A precision measurement of the mass of the top quark

The standard model of particle physics contains parameters such as particle masses - whose origins are still unknown and which cannot be predicted, but whose values are constrained through their interactions. In particular, the masses of the top quark (M-t) and W boson (M-W)(1) constrain the mass of the long-hypothesized, but thus far not observed, Higgs boson. A precise measurement of Mt can therefore indicate where to look for the Higgs, and indeed whether the hypothesis of a standard model Higgs is consistent with experimental data. As top quarks are produced in pairs and decay in only about 10(-24) s into various final states, reconstructing their masses from their decay products is very challenging. Here we report a technique that extracts more information from each top-quark event and yields a greatly improved precision (of +/- 5.3 GeV/c(2)) when compared to previous measurements(2). When our new result is combined with our published measurement in a complementary decay mode(3) and with the only other measurements available(2), the new world average for M-t becomes(4) 178.0 +/- 4.3 GeV/c(2). As a result, the most likely Higgs mass increases from the experimentally excluded(5) value(6) of 96 to 117GeV/c(2), which is beyond current experimental sensitivity. The upper limit on the Higgs mass at the 95% confidence level is raised from 219 to 251 GeV/c(2).