Search for promptly produced heavy quarkonium states in hadronic Z decays

A search has been made for direct production of heavy quarkonium states in more than 3 million hadronic Z(0) decays in the 1991-1994 DELPHI data, Prompt J/psi, psi(2S) and Upsilon candidates have been searched for through their leptonic decay modes using criteria based on the kinematics and decay vertex positions. New upper limits are set at the 90% confidence level for Br(Z(0) --> (Q (Q) over bar) X)/Br(Z(0) --> hadrons) for various strong production mechanisms of J/psi and Upsilon; these range down to 0.9 x 10(-4). The limits are set in the presence of a small excess (similar to 1% statistical probability of a background fluctuation) in the sum of candidates from prompt J/psi, psi(2S), Upsilon(1S), Upsilon(2S) and Upsilon(3S) relative to the estimated background.