WHAT IS THE MASS OF THE TOP QUARK? The top was discovered only in 1995 and its existence is based on no more than a few dozen events (above background) recorded in proton-antiproton collisions at Fermilab. Yet, ironically, the top mass is known with greater precision than are the masses of the light quarks that constitute most of the matter in the universe. The reason is that the top has a proportionately bigger mass, making experimental and theoretical uncertainty proportionately smaller. Physicists would especially like to have a more accurate top mass since this would help constrain the mass of the much-wanted Higgs boson, thereby helping in the search for the Higgs in upcoming accelerator experiments. To improve the current estimate for the top mass, Fermilab scientists at the two major collider experiments, known as D0 and CDF, continue to sift through their existing inventory of scattering events. Produced back to back in the fireball of a proton-antiproton collision, a top quark and its antitop twin quickly decay into a pair of W bosons and a pair of bottom quarks. Top event candidates are generally classified into three categories according to how the Ws decay: (1) the mode in which one W decays into a lepton (such as an electron or a muon) plus a neutrino (nu) while the other W decays into two quark jets. This mode is the basis of a new, record-high-accuracy top mass determination from D0 (S. Abachi et al., Physical Review Letters, 18 August). (2) The most common decay mode, but somewhat harder to distinguish from non-top particle decays, is the "all-hadronic" mode in which both Ws decay into a pair of quark jets. Finally (3) there is the rare "dilepton" mode in which both Ws decay via lepton+nu. Previous top mass studies used only mode 1 events, but a new CDF mass measurement (F. Abe et al., 15 Sept. Phys. Rev. Lett) is based on the first identification of all-hadronic (mode 2) top decays. Subsequently, D0 has reported calculations of the top mass based on the "dilepton" (mode 3) events. The fact that this result agreed so well with the others quenched speculations that some dilepton events could be caused by decays from some particles not predicted by the Standard Model. CDF and D0 are setting up a working group to combine their best top mass calculations, which currently stand at about 177 GeV for CDF and 172 GeV for D0. (Images at Physics News Graphics.)

WHITE BLOOD CELLS SORT THEMSELVES BY TYPE in an artificial environment with dimensions similar to those of human capillaries, the narrow blood vessels which connect arteries to veins. While red blood cells pass easily through a capillary (which is 4-5 microns in diameter), white blood cells (around 10 microns in diameter) must squeeze through the entrance with the help of complex hydrodynamic forces. Once inside the capillary, the cells stick somewhere on the capillary walls. Now, a research team (Bob Austin, Princeton, 609-258-4353) has constructed a series of artificial capillaries--microfabricated polyurethane channels, each with a width of 5 microns. Sending fluorescently labeled white blood cells through the channels, the researchers observed that T-lymphocytes (antibody-producing cells from the thymus) avoided sticking to regions occupied by groups of granulocytes (cells with grainy features surrounding the nucleus) and groups of monocytes (cells with a kidney-shaped nucleus). This unexpected self-sorting process suggests a sort of physical communication between the different types of cells. In addition, the artificial channels may potentially serve as a tool for identifying blood cell disorders. (Robert H. Carlson et al., 15 September Physical Review Letters; images at Physics News Graphics).