Speaker
Description
For the FCC-ee physics programme, a precise measurement of the luminosity is
essential. The high statistics scan of the Z line shape dictates the ambitions
goals of 10^-4 precision on the absolute luminosity measurement and 10^-5 on
the relative luminosity measurement between energy scan points. The reference
process for the measurement is small angle Bhabha scattering observed by a set
of two calorimeters centered around the two outgoing beam lines. The relevant
forward region is a very crowded one, and the very compact
silicon-tungsten-sandwich luminometers have their faces positioned only about
1 m from the IP. The Bhabha scattering cross section has a very strong angular
dependence, and the geometrical precision of the luminomitors shall allow the
radial coordinate of showers to be determined to about 1 micron
precison. Machine related background processes have been investigated and
found to be small. An important effect, which is under study, is the focussing
of the final state Bhabha electrons by the strong EM field of the opposing beam
resulting in a systematic shift of the scattering angle. It has been found
that this effect is correlated with a similar beam-beam effect which causes
the beam crossing angle to increase by ~0.3 percent. Hence, a precise study of
the beam crossing angle via the copiously produced dimoun events will give
valuable information on the detailed beam-beam interaction.
