Measurement of the Charge Asymmetry in Top Pair Production  

Measurement of the Charge Asymmetry in Top Pair Production with 1.9fb-1 

Public Page 

Thorsten Chwalek, Dominic Hirschbühl, Thomas Müller, Thomas Peiffer,

Jeannine Wagner-Kuhr, Wolfgang Wagner, Julia Weinelt

CDF public note: ps-file, pdf-file

summary of analysis: pdf-file , ppt-file , office-file


We present a measurement of the charge asymmetry in top pair production using an integrated luminosity of 1.9/fb collected during the years 2003 to 2007 with the CDF detector. Top pair candidates with a signature of a charged lepton, missing transverse momentum and at least four jets are selected. In order to measure the charge asymmetry the rapidity difference between the semileptonically decaying and the hadronically decaying top quark multiplied with the charge of the charged lepton is used. A measurement of the inclusive asymmetry, yielding A=0.24±0.13±0.04, as well as of the asymmetry in events with exactly 4 or at least 5 reconstructed jets is performed. The measured asymmetries are higher than the asymmetries predicted by the next-to-leading order Monte Carlo generator MC@NLO, but within the uncertainties the maesured values are consistent with the NLO predictions.

Final Results for 1.9 fb-1:


   Inclusive asymmetry (corrected) : A = 0.24 ± 0.13 (stat.) ± 0.04 (syst.)

   At least 4 jets (bkg subtracted): A = 0.119 ± 0.064 (stat.)
   Exact 4 jets (bkg subtracted)   : A = 0.132 ± 0.075 (stat.)
   At least 5 jets (bkg subtracted): A = 0.079 ± 0.123 (stat.)

Introduction into Top Charge Asymmetry

LO qqbar->ttbar Feynman diagram, (EPS)

Box diagram,

Initial state radiation (ISR) diagram, (EPS)

Final state radiation (FSR) diagram, (EPS)
Interference between LO and box diagram leads to a positive asymmetry value
Interference between ISR and FSR diagram leads to a negative asymmetry value

Sensitive Variable

Definition of the sensitive varaible, (EPS)

(Δ y*Ql) distribution on generator level.
No cuts are applied. (EPS)

Dependence on Jets

Asymmetry as a function of the number of reconstructed jets, (EPS)

Impact of jet selections cut on the visible asymmetry, (EPS)

Modeling of the Background:

To check the modeling of the background a sample with with a b-tag veto is used, (EPS).

Correction of Asymmetry

Relative efficiencies, (EPS)

Smearing matrix, (EPS)

(Δ y*Ql) residual distribution, (EPS)

Corrected (Δ y*Ql) distribution, (EPS)

Check of correction method: reweighted (Δ y*Ql)gen distribution, (EPS)

Result plots:

Reconstructed (Δ y*Ql) distribution for Njets>=4, (EPS)

Reconstructed (Δ y*Ql) for Njets=4, (EPS)

Reconstructed (Δ y*Ql) distribution for Njets>=5, (EPS)

Background subtracted asymmetry as a function of Njets, (EPS)


Systematic uncertainties of the corrected asymmetry value (inclusive analysis), (EPS)

Asymmetries of the backgrounds in the tagged sample (inclusive analysis), (EPS)

Check of the method: result of the linear reweighting, (EPS)

Check of the method: result of the more exotic reweighting for A=20% and A=30%, (EPS)

Check of method using Monte Carlos with different initial asymmetries, (EPS)

Last modified: Mon May 5 06:10:35 CDT 2008 by J. Wagner-Kuhr