Higher order QED corrections to Bhabha scattering at low angles

We calculate the QED bremmstrahlung corrections beyond O() for small-angle scattering. These corrections are of vital importance for measuring the luminosity and consequently the total cross sections in LEP experiments. They are dominated by the second order leading-logarithmic contributions. We calculate them with the high technical precision 0.02% and we show that they depend strongly of the details of experiment cut-off. We propose a practical method of introducing the O(²) correction calculated in this paper into current procedures of the luminosity measurement at LEP. The remaining overall QED uncertainty is estimated to be 0.3%. This estimate includes 0.2% due to pure bremsstrahlung and 0.1% due to light pair production but does not include hadronic uncertainty in the photon vaccum polarization.     

Measurement of QED structure functions of the photon using azimuthal correlations at LEP

We have studied azimuthal correlations in singly-tagged e⁺e^? → e⁺e^?μ⁺μ^? events at an average Q ² of 5.2 GeV². The data were taken with the OPAL detector at LEP at e⁺e^? centre-of-mass energies close to the Z⁰ mass, with an integrated luminosity of approximately 100 pb^?1. The azimuthal correlations are used to extract the ratio $F_{B}^{αmma}/F_{2}^{αmma}$ of the QED structure functions $F_{B}^{αmma}(x,Q^{2})$ and $F_{2}^{αmma}(x,Q^{2})$ of the photon. In leading order and neglecting the muon mass $F_{B}^{αmma}$ is expected to be identical to the longitudinal structure function $F_{L}^{αmma}$. The measurement of $F_{B}^{αmma}/F_{2}^{αmma}$ is found to be significantly different from zero and to be consistent with the QED prediction.     

Measurement of antiproton-proton small-angle elastic scattering at low momentum

Two high statistics measurements of antiproton-proton small-angle elastic scattering, at p = 233 MeV/c and p = 272 MeV/c, are presented. The measurements were carried out at the LEAR facility at CERN. By the Coulomb-nuclear interference method, values are obtained for the real-to-imaginary ratio ρ of the p̄p forward nuclear scattering amplitude and for its exponential slope b: ρ = + 0.041 ± 0.026 and b = 71.5 ± 4.5 (GeV/c)⁻² at 233 MeV/c and ρ = −0.014 ± 0.027 and b = 47.7 ± 2.7 (GeV/c)⁻² at 272 MeV/c. The method to derive these values is discussed in detail and so are the uncertainties contributing to their systematic error. The results are compared with predictions from forward dispersion relation calculations and with predictions from p̄p potential models.     

Resonant Bhabha scattering at MeV energies

We discuss, on a phenomenological level, the possible appearance of resonances ine ⁺+e ^? scattering at energies in the MeV region. The expected resonance cross section and angular distribution are examined. The observability depends crucially on the attainable energy resolution which is limit by the momentum distribution of the electrons contained in the target.     

Measurement of the hadronic cross-section for the scattering of two virtual photons at LEP

The interaction of virtual photons is investigated using the reaction hadrons based on data taken by the OPAL experiment at centre-of-mass energies GeV, for GeV and at an average of 17.9 GeV. The measured cross-sections are compared to predictions of the Quark Parton Model (QPM), to the Leading Order QCD Monte Carlo model PHOJET, to the NLO prediction for the reaction , and to BFKL calculations. PHOJET, NLO , and QPM describe the data reasonably well, whereas the cross-section predicted by a Leading Order BFKL calculation is too large. Received: 3 September 2001 / Published online: 5 April 2002     

The running of the electromagnetic coupling $\alpha$ in small angle Bhabha scattering

A method to determine the running of from a measurement of small angle Bhabha scattering is proposed and worked out. The method is suited to high statistics experiments at e ⁺ e^--colliders which are equipped with luminometers in the appropriate angular region. A new simulation code predicting small angle Bhabha scattering is also presented.Received: 22 January 2004, Published online: 16 March 2004L. Trentadue: Permanent address: Dipartimento di Fisica, Universitá di Parma and INFN, Gruppo Collegato di Parma, 43100 Parma, Italy     

Higher order QED corrections toW pair production at LEP II

The problem of evaluating initial state QED corrections to theW pair production cross section, taking into account the effects of finiteW width, is investigated. The contributions of higher order corrections compared with the ones, hard bremsstrahlung and experimental photon energy cuts are computed within the structure functions formalism both in the threshold and peak region. The numerical impact of the corrections on theW mass measurement is studied.     

Measurement of the Tau Polarisation at LEP

The polarisation of 's produced in Z decay is measured using 160 pb of data accumulated at LEP by the ALEPH detector between 1990 and 1995. The variation of the polarisation with polar angle yields the two parameters and which are consistent with the hypothesis of e- universality. Assuming universality, the value is obtained from which the effective weak mixing angle is derived. Received: 2 March 2001 / Published online: 8 June 2001     

QED interconnection effects on W momentum distributions at LEP 2

The process ${? e}^{+}{? e}^{-}?ghtarrow {? W}^{+}{? W}^{-}?ghtarrow {? f}_{1}{?erline {? f}}_{2}{? f}_{3}{?erline {? f}}_{4}$ contains charges in the initial, intermediate and final stages. This gives a rich selection of possible QED interconnection effects. Coulomb interaction is the simplest of these, and can thus be used to explore consequences. We study a number of experimental observables, with emphasis on those related to the W momentum distribution. Second-order Coulomb effects are shown to be practically negligible. The limited LEP 2 statistics will not allow detailed tests, so any theory uncertainty will be reflected in the systematic error on the W mass. Currently the uncertainty from this source may be as high as 20 MeV.     

Measurement of the proton structure function F2 in QED Compton scattering at HERA

The European Physical Journal C - QED Compton scattering at HERA is proposed for a measurement of the proton structure function F2 at low momentum transfers Q2. It is shown that the analysis of...     

Bhabha scattering near the Z0

A complete analysis of EM radiative corrections to Bhabha scattering near the Z₀ is presented Compact analytic formulae are given which include exact one-loop results and soft-and-collinear photon effects resummed to all orders. Detection of back-to-back e⁺e⁻ pairs can be therefore used as a high precision monitor of luminosity at LEP/SLC energies.     

Measurement of the Z resonance parameters at LEP

Abstract. The properties of the Z resonance are measured from the analysis of 4.5 million Z decays into fermion pairs collected with the Aleph detector at LEP. The data are consistent with lepton universality. The resonance parameters are measured to be , GeV, nb and, combining the three lepton flavours, . The corresponding number of light neutrino species is and the strong coupling constant is . The lepton pair forward-backward asymmetry is measured to be from which the effective weak mixing angle is derived: . The measurement of the leptonic width MeV leads to a determination of the effective parameter . The data support the Standard Model and favour a light Higgs. Received: 16 July 1999 / Published online: 17 March 2000     

Radiative corrections to the Bhabha scattering

In this review we consider the high energy processes in QED frames in peripherical kinematics. The key feature of this kinematics is that processes have a large cross section which do not decrease with the increasing of the initial center of mass energy. Two purposes to study peripheral processes are: the background processes with large total cross sections and the structure of jets in the fragmentation region.