Number 124 (Story #2), April 16, 1993 by Phillip F. Schewe and Ben Stein THE TOP QUARK has not been discovered yet in high energy proton-antiproton collisions at Fermilab, and researchers there have pushed up their estimate of the minimum mass the top must have, if indeed it exists. Speaking at the APS meeting, Tony Liss of the University of Illinois reported that data from the CDF detector preclude a top quark mass less than 108 GeV. David Buchholz of Northwestern reported a comparable value, 103 GeV, for the D0 detector. Both of the mammoth detectors (each employing more than 400 physicists) have recorded a small number of events (officially 2 for CDF and 1 for D0) suggestive of top-quark production, events in which a high energy muon or electron, along with jets of other particles, emerges from the proton-antiproton collision. The Tevatron scientists admit, however, that such events might also be ascribed to a variety of non-top background reactions. Fermilab director John Peoples said that an inventory of at least 10 events in each detector would be necessary before one could even consider declaring that the top had been produced unambiguously. This underscores the statistical nature of the search for the top; the Tevatron has more than enough energy to create quarks with masses of 200 GeV or more, but the basic probability (the cross section) of this happening in any one interaction is extremely small. What is needed is much more data, and an important way of doing that is to increase the luminosity, the rate at which beam particles can be brought to bear at the interaction point. After a scheduled shutdown (June-Oct) the Tevatron is expected to operate with a larger luminosity and at a higher energy, 2 TeV.