O(α2){\cal O} (\alpha^2) Next-to-Leading Photonic Corrections to Small-Angle Bhabha Scattering in the Structure Function Formalism

A general method for computing ${\cal O} (\alpha^2)$ and higher-order next-to-leading photonic corrections is presented and applied to the precision calculation of the small-angle Bhabha scattering cross section in the phase-space region of interest for the luminosity measurement at LEP. The formulation is based on a proper matching of exact $\cal O (\alpha)$ results with higher-order corrections in the Structure Function formalism. The results of the approach are analytically compared with theoretical calculations, both for $s$- and $t$-channel processes, available for simple Event Selections. Numerical predictions for realistic Event Selections are also provided and critically compared with the ones existing in the literature.Comment: 16 pages, gzip'ed uuencoded tar fil

Six-Fermion Calculation of Intermediate-mass Higgs Boson Production at Future e+e−e^+ e^- Colliders

The production of an intermediate-mass Higgs boson in processes of the kind $e^+ e^- \to 6$ fermions at the energies of future linear colliders is studied. The recently developed and fully automatic algorithm/code ALPHA is used to compute the tree-level scattering amplitudes for the reactions $e^+ e^- \to \mu^+ \mu^- \tau^- \bar\nu_{\tau} u \bar d, \mu^+ \mu^- e^- \bar\nu_{e} u \bar d$. The code has been interfaced with the Monte Carlo program HIGGSPV/WWGENPV, properly adapted to 6-fermion production, in order to provide realistic results, both in the form of cross sections and event samples at the partonic level. Phenomenological results, that incorporate the effects of initial-state radiation and beamstrahlung, are shown and commented, emphasizing the potentials of full six-fermion calculations for precise background evaluation as well as for detailed studies of the fundamental properties of the Higgs particle.Comment: 31 pages, LaTeX, 15 eps low-resolution figures include

Radiative four-fermion processes at LEP2

The production of four fermions plus a visible photon in electron-positron collisions is analyzed, with particular emphasis on the LEP2 energy range. The study is based on the calculation of exact matrix elements, including the effect of fermion masses. In the light of the present measurements performed at LEP, triple and quartic anomalous gauge couplings are taken into account. Due to the presence of a visible photon in the final state, particular attention is paid to the treatment of higher-order QED corrections. Explicit results for integrated cross sections and differential distributions are shown and commented. The features of the Monte Carlo program WRAP, used to perform the calculation and available for experimental analysis, are described.Comment: LaTeX, 14 pages, 6 tables, 13 figures. Numerical results added, version to appear in Eur. Phys. J.

Matrix elements and Parton Shower in the event generator BABAYAGA

A new version of the event generator BABAYAGA is presented, which is based on an original matching of the Parton Shower approach with the complete exact O(alpha) matrix element for the inclusion of the QED radiative corrections to the Bhabha process at flavour factories. The theoretical accuracy of the improved generator is conservatively estimated to be 0.2%, by comparison with independent calculations. The generator is a useful tool for precise luminosity determination at flavour factories, for center of mass energies below 10 GeV.Comment: 4 pages, 2 figures. To appear in the proceedings of the International Workshop e+ e- Collisions from phi to psi, Novosibirsk (Russia), 27 Feb - 2 Mar 200

Higher-order QED corrections to single-W production in electron-positron collisions

Four-fermion processes with a particle lost in the beam pipe are studied at LEP to perform precision tests of the electroweak theory. Leading higher-order QED corrections to such processes are analyzed within the framework of the Structure Functions (SF) approach. The energy scale entering the QED SF is determined by inspection of the soft and collinear limit of the O(alpha) radiative corrections to the four-fermion final states, paying particular attention to the process of single-W production. Numerical predictions are shown in realistic situations for LEP experiments and compared with existing results. A Monte Carlo event generator, including exact tree-level matrix elements, vacuum polarization, higher-order leading QED corrections and anomalous trilinear gauge couplings, is presented.Comment: LaTeX (using elsart), 21 pages, 8 .ps figure