\(\newcommand{\ttbar}{\rm{t\bar{t}}}\) \(\newcommand{\DYtt }{\rm{DY\rightarrow\tau\tau}}\) \(\newcommand{\DYeemm }{\rm{DY\rightarrow ee/\mu\mu}}\) \(\newcommand{\gev}{\rm{GeV}}\) \(\newcommand{\pt}{p_{T}}\) \(\newcommand{\mt}{m_{T}}\) \(\newcommand{\Ht}{H_T}\) \(\newcommand{\MET}{\mbox{$E\kern-0.50em\raise0.10ex\hbox{/}_{T}$}}\) \(\newcommand{\met}{\mbox{$E\kern-0.50em\raise0.10ex\hbox{/}_{T}$}}\) \(\newcommand{\sys}{\text{syst.}}\) \(\newcommand{\syst}{\text{syst.}}\) \(\newcommand{\stat}{\text{stat.}}\) \(\newcommand{\et}{E_{\text{T}}}\) \(\newcommand{\Et}{E_{\text{T}}}\) \(\newcommand{\mll}{\rm{m_{ll}}}\) \(\newcommand{\mtt}{m_{\ttbar}}\) \(\newcommand{\afb}{A_{\text{FB}}}\) \(\newcommand{\AFB}{A_{\text{FB}}}\) \(\newcommand{\qeta}{q_{\ell}\eta_{\ell}}\) \(\newcommand{\deta}{\Delta\eta_{\ell}}\) \(\newcommand{\afblep}{A_{\text{FB}}^{\ell}}\) \(\newcommand{\afbdeta}{A_{\text{FB}}^{\ell\ell}}\) \(\newcommand{\afbtt}{A_{\text{FB}}^{\ttbar}}\) \(\newcommand{\fbm}{\rm{fb}^{-1}}\) \(\newcommand{\atanh}{a\cdot\tanh(\frac{1}{2}\qeta)}\) \(\newcommand{\atanhdeta}{a\cdot\tanh(\frac{1}{2}\deta)}\) \(\newcommand{\dy}{\Delta y_{t}}\) \(\newcommand{\pztt}{p_{z, \ttbar}}\) \(\newcommand{\pttt}{p_{T, \ttbar}}\) \(\newcommand{\Mtt}{m_{\ttbar}}\)

Measurement of the Forward-Backward Asymmetry of \(\ttbar\) Production in the Dilepton Final State

Ziqing Hong, Dave Toback, and Jon S. Wilson
Texas A&M University

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May 2015

Public note: CDF11161

We measure the forward-backward asymmetry of the top quark--antiquark pair events (\(\afbtt\)) in the dilepton final state with the full CDF run II data, corresponding to an integrated luminosity of \(9.1~\fbm\). The inclusive \(\afbtt\) is \(\afbtt = 0.12 \pm 0.11 (\stat) \pm 0.07 (\syst) = 0.12 \pm 0.13\). This result is consistent with the NNLO standard model expectation of \(\afbtt = 0.095 \pm 0.007\) and the CDF measurement in the lepton + jets final state of \(\afbtt = 0.164 \pm 0.047\). We also measure the \(\afbtt\) as a function of the rapidity difference of the top pairs (\(\dy\)). The results are \(\afbtt(|\dy|<0.5)=0.12\pm0.33(\stat)\pm0.20(\syst) = 0.12\pm0.39\) and \(\afbtt(|\dy|>0.5)=0.13\pm0.13(\stat)\pm0.11(\syst)=0.13\pm0.17\), which can be compared to the predictions from the Powheg Monte Carlo at 0.017 and 0.081, respectively. The slope of the \(\afbtt\) vs. \(|\dy|\) is estimated to be \(\alpha=0.14\pm0.15\), consistent with the NNLO SM prediction.


CutOpt_mlb.png CutOpt_jQ.png
CutOpt_jD.png CutOpt_dR.png
Fig. 1: Optimization point chosen based on expected statistical + background systematic uncertainty.

Top reconstruction performance
derived from Powheg MC

Fig. 2: Distribution of \(\dy\)(reconstructed)-\(\dy\)(generated).
Fig. 3: \(\dy\) smearing matrix.
Fig. 4: Efficiency of the four \(\dy\) bins, as a function of the parton-level \(\afbtt\).

Unfolding validation

Fig. 5: \(\afbtt\) measured vs. \(\afbtt\) generated with reweighted Powheg. All points are correlated. Good agreement.
Fig. 6: \(\afbtt\) measured vs. \(\afbtt\) generated with reweighted Powheg, LO SM and benchmark BSM samples. All the reweighted Powheg points are correlated.


Table 1: Expected number of events in data along with the observed number of events, passing all top dilepton event selections and top reconstruction quality selections.
gh_pTtt.png gh_pztt.png gh_mtt.png
Fig. 7: Distributions of \(\pttt\), \(\pztt\), and \(\Mtt\) from data compared with SM expectations. The agreement is good.
Fig. 8: \(\dy\) distribution from data compared with SM expectations.
Fig. 9: Posterior of \(\afbtt\) from data. A Gaussian fit is performed to extract the result.
Table 2: Table of uncertainties for the \(\afbtt\) measurement.
Fig. 10: A comparison of all top \(\afb\) results from the Tevatron with the NLO and NNLO SM predictions.

\(\afbtt\) vs. \(\dy\)

AFBMeasured_AFBGenerated_Ain_band.png AFBMeasured_AFBGenerated_Aout_band.png
Fig. 11: \(\afbtt\) measured vs. \(\afbtt\) generated in two \(\dy\) bins with reweighted Powheg.
DATA_P38_Ain.png DATA_P38_Aout.png
Fig. 12: Posterior probability distribution of \(\afbtt(|\dy|<0.5)\) and \(\afbtt(|\dy|>0.5)\). Gaussian fits are performed to extract the results.
Fig. 13: Two-dimensional posterior probability distribution of \(\afbtt(|\dy|>0.5)\) vs. \(\afbtt(|\dy|<0.5)\), with statistical uncertainties only. The correlation between the two observables (statistical uncertainty only) is estimated to be -0.44.
Table 3: Table of uncertainties for the \(\afbtt(|\dy|<0.5)\) and \(\afbtt(|\dy|>0.5)\) measurement.
Fig. 14: \(\afbtt\) vs. \(\dy\). The inner error bars represent the statistical uncertainties, and the whole error bars represent the total uncertainties. The data points are placed at the bin centroids predicted by the Powheg MC. The blue line with corresponding filled band represents a linear fit to the two data points with zero intercepts. The slope \(\alpha\) is estimated to be \(0.14\pm0.15\).

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