A Precision Measurement of the Mass of the Top Quark
  The Standard Model of particle physics contains about two dozen parameters - such as particle masses - whose origins are still unknown and cannot be predicted, but whose values are constrained through their interactions. In particular, the masses of the top (t) quark (M_t) and W boson constrain the mass of the long-hypothesized, but thus far not observed, Higgs boson. A precise measurement of the top-quark mass can therefore point to where to look for the Higgs, and indeed whether the hypothesis of a SM Higgs is consistent with experimental data. Since top quarks are produced in pairs and decay in only ~10^-24 s into various final states, reconstructing their mass from their decay products is very challenging. Here we report a technique that extracts far more information from each top-quark event and yields a greatly improved precision on the top mass of 5.3 GeV/c^2, compared to previous measurements. When our new result is combined with our published measurement in a complementary decay mode and with the only other measurements available, the new world average for M_t becomes 178.0 +- 4.3 GeV/c^2. As a result, the most likely Higgs mass increases from the experimentally excluded value of 96 GeV/c^2 to 117 GeV/c^2, which is beyond current experimental sensitivity. The upper limit on the Higgs mass at 95% confidence level is raised from 219 GeV/c^2 to 251 GeV/c^2. 
D0 Collaboration