CMS-TOP-15-016

CMS-TOP-15-016 ; ATLAS-TOPQ-2016-16 ; CERN-EP-2017-236
Combination of inclusive and differential $\mathrm{t\bar{t}}$ charge asymmetry measurements using ATLAS and CMS data at $\sqrt{s} = $ 7 and 8 TeV
ATLAS and CMS Collaborations
15 September 2017
JHEP 04 (2018) 033
Abstract: This paper presents combinations of inclusive and differential measurements of the charge asymmetry ($A_{\mathrm{C}}$) in top quark pair ($\mathrm{t\bar{t}}$) events with a lepton+jets signature by the ATLAS and CMS Collaborations, using data from LHC proton-proton collisions at centre-of-mass energies of 7 and 8 TeV corresponding to integrated luminosities of about 5 and 20 fb$^{-1}$ for each experiment, respectively. The resulting combined LHC measurements of the inclusive charge asymmetry are $A_{\mathrm{C}}^{\mathrm{LHC7}} =$ 0.005 $\pm$ 0.007 (stat) $\pm$ 0.006 (syst) at 7 TeV and $A_{\mathrm{C}}^{\mathrm{LHC8}} =$ 0.0055 $\pm$ 0.0023 (stat) $\pm$ 0.0025 (syst) at 8 TeV. These values, as well as the combination of $A_{\mathrm{C}}$ measurements as a function of the invariant mass of the $\mathrm{t\bar{t}}$ system at 8 TeV, are consistent with the respective standard model predictions.
Links: e-print arXiv:1709.05327 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Summary of the single inclusive measurements and the LHC combination at $\sqrt {s} = $ 7 TeV compared to the theoretical prediction calculated at NLO precision in the strong coupling constant (including NLO electroweak corrections). The inner bars indicate the statistical uncertainty, while the outer bars indicate the total uncertainty. The uncertainty in the theoretical prediction is dominated by uncertainties due to scale variations.
png pdf	Figure 2: Summary of the single inclusive measurements and the LHC combination at $\sqrt {s} = $ 8 TeV compared to theoretical predictions at NLO and NNLO precision in the strong coupling constant (including NLO electroweak corrections). The inner bars indicate the statistical uncertainty, while the outer bars indicate the total uncertainty. The uncertainty in the theoretical prediction is dominated by uncertainties due to scale variations.
png pdf	Figure 3: Measured inclusive charge asymmetries $A_{\mathrm {C}}$ at the LHC at $\sqrt {s} = $ 8 TeV (horizontal line) versus forward-backward asymmetries $A_{\mathrm {FB}}$ (vertical lines) at the Tevatron [1,2], compared with the SM prediction at QCD NNLO (+EW NLO) [19,20,21] and predictions incorporating various potential BSM contributions [62,63]: a $\mathrm{W'} $ boson, a heavy axigluon ($G_{\mu}$), a scalar isodoublet ($\phi $), a colour triplet scalar ($\omega ^{4}$), and a colour sextet scalar ($\Omega ^{4}$).
png pdf	Figure 4: Charge asymmetry in six bins of the invariant mass of the ${\mathrm{t} {}\mathrm{\bar{t}}} $ system as measured in the ATLAS and CMS analyses and the combined results. The last bin includes the overflow. The gray band indicates the uncertainty in the combined result.
png pdf	Figure 5: The combined ATLAS+CMS charge asymmetry in six bins of the invariant mass of the ${\mathrm{t} {}\mathrm{\bar{t}}} $ system in comparison with theoretical predictions for the SM [18,19,20,21] and two versions of a colour-octet model [64]. The last bin includes the overflow. The uncertainties, indicated by the shaded areas, reported for the SM predictions are dominated by scale variations and are small, while the uncertainties reported for the colour-octet model are statistical uncertainties in the simulation.

Tables
png pdf	Table 1: Overview of the most recent theoretical predictions for the inclusive $A_{\mathrm {C}}$ at the LHC at $\sqrt {s} = $ 7 and 8 TeV, along with the experimental results from the ATLAS and CMS Collaborations. The uncertainties in all theoretical predictions are dominated by the uncertainties due to scale variations. Wherever available, the uncertainties in the experimental results are given separately as statistical (first contribution) and systematic (second contribution) uncertainties, otherwise the total uncertainty is quoted. The ATLAS and CMS experimental results are described in Section 2.
png pdf	Table 2: Uncertainties in the input measurements, assumed correlations $\rho $ between the uncertainties, and the resulting values for the uncertainties in the inclusive combination at $\sqrt {s} = $ 7 TeV. Systematic uncertainties smaller than 0.001 are ignored.
png pdf	Table 3: Uncertainties in the input measurements, assumed correlations $\rho $ between the uncertainties, and the resulting values for the uncertainties in the inclusive combination at $\sqrt {s} = $ 8 TeV. The breakdown of the systematic uncertainties of the ATLAS analysis (for the systematic uncertainties that are marginalised) corresponds to the expected uncertainties after marginalisation.
png pdf	Table 4: ATLAS and CMS charge asymmetry results at 8 TeV in six bins of $m_{{\mathrm{t} {}\mathrm{\bar{t}}}}$ and the combined values along with statistical and systematic uncertainties. In addition the predictions from QCD calculations at NLO [18] and NNLO [19,20,21] are given.
png pdf	Table 5: Correlation matrix for the six correlated bins of the combined result. "L$i$'' corresponds to bin $i$ of the LHC combination.

Summary

Combinations of ATLAS and CMS measurements of the $\mathrm{t\bar{t}}$ charge asymmetry in lepton+jets final states are reported using proton-proton collision data collected at the LHC at centre-of-mass energies of 7 and 8 TeV and corresponding to integrated luminosities for each experiment of up to 5 and 20 fb$^{-1}$ , respectively. Inclusive charge asymmetry results are combined at 7 and 8 TeV, and differential measurements of the charge asymmetry as a function of the invariant mass of the $\mathrm{t\bar{t}}$ system at 8 TeV are combined. Detailed studies of the correlations between the different measurements and systematic uncertainties have been performed. The precision of the resulting combinations is significantly improved with respect to the corresponding individual measurements. The individual results and the combinations are in agreement with standard model calculations at next-to-leading-order and next-to-next-to-leading-order precision and also compatible with zero asymmetry. They uniquely restrict the phase space of possible new physics phenomena which would produce asymmetries larger than the standard model ones.

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