Top Mass from the leptons’ PT in the dilepton channel using b-tagging at 2.8 fb-1
1University of Athens
2University of Michigan
3FNAL
Abstract:
A new measurement of the top quark mass at 2.8 fb-1 integrated luminosity, using the leptons' PT is presented. The events are selected in the dilepton decay channel where at least one tagged jet is required. A top quark mass of Mtop = 154.6±13.3(stat)±2.3(syst) GeV/c2 is obtained.
Method:
This analysis presents a measurement of the top quark mass in the dilepton channel using 2.8 fb-1 of data. The top quark mass is measured using only the lepton's PT information. The leptons' PT is a variable that can be measured very well in the tracker and the calorimeter and can be accurately calibrated against Z→dilepton decays. Jets have a minimal involvement in this analysis, i.e. only in the criteria used for the event selection. Therefore the top quark mass as extracted through this method, is associated with a low JES uncertainty.
Similar measurements have been presented and approved by the CDF for the top-antitop lepton+jets and dilepton decay channels. It was first implemented for the lepton+jets channel at the low luminosity of 340 pb-1 and again at 2.7 fb-1 using b-tagging, and improving significantly both the statistical and the systematic uncertainties to Mtop = 172.1±7.9(stat)±3.0 (syst) GeV/c2. The first measurement in the dilepton channel used 1.8 fb-1 of data and no b-tagging. This top mass measurement gave Mtop = 156.2 ±20(stat)±4.6 (syst) GeV/c2.
This analysis has been based upon the observation that the leptons' transverse momentum PT is sensitive to the top mass and the dependence is linear.
In Figure 1 the lepton mean PT vs the top mass is shown. The leptons' mean PT is derived from the PT distributions of the mass signal templates generated for different input top masses. In Figure 2 each signal template has been combined to the total background template taking into account that the purity of the total sample, as calculated for Mtop =175 GeV/c2, is ρ = signal/(signal+background) = 0.94.
FIG 1: Lepton mean PT sensitivity to the top mass form signal only PT distributions, where b-tagged dilepton selection was applied. The sensitivity of the PT of the signal leptons to the top mass is 15 ± 0.5 %.
FIG 2: Lepton mean PT sensitivity to the top mass form combined signal and background PT distributions,where b-tagged dilepton selection was applied. The sensitivity of the PT of the signal & background leptons to the top mass is 14 ± 0.4 %
We model the leptons' PT distribution with an analytical function to examine how this function depends on the top mass. We found that such a function can be the product of a Gamma times a Fermi function.
This function models successfully the shape of both the signal and the background PT distributions (Figures 3 and 4). It has two free parameters p, q, where p is related to the expected rate of leptons with the average PT and q can be interpreted as the expected average PT per lepton.
FIG 3: (left) Fit of the Gamma x Fermi function to the PT distribution of the signal MC leptons generated for Mtop = 175 GeV/c2
FIG 4: (right) Fit of the Gamma x Fermi function to the PT distribution of the total background
The top mass is measured by employing the likelihood minimization procedure. The Gamma x Fermi function is used as a probability density function (p.d.f).
Systematic uncertainties:
The total systematic uncertainty to the top mass is derived by adding in quadrature the partial errors. Table 1 summarizes the systematic uncertainties as well as the symmetrized (i.e ±(|negative|+positive)/2).
TABLE 1: Partial and total systematic uncertainty on the top mass. The second column lists the asymmetric errors and the third column the symmetrized ones, taken as the average of the asymmetric counterparts.
Results:
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
Additional Plots:
FIG 7: Signal sanity test with 100 PE, each corresponding to 2.8 fb-1.
FIG 8: Signal & background sanity test with 75 PE, each corresponding to 2.8 fb-1.
FIG 9: PT distribution of signal leptons generated for Mtop = 150 GeV (blue) and Mtop = 175 GeV (magenta)
Measured value:
Mtop = 154.6 ± 13.3(stat) ± 2.3(syst) GeV/c2
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