Number 213, February 7, 1995 by Phillip F. Schewe and Ben Stein THE D0 EXPERIMENT AT FERMILAB has added new information to the search for the top quark. In April 1994, scientists from D0's neighbor, the CDF detector collaboration, reported (eventually in Physical Review Letters, 11 July 1994) a tentative sighting of the top quark; in support of their claim they exhibited a sample of 12 candidate events and deduced a cross section (related to the likelihood of producing a top-antitop pair in proton-antiproton collisions) of 14 pico-barns (1 barn equals 10**-24 cm**2). At that time the D0 group, with similar sensitivity, did not have significant evidence for the top. They had previously published a lower limit on the mass of the top quark (Phys. Rev. Lett., 4 April 1994). A year later, and using the same data as before, the D0 scientists have re-optimized their analysis by expanding their search to additional top-decay modes and by better understanding possible background phenomena. With this new perspective they now report a sample of 9 candidate top events with an expected background of 3.8 events. If one assumes that the excess events are top-quark events, and if one assumes their mass is 180 GeV, then the cross section for top production would be 8.2 (with an error of 5.1) pico-barns (S. Abachi et al., upcoming article in Phys. Rev. Lett.). The D0 co-spokesmen, Paul Grannis and Hugh Montgomery, caution that this analysis does not yet demonstrate the existence of the top quark. More recently, a D0 paper presented at a physics meeting in Aspen, Colorado in January 1995 provides supplementary information. Using data from run Ia (Aug. '92--Jun. '93) and a preliminary analysis of partial data from run Ib (1994-95), the D0 inventory now consists of 18 candidate events with an expected background of 8.2, still short of what they feel is needed for a significant demonstration of a top signal. This sample of events suggests (assuming the top exists) a mass of about 200 GeV. INDIRECT EVIDENCE FOR NEUTRINO MASS comes from a Los Alamos experiment in which muon antineutrinos are perhaps transmuting into electron antineutrinos in a process called "neutrino oscillation." Los Alamos uses a proton beam to produce pions whose decays result in streams of various daughter particles, including muon antineutrinos. The pion decay process does not produce any electron antineutrinos, so any that turn up further downstream must, the researchers believe, come from the metamorphosis of another neutrino type, probably muon antineutrinos. Neutrinos, regardless of their type, interact very feebly. During the five months of data taking, the Los Alamos scientists looked for rare interactions in which the newly minted electron antineutrino enters the reaction vessel (filled with 180 tons of mineral oil) and collides with a proton, creating a positron and a neutron. The apparatus is designed to search for characteristic light (Cerenkov radiation) from the positron; meanwhile, the 2-MeV neutron eventually combines with a proton to make a deuteron and a gamma ray. From the sample size one can calculate the oscillation rate. From that, one can infer not a value for neutrino mass directly but rather the difference of the squares of the masses for the two neutrino species. Current theoretical models hold that if oscillation is occurring, at least one of the neutrino types has mass. According to D. Hywel White and William Louis of Los Alamos, the observed rate of electron antineutrino interactions suggests a neutrino mass range of 0.5 and 5 eV. The results are not statistically sufficient to settle the issue of neutrino mass and more tests are needed. The issue is important for particle physicists and for cosmologists, who suspect that neutrinos with even a very small mass may play a role in organizing matter into galaxies.