Three years ago, the top quark–the sixth and final quark predicted by the Standard Model of particle physics–was discovered after nearly two decades of searching. The 20-year-old theory remains intact, but high energy physicists continue to search for signs of a more general theory beyond the Standard Model. To do that, and learn about the elusive Higgs particle the theory predicts, they must measure the top mass with as much precision as possible. In a series of three papers in PRL, the two collaborations that discovered the top report their latest results on the top mass.
The two large collaborations, known as CDF (Collider Detector at Fermilab) and D0 (their position on the accelerator ring), detected the debris from 1800 GeV collisions of protons and anti-protons at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, IL. In April, 1995, the teams simultaneously published the top discovery  finding its mass at over much larger than once expected, and similar to the mass of a gold nucleus, which contains 197 protons and neutrons. The top also has the shortest lifetime among quarks–less than seconds–and decays as a free particle, the only quark to do so. All other quarks created in such a collision live long enough to pull more quarks from the vacuum and make complicated “jets” composed of many particles. The top’s independence has allowed the Fermilab teams to determine its mass to far greater precision than the mass of any other quark.
In the events when the proton-anti-proton collision energy spawns a top-anti-top pair, each of the quarks immediately decays into a W boson and a bottom (or anti-bottom) quark. The top event is classified according to the decays of the two W’s: (1) The “all hadronic” mode occurs when each W decays into a pair of quark jets. This is the most common mode but also the trickiest to separate from top-imitating events. (2) If each W decays into a lepton (electron, muon, or tau) plus a neutrino, it’s the “dilepton” mode–the cleanest to identify but the most rare. (3) In the “lepton plus jets” mode, one W decays leptonically and the other becomes a pair of quark jets–for precision top mass measurements this is the optimal compromise.
After the top discovery publications in 1995, which were based mainly on the lepton plus jets mode, both groups roughly doubled their data sets with another year of running the collider. The independent data from each decay mode can be used to calculate separate mass values, and the teams consider this a critical cross-check on their methods. In August last year D0 published  an updated top mass estimate based on the lepton plus jets mode, and CDF followed  in September with their analysis of the all hadronic mode. In the new PRL papers, D0 gives their mass estimate from the dilepton mode, and CDF publishes their dilepton value and their result from the far more precise lepton plus jets mode. D0 now puts the top mass at the CDF value is and both groups have total uncertainties of less than The most important tests of the Standard Model–and perhaps clues to what lies beyond it–are expected when the last undiscovered particle of the theory is found: the Higgs particle. A precise top mass will help theorists constrain the Higgs mass, improving its chances of detection at Fermilab or CERN’s Large Hadron Collider in Geneva, Switzerland, in the next decade. Unfortunately, even the latest top data leave the Higgs mass uncertainty very large indeed: it could be less than half or more than twice the top mass. The next collider run at Fermilab, beginning in 2000, should provide 20 times as many top quarks as the last one, and high energy physicists hope it will bring the mighty Higgs even closer to detection.