The final top mass result is corrected for the PDF reweighting, the local and global lepton PT corrections and an observed overestimation of the top mass of -1 GeV. For the PDF reweighting an event reweighting scheme was applied from the LO CTEQ5L set to the NLO CTEQ6M set, forcing the correct NLO fraction of gg→ttbar events relative to qqbar→ttbar events. The lepton PT values were also corrected using the global and local correction coefficients documented in CDF note 9579. The corrections applied are listed in Table 2.
TABLE 2: List of the corrections to the top mass.
The b-tagged dilepton selection on the 2.8 fb-1 of data gives 80 dilepton pairs. The 160 leptons, give a mean PT = 49.7 ± 2 GeV/c. Figure 5 illustrates the lepton PT distribution of the data in comparison with the Standard Model signal + background expectation. The Kolmogorov-Smirnov test between the two gives 0.86.
FIG 5: Data leptons' PT at 2.8 fb-1 (blue points). The light blue spectrum corresponds to the leptons' PT of the MC signal leptons at Mtop =150 GeV and the corresponding background. We expect 130.4 signal leptons, estimated for Mtop = 175 GeV and 8±3.4 background leptons. The total signal & background SM expectation is 138.4 leptons, while the 2.8 fb-1 data give 160 leptons.
Figure 6 shows the Gamma x Fermi fit to the data before any correction applied. After all corrections listed in Table 2 the top quark mass measured in the b-tagged dilepton channel at 2.8 fb-1, using the full shape of the leptons' PT is:
Mtop = 154.6 ± 13.3(stat) ± 2.3(syst) GeV/c2
This is one of the best few top mass measurements with regards to the systematic error. The method can potentially be of more interest for the LHC experiments, where a huge number of top candidate events will be registered and the dominating error will, then, be the systematic one. In the circumstance that the leptons will have an accurate calibration soon after the start of data taking, and the energy of the jets a large uncertainty this method is promising one of the first/best top mass measurements.
FIG 6: Fit to the 2.8 fb-1 b-tagged dilepton data