Analysis of the process $e^+ e^- \to \mu ^+ \mu^- (n\gamma)$ at centre-of-mass energies from 60 to 140 GeV with the ALEPH detector at LEP

The total cross section and the forward-backward asymmetry for the process $e^+ e^- \rightarrow \mu^+ \mu^- (n \gamma)$ are measured in the energy range 60-140 GeV by reconstructing the effective centre-of-mass energy $\sqrt{s'}$ after initial state radiation. The analysis is based on the data recorded with the ALEPH detector at LEP between 1990 and 1995, corresponding to a total integrated luminosity of 143.5 $\mathrm{pb}^{-1}$. The full statistical power of the event sample has been used and a precise measurement of the energy dependence of $\sigma^0$ and $A_{\mathrm{FB}}^0$ is obtained from a model independent fit, enabling constraints to be placed on models with extra Z bosons.