DIRECT SEARCH FOR PAIR PRODUCTION OF HEAVY STABLE CHARGED-PARTICLES IN Z-DECAYS - SODERSTROM, E; MCKENNA, JA; ABRAMS, GS; ADOLPHSEN, CE; AVERILL, D; BALLAM, J; BARISH, BC; BARKLOW, T; BARNETT, BA; BARTELT, J; BETHKE, S; BLOCKUS, D; et al. (PHYSICAL REVIEW LETTERS, 1990)