On Large-Angle Bhabha Scattering at LEP

The theoretical accuracy of the program TOPAZ0 in the large-angle Bhabha channel is estimated. The physical error associated with the full Bhabha cross section and its forward and backward components separately is given for some event selections and several energy points of interest for LEP1 physics, both for the s and non-s contributions to the cross section. E-mail: [email protected] [email protected] [email protected] FNT/T-99/05 April 19, 1999 One of the open issues of precision physics at LEP is the determination of the accuracy of the theoretical predictions for the large-angle Bhabha scattering cross section. At present, several computer codes developed for large-angle Bhabha scattering studies can be found in the literature, ranging from semi-analytical to truly Monte Carlo ones. A detailed account of them has been presented in refs. [1, 2]. In particular, in ref. [1] several comparisons have been performed, both for “academic” and realistic event selections (ES’s), both for LEP1 and LEP2 energies. After the publication of ref. [1], a new analysis concerning specifically two codes, namely ALIBABA [3] and TOPAZ0 [4], has been performed [5], where a very detailed comparison between the two programs is developed and the estimate of the theoretical error associated with their predictions is given. If, on the one hand, the comparison is very careful, on the other hand the study is, in the opinion of the authors of the present note, lacking for the following aspects: it considers, as the main source of information on large-angle Bhabha observables, only ALIBABA and TOPAZ0; it is based on a comparison for a “bare” ES, which is far from being realistic, and ignores more realistic ES’s such as the ones considered in ref. [1]; it does not fully exploit the detailed comparisons for s-channel annihilation processes, that can be found in the literature [6] and give valuable pieces of information on a significant part of the full Bhabha cross section; it considers only centre of mass energies around the Z resonance, leaving aside LEP2 energies, from which additional information can be extracted concerning the accuracy of the non-s component of the full Bhabha cross section; it addresses the problem of assigning a theoretical error to the full Bhabha cross section both for ALIBABA and TOPAZ0, but the error for the non-s part of the Bhabha cross section is given for ALIBABA only; moreover no information is given concerning the forward and backward components of the cross section itself. √ s (GeV) 88.45 89.45 90.20 91.19 91.30 91.95 93.00 93.70 σ (pb) 457.08 644.86 912.06 1185.7