The ratio Rl of the number of Z hadronic decays to the number of Z leptonic decays can be potentially measured at FCC-ee with a relative statistical precision of O(3 × 10−6) for each lepton type. It is a key quantity [4] that serves – in conjunction with the total Z decay width and the peak hadronic cross section – as input to several fundamental quantities: (i) the measurement of the leptonic Z partial width Γll, a very clean electroweak observable whose relation to the Z mass is the ρ (or T) parameter, and unaffected by αQED(m2Z), with a 10−5 relative precision; and (ii) the measurement of the strong coupling constant αS(m2Z) with an absolute experimental uncertainty below 0.0001. Experience from LEP showed that a limiting systematic uncertainty comes from the knowledge of the geometrical acceptance for lepton pairs. The requirements on the detector design to match the statistical precision will be studied in the full context of the constraints from the interaction region layout. As a by-product, the determination of the geometrical acceptance for the e+e− → γγ process, which may be used for the measurement of the absolute luminosity, potentially with a statistical precision of a few 10−5, will be investigated. The knowledge, at the same level of precision, of the acceptance for the more abundant hadronic Z decays, a much easier problem at LEP, will need to be verified.
- The corresponding Snowmass LOI