A precision measurement of the mass of the top quark

*A list of authors and their affiliations appear at the end of the paper

Received 23 January; accepted 21 April 2004; doi:10.1038/nature02589.

Correspondence and requests for materials should be addressed to J. Estrada ([email protected]).

Nature 429(6992):p 638-642, June 10, 2004.

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) andWboson (M_W)constrain the mass of the long-hypothesized, but thus far not observed, Higgs boson. A precise measurement ofM_tcan 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 10⁻²⁴s 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/c²) when compared to previous measurements. When our new result is combined with our published measurement in a complementary decay modeand with the only other measurements available, the new world average forM_tbecomes178.0 ± 4.3 GeV/c². As a result, the most likely Higgs mass increases from the experimentally excludedvalueof 96 to 117 GeV/c², 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/c².