Measurement of the tt production cross section in the fully hadronic decay channel in pp collisions at 7 TeV

Abstract

This note presents a first measurement of the top quark pair production cross section in the fully hadronic decay channel at a center-of-mass energy of 7 TeV using data corresponding to an integrated luminosity of 1.09 fb-1 taken with the CMS detector. The cross section is determined from an unbinned maximum likelihood fit to the reconstructed top quark mass. The reconstruction of ttbar candidates is performed after a cut-based event selection using a kinematic fit. A data-driven technique is used to estimate the dominant background from QCD multi-jet production. The cross section measurement yields 136 20 (stat.) 40 (sys.) 8 (lumi.) pb. This result is consistent with measurements in other decay channels and with the Standard Model prediction.

Main Document

Detailed record in CDS, and the full text in PDF.

Approved Results from TOP-11-007 ( click on plot to get .pdf )

Main result Abbreviated Caption
Result Measured tt cross section in the fully hadronic decay channel. PNG

Cross check Abbreviated Caption
CrossCheck Measured tt cross section in the fully hadronic decay channel. PNG

Approved Plots from TOP-11-007 ( click on plot to get .pdf )

Figures main analysis Abbreviated Caption
Figure 1 Figure 1: Result of the fit of the reconstructed top quark mass for the tt simulation (solid red line) and the multi-jet QCD estimated from data (dashed blue line). The uncertainty stated on the signal fraction fsig is only statistical. PNG
Figure 2 Figure 2: Ratio R(pT,abs(η)) of b-tagged jets in events with at least one b-tagged jet and all non b-tagged jets as function of pT and abs(η). PNG
Figure 3a Figure 3a: Validation of the background estimation method using simulated QCD multi-jet events: comparison of the top mass templates obtained directly from events in the signal region (black circles) and from weighted events in the control region (red triangles). The error bands indicate the statistical uncertainties. PNG
Figure 3b Figure 3b: Comparison of the background estimates derived from data and derived from simulation. Gamma distributions are fitted to these estimates. PNG

Figures cross check analysis Abbreviated Caption
Figure 4a Figure 4a: Neural network output for simulated tt events and multi-jet events from data shown normalized to unity. PNG
Figure 4b Figure 4b: Reconstructed top quark mass mtop for b-tagged jet combinations observed in data (circles) passing the full event selection. For comparison, the expected background (dashed blue line) and tt signal (solid red line) are shown, both normalized to the yields from the fit. PNG

Tables from TOP-11-007 ( click on plot to get .pdf )

Tables main analysis Abbreviated Caption
Table 1 Table 1: Number of events and the expected signal fraction in the data sample after each selection step. The expected signal fraction is taken from the simulation, assuming a cross section of 163 pb. PNG
Table 2 Table 2: List of all uncertainties and their values considered for this measurement. PNG

-- EikeSchlieckau - 8-Aug-2011

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Topic revision: r12 - 2011-08-08 - FrankPeterSchilling
 
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