Number 216, March 3, 1995 by Phillip F. Schewe and Ben Stein THE TOP QUARK AT LAST! After months and years of speaking guardedly about tentative, preliminary evidence for the top quark, the rival CDF and D0 collaborations at Fermilab finally, jointly, announced yesterday that they had indeed discovered the top quark. This particle is the sixth and supposedly last of a class of quarks which, along with the leptons, form the basic alphabet from which all matter is constructed. With two to three times more data available than a year ago, researchers at both groups are now confident that their inventory of top quark events (rare events in which top quarks are created out of the annihilation of high energy protons and antiprotons) represents a true signal and not just a spurious effect due to some background phenomenon. The new results were announced at a colloquium held yesterday at Fermilab and are also circulating in the form of preprints of articles submitted by both groups to Physical Review Letters. According to Drew Baden of the University of Maryland (a member of the D0 team), both experiments sampled about 6 trillion collision events, of which data was collected for about 40 million events. This vast mountain of information was sifted through a myriad of computer programs seeking just the right configurations of daughter particles. In the end the CDF turned up 37 top candidate events as against an expected background of 12 events. (In this business, one must always specify both the number of measured events and suspected background events.) The D0 collaboration found 17 top candidate events and estimated a background of about 4 events (with an uncertainty of 0.6 events in the background). Establishing a mass for the top quark and a cross section for producing tops in proton- antiproton collisions are also important. CDF reports a mass of 176 GeV, with an uncertainty of 8 GeV arising from the statistical size of the data sample and 10 GeV from possible systematic errors associated with the measurement process. They report a cross section of 6.8 pico-barns (a barn is a unit equalling 10**-24 sq cm). The uncertainty in this cross section ranges from +3.6 pb to -2.4 pb. D0's somewhat different numbers are 199 GeV (statistical uncertainty of 20 GeV and systematic uncertainty of 22 GeV) for the mass and 6.4 pb (with an uncertainty of 2.2) for the cross section. The statistical significance of the new top quark measurement is (expressed in units of likelihood) about 4.7 standard deviations. Put another way, the overall possibility of the observed top quark events being purely due to some background phenomenon is less than one part in a million for both experiments. This assurance of a genuine effect should improve as more events are recorded. The researchers expect to have twice the current data by December when a planned shutdown of the beams will occur. Quarks have hitherto never been seen in isolation; instead they always are seen in clumps of two (particles called mesons) or three (baryons). In order to split up such clumps (collectively called hadrons) the gluon lines which, according to the theory of quantum chromodynamics, yoke the quarks together must be severed. But in practice doing this only results in the creation of a new quark-antiquark pair, each of which teams up with the newly unattached quarks to form more hadrons. In the case of the top quarks, however, the top decays so quickly that it never has time to pair up with another quark. Therefore, Drew Baden asserts, the D0 and CDF observations constitute, in effect, the first detection of a free quark.