ALL HADRONIC TOP MASS USING THE IDEOGRAM TECHNIQUE

Petteri Meht�l�, Timo Aaltonen, Nick van Remortel
Helsinki Institute of Physics and Department of Physical Sciences,
P.O. Box 64, FIN-00014 University of Helsinki, Finland

mehtala@mappi.helsinki.fi
taaalton@mappi.helsinki.fi
remortel@fnal.gov

Using 310 pb-1 of CDF data collected between 2002-2004, we have measured the mass of the top quark in the all hadronic decay channel of tt pairs. This channel is characterized by a 6 jet final state topology with little missing energy in the event. The reconstruction of the top quark mass in this channel is challenging for two main reasons: the large jet ambiguities to reconstruct two top quarks out of 6 jets and a large amount of QCD multi-jet background of which the cross sections are badly known.

The Ideogram method tackles this problem by combining both template and matrix element approaches to extract the maximum amount of mass information out of a tt candidate event. This is realized by construction a two-dimensional event-by-event likelihood in which the signal purity (or in other words, signal cross section) is left as a free parameter, in addition to the top quark mass itself. The event likelihood takes into account all 90 possible independent jet combinations. Its signal part is purely data driven and consists of the two dimensional convolution of natural Breit-Wigners with Gaussian resolution functions, using fitted masses and their variances obtained from a 2-C kinematical fit. The QCD multi-jet background is parametrized by a 2D mass template. Each of the 90 jet configurations is weighted by it's goodness of fit and a jet probability weight, giving the probability that exactly two out of the 6 final state jets correspond to a b-quark.

The current measured top quark mass in the all hadronic channel amounts to M_top = 177.1 ± 4.9 (stat.) ± 4.7 (syst.) GeV/c². The statistical uncertainty is expected to scale with 1/sqrt(int lum.). The dominant source for the systematic uncertainties are the Jet energy scale corrections, the background shape and the background fraction. The latter two are correlated with the JES and taken into account in the calculation of the total systematic uncertainty of this measurement.

Public note:

< CDF/PHYS/TOP/PUBLIC/8233> (April 26 2006)

Blessed Result (310 pb^-1)

**M_top = 177.1 ± 4.9 (stat.) ± 4.7 (syst.) GeV/c²**

Blessed Tables and Plots

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Description of Plots	gif	eps
Most probable top mass value for the signal term of the total likelihood.		eps
Final Calibrated 1-D top mass likelihood.		eps
Final Calibrated 1-D top mass likelihood zoomed.		eps
2-D top mass likelihood (mass calibrated only).		eps
Most likely sample purity value for each test mass (mass-purity contour followed in 2D purity-mass likelihood plane).		eps
Expected stat. uncertainty for Mtop=178 GeV.		eps
Expected stat. uncertainty for various integrated luminosities.		eps
Calibration (read:sensitivity) curves for various background fractions. The green line corresponds to our current working point.		eps
Calibration curve slope as function of background fraction. The black line indicates our current working point.		eps
Calibration curve offset (bias at Mt=175 GeV) as function of background fraction. The black line indicates our current working point.		eps
The width of the pull distribution (before correction) for various input top masses.		eps
Calibration curve for the sample purity at Mt=178 GeV		eps
Calibrated purity for MC Pseudo experiments including background. The yellow band is the purity fitted from real data and its 1 sigma uncertainty.		eps
Efficiency to match 6 final state jets with 6 generator level partons, using cone dR=0.4.		eps
2D mass template for background events (used in likelihood).		eps
2D mass template for non-matched events (used in likelihood).		eps
2D mass template for correct jet combination of signal MC events with Mt=178 GeV. (Not used in likelihood, for illustrational purposes only).		eps
2D mass template for incorrect jet combination of signal MC events with Mt=178 GeV. (Not used in likelihood, for illustrational purposes only).		eps
First fitted mass, obtained from kinematical fit.		eps
Second fitted mass, obtained from kinematical fit.		eps
First fitted mass error, obtained from kinematical fit.		eps
Second fitted mass error, obtained from kinematical fit.		eps
Chisquared obatined from kinematical fit.		eps
Jet probability weight.		eps
Table of systematic uncertainties. The correlation between the JES , background shape and background fraction systematics is already taken into account.		eps