Top mass in DIL using the leptons' P_T at 1.8fb^-1

Authors:

V. Giakoumopoulou, N. Giokaris, A. Manousakis-Katsikakis, C. Vellidis
University of Athens, Greece
J. Budagov, G. Chlachidze, V. Glagolev, A. Sissakian
Joint Institute for Nuclear Research, Dubna, Russia

Abstract:

A new measurement of the top quark mass using dilepton events is reported, using the leptons' Pt information. The data were collected by the CDF II Detector. The preliminary result in a dataset corresponding to an integrated luminosity of 1.8 fb^-1 is:

M_top=156±20(stat)±4.6(syst) GeV/c² (Likelihood method) and 

M_top=149±21(stat)±5(syst) GeV/c² (Straight Line method) 

where the first uncertainty is statistical and the second one systematic

Method:

The analysis has been based upon the observation that the leptons' Pt is linearly dependent to the top mass: Pt=l*Mtop+k. The top mass is estimated with two independent and compatible methods.

1. Likelihood method (LH): The full shape information of the leptons' Pt distribution is used. This information is taken from the parameterization of the spectra with the GammaxFermi function. The top mass is estimated using the modeling function for signal and background as p.d.fs and following the likelihood minimization procedure

2. Straight Line Method (SL): The leptons' mean Pt of the combined signal+background distribution is used. The top quark mass is estimated using the formula of the linear dependence of the top mass vs the leptons' Pt.

More information about the methodology of the analysis is available in cdfnote 8959

Results:

The dilepton selection on the 1.8fb^-1 data gives 125 dilepton events. The 250 leptons of this dataset have a mean P_T=52.96±1.97 GeV. Figure 1 illustrates the mean P_T of the data in comparison with the Standard Model signal+background expectation.

Figure1:Lepton Pt distribution of the 1.8fb^-1 data (blue points). The MC top signal is for a top mass of 156 GeV/c²

Figure 2 illustrates the fit to  the data (blue line). The red line is the fit to the signal and the black line the fit to the background.

Figure 2: Fit to the 1.8fb^-1 data

The Likelihood (LH) method gives M_top=156±20(stat)±4.6(syst) GeV/c²  

The Straigh Line (SL) method estimates that M_top=149±21(stat)±5(syst) GeV/c²

Figures 3,4 and 5 illustrate the mean P_T of the ee, mm and em data in comparison with the Standard Model signal+background expectation.

Figure3: Lepton Pt distribution of the 1.8fb^-1 ee data (blue points). The MC top signal is for a top mass of 156 GeV/c²

Figure4: Lepton Pt distribution of the 1.8fb^-1 mm data (blue points). The MC top signal is for a top mass of 156 GeV/c²

Figure5: Lepton Pt distribution of the 1.8fb^-1 em data (blue points). The MC top signal is for a top mass of 156 GeV/c²

Figures 6, 7 and 8 illustrate the fit to  the ee, mm and em data (blue line) respectively. In each case the red line is the fit to the signal and the black line the fit to the background.

Figure 6: Fit to the 1.8fb^-1  ee data

Figure 6: Fit to the 1.8fb^-1  mm data

Figure 6: Fit to the 1.8fb^-1  em data

Table I presents the mass estimation for the ee, mm and em data events.

Table I: Top mass measurement for the 1.8fb^-1 ee, mm and em data.

Table II includes the eps files for the plots related to the analysis: