CONSTRAINTS ON THE NUMBER OF HEAVY GENERATIONS FROM THE S-PARAMETERS AND T-PARAMETER

We show that precision measurements of the oblique correction paramete rs S and T can be used to derive an upper bound on the number of extra generations of heavy fermions N-eg. The bound depends on the type of heavy neutrino masses. We have obtained N-eg less than or equal to 2 i n the case of Dirac neutrinos and N-eg less than or equal to 5 in the case of neutrinos with Majorana masses, under some assumptions on the flavor and color contents of extra generations.