Abstract
In e+e− collisions, electromagnetic effects caused by large charge density bunches modify the effective acceptance of the luminometer system of the experiments. These effects consequently bias the luminosity measurement from the rate of low-angle Bhabha interactions e+e−→e+e−. Surprisingly enough, the magnitude of this bias is found to yield an underestimation of the integrated luminosity measured by the LEP experiments by about 0.1%, significantly larger than the reported experimental uncertainties. When accounted for, this effect modifies the number of light neutrino species determined at LEP from the measurement of the hadronic cross section at the Z peak.
Highlights
The Large Electron-Positron (LEP) collider was operated at CERN between 1989 and 2000, and delivered e+e− collisions to four experiments, at centre-of-mass en√ergies that covered the Z resonance, the WW threshold, and extended up to s = 209 GeV
Where R0 is the ratio of the hadronic-to-leptonic Z branching fractions; δτ is a small O(m2τ /m2Z) correction; and (Γνν/Γ )SM is the ratio of the massless neutral-to-charged leptonic Z partial widths predicted by the Standard Model (SM)
The bias of the luminometer acceptance, induced by the focusing of the final state electrons and positrons from small angle Bhabha scattering by the opposite-charge bunches, has been quantified for the four experiments operating at LEP at and around the Z pole
Summary
The final state e+ (e−) in a Bhabha interaction, emitted at a small angle off the e+ (e−) beam, feels an attractive force from the incoming e− (e+) bunch, and is focused towards the beam axis.2 This effect, illustrated, leads to an effective reduction of the acceptance of the LumiCal, as particles that would otherwise hit the detector close to its inner edge are focused to lower polar angles and may miss the detector. Detailed results of these calculations are presented, for a representative situation corresponding to the OPAL luminosity measurement performed in 1994, during which half of the LEP data at the Z peak was collected This illustrative example assumes a polar angle acceptance between θmin = 31.3 mrad and θmax = 51.6 mrad (Table 1), and the set of beam parameters given in the third row of Table 2.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
