Number 176 (Story #1), April 26, 1994 by Phillip F. Schewe and Ben Stein|
FERMILAB SEES SOME EVIDENCE FOR THE TOP QUARK at a mass of 174 GeV. The standard model of particle physics holds that all matter is made from a small alphabet of elementary particles consisting of six quarks and six leptons. The heaviest of these, the top quark, is unstable and only exists when it is created artificially in the high-energy proton- antiproton collisions carried out at the Fermilab Tevatron. Finding the top quark, a major goal of Fermilab scientists for more than a decade, is important since it is the only quark not yet observed. Failure to find the top quark would disrupt the current theory of basic particle physics. Produced in conjunction with an anti-top, the top quickly decays into a variety of daughter particles. The best way to search for the top, theorists say, is to look for its decay into a W boson and the next lightest quark, the b quark. One of the chief problems, in this regard, is the fact that energetic b's and W's are also unstable and quickly decay into the kind of particle jets that emanate from less-interesting background collisions. For the purpose of identifying a top quark, the cleanest event (a di-lepton event) is one in which both W's decay leptonically, that is, into an electron or muon, plus a neutrino. But the W's can also decay into hadrons, the collective name for protons, neutrons, and mesons. In general the lepton decay events are less liable to misinterpretation than are the hadron-decay events. A third category of event (a semi-leptonic event) is one in which one W decays leptonically and the other hadronically. A claim for the top quark would be the observation of a number of events well above the number of events one would expect from background processes. Fermilab's claim, which it carefully cites as "evidence for" and not a "discovery of" the top quark, is of this type. Speaking at a seminar today at Fermilab, Melvyn Shochet of the University of Chicago, chief spokesman of the 440-person CDF collaboration, and William Carrithers of LBL, reported that an analysis of a trillion collisions had netted a dozen candidate top events in an energy range around 170 GeV: 2 di-lepton events (with an estimated background of 0.13 events), 7 semi-leptonic events (compared to an expected 3 background events), and 6 non-leptonic events (2 background events); 3 events were counted twice. Fermilab's way of assessing the "signal-to-noise ratio" for this data sample is to say that there is only a 1-in-400 chance that the 12 events, compared with the expected background, do not represent a top-quark signal. The CDF scientists have calculated the top mass to be 174 GeV with an uncertainty of about 10% and the cross section (the inherent likelihood) for producing top pairs at that mass to be about 14 pico-barns (1 barn equals 10**-24 cm**2). In addressing the question of whether there is enough statistical significance for an announcement at this time, Carrithers said that CDF was on a "middle ground" where they could not claim a discovery nor yet dismiss the presence of what looks like an excess of events at an interesting energy range. Meanwhile, Hugh Montgomery, a spokesman for the D0 collaboration, CDF's rival at the Tevatron, reported at the same seminar that his team had recorded a few interesting events but that these were comparable in number to the expected background. The data for both groups were recorded during a run which stretched from August 1992 to June 1993. They expect to take several times more data in a run currently underway.