Beam-beam effects on the luminosity measurement at FCC-ee

Abstract

The first part of the physics programme of the integrated FCC (Future Circular Colliders) proposal includes measurements of Standard Model processes in e + e − collisions (FCCee) with an unprecedented precision. In particular, the potential precision of the Z lineshape determination calls for a very precise measurement of the absolute luminosity, at the level of 10 −4 , and the precision on the relative luminosity between energy scan points around the Z pole should be an order of magnitude better. The luminosity is principally determined from the rate of low-angle Bhabha interactions, e + e − → e + e − , where the final state electrons and positrons are detected in dedicated calorimeters covering small angles from the outgoing beam directions. Electromagnetic effects caused by the very large charge density of the beam bunches affect the effective acceptance of these luminometers in a nontrivial way. If not corrected for, these effects would lead, at the Z pole, to a systematic bias of the measured luminosity that is more than one order of magnitude larger than the desired precision. In this note, these effects are studied in detail, and methods to measure and correct for them are proposed.

Precision measurement of the Z boson to electron neutrino coupling at the future circular colliders

“…We keep in mind that for Z s ⊗ W t interference (dominant in S(η)), the present version of KKMC features virtual EW+QCD corrections of DIZET only for Z s but not yet for W t . Even for Z s exchange, in the presence of hard photon, the EW virtual corrections of KKMC are still incomplete 10 .…”

“…(Although, corrections due to beam-beam effects of recent Ref. [10] change this result to N ν = 2.992 ± 0.008.) Therefore, to shed further light in this area and complement these tests, the precision of which should be considerably improved in the near and long term future at HL-LHC, SuperKEKB and FCC, it might be very useful to study universality in Z → ν ν decays.…”

“…Near the Z peak W t ⊗ Z s interference dominates this difference and W contribution squared is negligible. 10 Strictly speaking they are extrapolated out from the soft photon regime. The energy resolution of photons can be extracted experimentally from the reconstructed µµγ events, for which the muons reconstructed angles are very precise.…”

Beam-beam effects on the luminosity measurement at LEP and the number of light neutrino species

“…A detailed analysis of beam-induced effects on the luminosity measurement at the Future Circular Collider (FCC) has been recently carried out and is reported in Ref. [9], together with methods to measure and correct for them. In the context of that study, it has been realised that these effects were already significant at LEP.…”

Measurement of the weak mixing angle at a Super Charm-Tau factory with data-driven monitoring of the average electron beam polarization

“…Λ spin rotation in this field is negligible, but such a bunch field can disturb the Bhabha distribution and introduce a bias to the luminosity measurement, as is discussed in detail in ref. [20].…”