A Precision Measurement of the Mass of the Top Quark

Abstract

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.

Figures (4)

• Figure 1: Feynman diagrams for $t\bar{t}$ production in $p\bar{p}$ collisions, with subsequent decays into an electron, neutrino, and quarks. Diagram (a) (quark-antiquark production) is dominant, but diagram (b) (gluon fusion) contributes $\approx 10\%$ to the cross section. This particular final state ($e\bar{\nu} u \bar{d} b\bar{b}$) is one of the channels used in the analysis.
• Figure 2: Relative importance of various $t\bar{t}$ decay modes. The "lepton $+$ jets" channel used in this analysis corresponds to the two offset slices of the pie-chart and amounts to 30% of all the $t\bar{t}$ decays.
• Figure 3: Current experimental constraints on the mass of the Higgs boson. The $\chi^2$ for a global fit to electroweak data using the procedure of Ref. lepEWWG4, is shown as a function of the Higgs mass. The solid line corresponds to the result for the previous world average for the top-quark mass of $174.3 \pm 5.1$ GeV/$c^2$, with the blue band indicating the impact of theoretical uncertainty. The dotted line shows the result for the new world-averaged $M_t$ of $178.0 \pm 4.3$ GeV/$c^2$, while the dashed line corresponds to using just the new DØ average of $179.0 \pm 5.1$ GeV/$c^2$. The yellow shaded area on the left indicates the region of Higgs masses excluded by experiment ($> 114.4$ GeV/$c^2$ at the 95% confidence level lepdirect). The improved top mass measurement shifts the most likely value of the Higgs mass above the experimentally excluded range.
• Figure 4: Determination of the top-quark mass using the maximum likelihood method. The points represent the likelihood of the fit used to extract the top mass, divided by its maximum value, as a function of the mass of the top quark (after a correction for a $-0.5$ GeV/$c^2$ mass bias, see text). The solid line shows a Gaussian fit to the points. The maximum likelihood corresponds to a mass of 180.1 GeV/$c^2$, which is the new DØ measurement of the top mass in the lepton $+$ jets channel. The hatched band corresponds to the range of $\pm 1$ standard deviation, and indicates the $\pm 3.6$ GeV/$c^2$ statistical uncertainty of the fit.