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Register a new userQED radiative corrections to e+e- --> f+f- with realistic cuts at LEP energies and beyond
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M. Jack
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After 10 years of steadily increasing the experimental precision at LEP/SLC, there is a strong demand on an update of existing programs for fermion pair production. We present a rederivation of the O(alpha) Bremsstrahlung corrections to e+e- --> f+f- for the semi-analytic program ZFITTER. We focus on observables like total cross section and forward-backward asymmetry in the leptonic case with combined cuts on acollinearity angle, acceptance angle, and minimal energy of the fermions. The outcome of our analysis is a shift of the predictions by ZFITTER at LEP 1 energies off-resonance of a few per mil while at the Z resonance numerical changes can be neglected. Thus we obtain for cross sections and asymmetries at LEP 1 a level of agreement with other programs of better than per mil, like for the kinematically simpler s cut option. A preliminary analysis of ZFITTER, TOPAZ0, and other codes at LEP 2 energies showing deviations of several per cent with acollinearity cuts enforce a future examination of higher order effects with different cuts. The predictions by LEP/SLC data, however, are not affected within the experimental errors.
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The past ten years of physics with e+e- colliding experiments at LEP and SLAC have shown the success of these experiments on not only impressively proving the theoretical predictions of the Standard Model (SM), but also to help provide stringent bounds on physics beyond the SM. With this experience in mind, there appear two equally fascinating opportunities for studying fermion-pair production processes at a future Linear Collider (LC). On the one hand, performing high precision measurements to the SM, for example, when running with high luminosity at the Z boson resonance, could be a quick and feasible enterprise in order to pin down the symmetry breaking mechanism of the electroweak sector through indirectly determining the masses of a light SM or MSSM Higgs boson or supersymmetric particles via virtual corrections. On the other hand, looking for such particles in direct production or other `New Physics effects at energies between, for example, roughly 500 and 800 GeV will naturally be the main motivation to pursue the challenging endeavor of building and utilizing such a unique facility. These two scenarios for the LC shall be sketched here, with particular emphasis on the semi-analytical program ZFITTER for fermion-pair production in comparison with numerical programs like TOPAZ0, KK2f, and others.
In this dissertation, a complete calculation of QED radiative corrections is presented for total cross sections and forward- backward asymmetries for s-channel fermion pair production in e+e- annihilation with kinematical cuts to the final state. This includes cuts on the maximal acollinearity angle theta_acol and on the minimal energies E_min of the final state fermion pair and on the cosine of the scattering angle of one fermion, cos{theta}. The applied cuts pose a realistic alternative for leptonic final states compared to cuts on the invariant mass squared s of the fermion pair and on cos{theta}. The new formulae and the analysis presented in this thesis for high energies and luminosities form an essential building block for an upgrading of two-fermion codes like ZFITTER for a future e+e- Linear Collider.
We present a description of calculations of the amplitude for E+ E- --> F anti-F process with account of electroweak and QED one-loop corrections. This study is performed within the framework of the project SANC. The calculations are done within the OMS (on-mass-shell) renormalization scheme in two gauges: in Rxi, which allows an explicit control of gauge invariance by examining cancellation of gauge parameters and search for gauge-invariant subsets of diagrams, and in the unitary gauge as a cross-check. The formulae we derived are realized in two independent FORTRAN codes, eeffLib, which was written in an old fashioned way, i.e. manually, and another one, created automatically with an aid of s2n.f (symbols to numbers) software -- a part of SANC system. We present a comprehensive comparison with the results of the well-known program ZFITTER for all the light fermion production channels, as well as with the results existing in the world literature for the process E+ E- --> F anti-F;
Processes of electron-positron annihilation into charged pions and kaons are considered. Radiative corrections are taken into account exactly in the first order and within the leading logarithmic approximation in higher orders. A combined approach for accounting exact calculations and electron structure functions is used. An accuracy of the calculation can be estimated about 0.2%.
We suggest returning to a different presentation of the e+e- to bar{f} f data off the Z peak, with the hope of using zeroes of specific amplitudes to enhance the sensitivity to new physics.
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