Measurement of the ZZ cross-section in $e^+ e^-$ interactions at 183-189 GeV

Abstract

Measurements of on-shell $ZZ$ production are described, using data collected by DELPHI in 1997 and 1998, at centre-of-mass energies $\sqrt s$ = 182.6 GeV and 188.6 GeV respectively. Results obtained in each of the final states $q \bar{q} q \bar{q}$, $\mu^+ \mu^- q \bar{q}$, $e^+e^- q \bar{q}$, $\nu \bar{\nu} q \bar{q}$, $l^+l^-l^+l^-$, and $\nu \bar{\nu} l^+l^-$ are presented. The measured cross-sections for on-shell $ZZ$ production via the tree-level doubly-resonant graphs (NC02) are: \begin{tabbing} ~~~~~~~~~~\=$\sigma_{\tiny NC02}$~(182.6 GeV) \== 0.38 $\pm$ $0.18 ~(stat)$ $\pm$ $0.04 ~(syst)$ pb, \\ ~~\\ \ $\sigma_{\tiny NC02}$~(188.6 GeV) \ = 0.60 $\pm$ $0.13 ~(stat)$ $\pm$ $0.07 ~(syst)$ pb. \end{tabbing} They are consistent with the Standard Model expectations of 0.25 pb and 0.65 pb at each energy