Measurement of Z/$\gamma^*$ production in compton scattering of quasi-real photons

Abstract.

The process $e^+ e^- \to e^+ e^- Z/\gamma^*$ is studied with the OPAL detector at LEP at a centre of mass energy of $\sqrt{s}$ = 189 GeV. The cross-section times the branching ratio of the Z/$\gamma^*$ decaying into hadrons is measured within Lorentz invariant kinematic limits to be $(1.2 \pm 0.3 \pm 0.1)$ pb for invariant masses of the hadronic system between 5 GeV and 60 GeV and $(0.7 \pm 0.2 \pm 0.1)$ pb for hadronic masses above 60 GeV. The differential cross-sections of the Mandelstam variables $\hat{s}, \hat{t}$, and $\hatu$ are measured and compared with the predictions from the Monte Carlo generators grc4f and PYTHIA. From this, based on a factorisation ansatz, the total and differential cross-sections for the subprocess ${\rm e} \gamma\to{\rm e Z}/\gamma^*$ are derived.