Measurement of the charge asymmetry in top-quark pair production in proton-proton collisions at sqrt(s) = 7 TeV

TL;DR

This study measures the ttbar charge asymmetry in proton-proton collisions at $\sqrt{s}=7$ TeV using the CMS detector, focusing on the observables $\Delta|\eta|$ and $\Delta y^{2}$. Events in the lepton+jets channel are fully reconstructed and corrected for detector effects via a regularized unfolding approach, with careful background estimation and systematic uncertainty evaluation. The final results, $A_C^{\eta} = -0.017 \pm 0.032\,(stat.)^{+0.025}_{-0.036}\,(syst.)$ and $A_C^{y} = -0.013 \pm 0.028\,(stat.)^{+0.029}_{-0.031}\,(syst.)$, are consistent with SM predictions at NLO and show no significant dependence on the ttbar invariant mass. The analysis demonstrates a robust methodology for extracting small asymmetries in a gluon-dominated environment and provides important constraints on BSM scenarios that could enhance the asymmetry.

Abstract

The difference in angular distributions between top quarks and antiquarks, commonly referred to as the charge asymmetry, is measured in pp collisions at the LHC with the CMS experiment. The data sample corresponds to an integrated luminosity of 1.09 inverse femtobarns at a centre-of-mass energy of 7 TeV. Top-quark pairs are selected in the final state with an electron or muon and four or more jets. At least one jet is identified as originating from b-quark hadronization. The charge asymmetry is measured in two variables, one based on the pseudorapidities (eta) of the top quarks and the other on their rapidities (y). The results A[C,eta] = -0.017 +/- 0.032 (stat.) + [+0.025/-0.036] (syst.) and A[C,y] = -0.013 +/- 0.028 (stat.) + [+0.029/-0.031] (syst.) are consistent within uncertainties with the standard-model predictions.

Figures (5)

• Figure 1: Comparison of the combined $\text{lepton}{+}\text{jets}$ data with simulated contributions for the distributions in $E_{\mathrm{T}}^{\text{miss}}\xspace$ (left) and M3 (right). The last bins include the sum of all contributions for $E_{\mathrm{T}}^{\text{miss}}\xspace > 200\,\text{Ge\spaceV}\xspace$ and $\mathrm{M3} > 800{\,\text{Ge\spaceV\space/\space}c^\text{2}}\xspace$, respectively. The simulated signal and background contributions are normalized to the results of the fits in Table \ref{['tab:FitResults']}.
• Figure 2: Reconstructed $\Delta\!\left|\eta\right|$ (left) and $\Delta y^{2}$ (right) distributions for the combined $\text{lepton}{+}\text{jets}$ channel. The last bins include the sum of all contributions for $\left|\Delta\!\left|\eta\right|\right| > 4.0$ and $\left|\Delta y^{2}\right| > 4.0$, respectively. The signal and background contributions are normalized to the results in Table \ref{['tab:FitResults']}.
• Figure 3: (left) Selection efficiency as a function of generated $\Delta\!\left|\eta\right|$, defined with respect to inclusive ${t}\overline{{t}}$ production. (right) Migration matrix between the true (generated) and the reconstructed values in $\Delta\!\left|\eta\right|$, after the event selection.
• Figure 4: Unfolded $\Delta\!\left|\eta\right|$ (left) and $\Delta y^{2}$ (right) normalized spectra. The NLO prediction is based on the calculations of Ref. Kuhn:2011ri. The last bins include the sum of all contributions for $\left|\Delta\!\left|\eta\right|\right| > 4.0$ and $\left|\Delta y^{2}\right| > 4.0$, respectively. The uncertainties shown on the data are statistical, while the uncertainties on the prediction account also for the dependence on the top-quark mass, PDF, and factorization and renormalization scales.
• Figure 5: Background-subtracted asymmetries for $\Delta\!\left|\eta\right|$ (left) and $\Delta y^{2}$ (right) as functions of the reconstructed ${t}\overline{{t}}$ invariant mass.