Measurement of the Forward Backward Asymmetry in Top Pair Production

Tom Schwarz1, Robin Erbacher1
Dante Amidei2

1University of California - Davis
2University of Michigan


Arxiv Paper (hep-ex/0806.2472)
Public Conference Note
Measurement in 695 pb-1 (FERMILAB-THESIS-2006-51)


Seminar Talk
One Slide Description

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A method of reconstructing t-tbar events in the lepton plus jets mode is applied to a measurement of the forward backward asymmetry in top pair production at CDF in the proton-antiproton rest frame. The measurement is a test of discrete symmetries of the strong interaction at high energy . In the present data set it is potentially sensitive to the the presence of parity-violating production channels such as a massive Z-like boson or new physics within Strong Interactions. Larger data sets will have sensitivity for an interesting charge asymmetry arising from pure QCD calculated at next-to-leading order. We measure the top quark forward-backward asymmetry in 1.9 fb-1 of CDF collision data, using 484 candidate t-tbar lepton plus jets events in the high Pt lepton trigger streams. We measure Afb = 0.17 +/- (0.07)stat +/- (0.04)syst which can be directly compared to the Standard Model prediction Afb = 0.04 +/- 0.01.

Synopsis of Method

  1. The t-tbar lepton plus jets events are selected from the high Pt lepton triggers (CEM/CMUP/CMX)

  2. The top and anti-top kinematics ( momentum, angles ) are reconstructed from the decay products using the kinematic fitter, a chi-squared minimization algorithm

  3. With the t-tbar kinematics in hand, the production angle of hadronically decaying top quark is reproduced. The production angle is defined as the angle between the top direction and the beamline in the laboratory rest frame.

  4. The predicted background shape is subtracted from the reconstructed production angle distribution to produce a shape which represents only t-tbar events

  5. An unfolding technique, performed via matrix inversion, is applied to correct for any bias or dilution from the detector, acceptance, or reconstruction

    where the corrected histogram is formed by unsmearing the effects of reconstruction (S) and correcting for the bias due to acceptance (A) of the background subtracted data.

  6. The Forward-Backward Asymmetry is calculated from the corrected production angle distribution.

    where Nf (Nb) represents the number of events where the production angle is greater(less) then zero. The measurement is statistically dominated, but the largest systematic uncertainty is the background shape and normalizations

Blessed Result

The measured top quark forward backward asymmetry in 1.9 fb-1 of CDF collected data is:

Afb = 0.17 +/- (0.07)stat +/- (0.04)syst

The value is directly comparable to the theoretical value, Afb = 0.04 +/- 0.01 [2].


Reconstructed rapidity distribution of the top quark production angle. The corrected result of the measurement is shown on the plot. Predicted backgrounds and ttbar signal (MC@NLO) are shown along with data for 1.9 fb-1. If you need just one plot to represent this measurement, this is probably it.
NOTE: Notice that the x-axis is labeled -Ql y. The reconstructed "hadronic" decaying top is always used to measure the production angle because it is more accurately reproduced. Because the hadronic side is sometimes top and sometimes anti-top you have to multiply by the sign of the top charge ( the opposite of the lepton ) to measure the angle with respect to the correct axis direction. We assume CP is conserved in the strong interaction. Please email me if you're confused by this.

Reconstructed production angle distribution of the top quark in Cos Theta. The corrected result of the measurement is shown on the plot. Predicted backgrounds and ttbar signal (MC@NLO) are shown along with data for 1.9 fb-1. This would be considered an alternative distribution to the rapidity distribution. Cos Theta is the variable used in correcting the backgrounds, matrices, etc. before the asymmetry is calculated.

Reconstructed production angle distribution of the top quark in Cos Theta where the Monte Carlo signal t-tbar sample has been reweighted to have a "true" asymmetry the same as measured (17%). This is a rough template method used as a x-check on the unfolding technique. Notice that the reconstructed asymmetry in data now agrees very well with the model prediction. A KS test before and after reweighting is shown also on the plot. After reweighting to 17% the predicted shape and data agree at KS = 45% compared to KS = 0.4%.

Reconstructed production angle distribution of the top quark in Cos Theta where the predicted t-tbar signal is now normalized to 6.7 pb xsection. The raw reconstructed asymmetry is shown, as opposed to the corrected asymmetry, along with the monte carlo prediction for the reconstructed asymmetry in backgrounds and t-tbar signal (MC@NLO).

Predicted Reconstructed production angle distribution for individual backgrounds. Each considered background and the corresponding asymmetry is shown, normalized to the method II background prediction for 1.9 fb-1. The asymmetries are the "reconstructed" asymmetries coming directly from the kinematic fitter.

Because the measurement method uses a simple form of unfolding it is fairly invariant to a number of different "true" production angle distributions for the top quark production angle. The method was tested against several exotic possibilities by reweighting t-tbar Monte Carlo samples and shown to get the correct asymmetry within less then a 1%. The plot here is an example of a few distributions tested. Each distribution has an asymmetry of 20%.

The anti-tag sample, which is very background dominated, is used as a check on the shape and asymmetry of the background model. This plot is the anti-tag sample for >= 4 Tight Jets. The shape and asymmetry for data and model are in good agreement.


Expected background contented in the signal region for 1.9 fb-1 as predicted by Method II For You.
Systematic uncertainties on the final measured asymmetry.

  1. T. A. Schwarz, Measurement of the Front Back Asymmetry in Top-Antitop Quark Pairs Produced in Proton-Antiproton Col lisions at Center of Mass Energy = 1.96 TeV, PhD thesis, FERMILAB-THESIS-2006-51 (Original Thesis Measurement)
  2. J.H. Kuhn and G. Rodrigo, Phys. Rev. Lett. 81, 49 (1998)
    J.H. Kuhn and G. Rodrigo, Phys. Rev. D 59, 054017(1999)

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