photo-production

We review on the calculation of the heavy-quark photo-production vector form factors, with the full dependence on the mass of the heavy-quark. The Feynman diagrams are evaluated within the dimensional regularization scheme and expressed in Laurent series of $\epsilon=(4-D)/2$, where $D$ is the space-time dimension. The coefficients of the expansion are expressed in terms of harmonic polylogarithms. The numerical evaluation of the analytical formulas and the threshold limit of our results are presented.Comment: Talk presented at the 11th International QCD Conference: QCD04, Montpellier, France, 5-10 July 2004; 4 pages, 4 figure

Two-Loop QCD Corrections to the Heavy Quark Form Factors: Anomaly Contributions

We present closed analytic expressions for the order $\alpha_s^2$ triangle diagram contributions to the matrix elements of the singlet and non-singlet axial vector currents between the vacuum and a quark-antiquark state. We have calculated these vertex functions for arbitrary momentum transfer and for four different sets of internal and external quark masses. We show that both the singlet and non-singlet vertex functions satisfy the correct chiral Ward identities. Using the exact expressions for the finite axial vector form factors, we check the quality and the convergence of expansions at production threshold and for asymptotic energies.Comment: 24 pages, 6 figure

Two-loop QED Corrections to Bhabha Scattering

Recent developments in the calculation of the NNLO corrections to the Bhabha scattering differential cross section in pure QED are briefly reviewed and discussed.Comment: 5 pages, 4 figures, to appear in the proceedings of the 7th International Symposium on Radiative Corrections (RADCOR05), Shonan Village, Japan, 200

Two-Loop N_F =1 QED Bhabha Scattering: Soft Emission and Numerical Evaluation of the Differential Cross-section

Recently, we evaluated the virtual cross-section for Bhabha scattering in pure QED, up to corrections of order alpha^4 (N_F =1). This calculation is valid for arbitrary values of the squared center of mass energy s and momentum transfer t; the electron and positron mass m was considered a finite, non vanishing quantity. In the present work, we supplement the previous calculation by considering the contribution of the soft photon emission diagrams to the differential cross-section, up to and including terms of order alpha^4 (N_F=1). Adding the contribution of the real corrections to the renormalized virtual ones, we obtain an UV and IR finite differential cross-section; we evaluate this quantity numerically for a significant set of values of the squared center of mass energy s.Comment: 24 pages, 15 figures. Formulas in Appendix B corrected, changes in Section 3, references adde

Electroweak top-quark pair production at the LHC with Z′Z' bosons to NLO QCD in POWHEG

We present the calculation of the NLO QCD corrections to the electroweak production of top-antitop pairs at the CERN LHC in the presence of a new neutral gauge boson. The corrections are implemented in the parton shower Monte Carlo program POWHEG. Standard Model (SM) and new physics interference effects are properly taken into account. QED singularities, first appearing at this order, are consistently subtracted. Numerical results are presented for SM and $Z'$ total cross sections and distributions in invariant mass, transverse momentum, azimuthal angle and rapidity of the top-quark pair. The remaining theoretical uncertainty from scale and PDF variations is estimated, and the potential of the charge asymmetry to distinguish between new physics models is investigated for the Sequential SM and a leptophobic topcolor model.Comment: 33 pages, 15 figures, 1 table. Discussion added on uncertainties from parton showers and NLL contribution