CMSTOP21014 ; CERNEP2022155  
Measurement of the $\mathrm{t\bar{t}}$ charge asymmetry in events with highly Lorentzboosted top quarks in pp collisions at $\sqrt{s} = $ 13 TeV  
CMS Collaboration  
4 August 2022  
Phys. Lett. B 846 (2023) 137703  
Abstract: The measurement of the charge asymmetry in top quark pair events with highly Lorentzboosted top quarks decaying to a single lepton and jets is presented. The analysis is performed using protonproton collisions at $ \sqrt{s}= $ 13 TeV with the CMS detector at the LHC and corresponding to an integrated luminosity of 138 fb$ ^{1} $. The selection is optimized for top quarks produced with large Lorentz boosts, resulting in nonisolated leptons and overlapping jets. The top quark charge asymmetry is measured for events with a $\mathrm{t\bar{t}}$ invariant mass larger than 750 GeV and corrected for detector and acceptance effects using a binned maximum likelihood fit. The measured top quark charge asymmetry of (0.42$_{ 0.69}^{+ 0.64}$)% is in good agreement with the standard model prediction at nexttonexttoleading order in quantum chromodynamic perturbation theory with nexttoleadingorder electroweak corrections. The result is also presented for two invariant mass ranges, 750900 and $ {>} $900 GeV.  
Links: eprint arXiv:2208.02751 [hepex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ; 
Figures  
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Figure 1:
Comparison between data and MC simulation for kinematic distributions based on events in the signal candidate sample (described in Section 6): $\Delta y$ (upper left), reconstructed $ {M_{{\mathrm{t} \mathrm{\bar{t}}}}} $ (upper right), distance between the lepton and the closest AK4 jet $ {\Delta R_{\text {min}}(\ell, j)} $ (lower left), and the number of AK4 jets (lower right). The vertical bars on the points show the statistical uncertainty in the data. The shaded bands represent the total uncertainty in the MC predictions (described in Section 5). The lower panels give the ratio of the data to the sum of the MC predictions. 
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Figure 1a:
Comparison between data and MC simulation for kinematic distributions based on events in the signal candidate sample (described in Section 6): $\Delta y$ (upper left), reconstructed $ {M_{{\mathrm{t} \mathrm{\bar{t}}}}} $ (upper right), distance between the lepton and the closest AK4 jet $ {\Delta R_{\text {min}}(\ell, j)} $ (lower left), and the number of AK4 jets (lower right). The vertical bars on the points show the statistical uncertainty in the data. The shaded bands represent the total uncertainty in the MC predictions (described in Section 5). The lower panels give the ratio of the data to the sum of the MC predictions. 
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Figure 1b:
Comparison between data and MC simulation for kinematic distributions based on events in the signal candidate sample (described in Section 6): $\Delta y$ (upper left), reconstructed $ {M_{{\mathrm{t} \mathrm{\bar{t}}}}} $ (upper right), distance between the lepton and the closest AK4 jet $ {\Delta R_{\text {min}}(\ell, j)} $ (lower left), and the number of AK4 jets (lower right). The vertical bars on the points show the statistical uncertainty in the data. The shaded bands represent the total uncertainty in the MC predictions (described in Section 5). The lower panels give the ratio of the data to the sum of the MC predictions. 
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Figure 1c:
Comparison between data and MC simulation for kinematic distributions based on events in the signal candidate sample (described in Section 6): $\Delta y$ (upper left), reconstructed $ {M_{{\mathrm{t} \mathrm{\bar{t}}}}} $ (upper right), distance between the lepton and the closest AK4 jet $ {\Delta R_{\text {min}}(\ell, j)} $ (lower left), and the number of AK4 jets (lower right). The vertical bars on the points show the statistical uncertainty in the data. The shaded bands represent the total uncertainty in the MC predictions (described in Section 5). The lower panels give the ratio of the data to the sum of the MC predictions. 
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Figure 1d:
Comparison between data and MC simulation for kinematic distributions based on events in the signal candidate sample (described in Section 6): $\Delta y$ (upper left), reconstructed $ {M_{{\mathrm{t} \mathrm{\bar{t}}}}} $ (upper right), distance between the lepton and the closest AK4 jet $ {\Delta R_{\text {min}}(\ell, j)} $ (lower left), and the number of AK4 jets (lower right). The vertical bars on the points show the statistical uncertainty in the data. The shaded bands represent the total uncertainty in the MC predictions (described in Section 5). The lower panels give the ratio of the data to the sum of the MC predictions. 
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Figure 2:
Comparison between data and MC simulation for $ {\Delta y} $ for each of the 12 analysis channels, both before (left) and after (right) the likelihood normalization. The plots in the upper row correspond to 750 $ < {M_{{\mathrm{t} \mathrm{\bar{t}}}}} < $ 900 GeV, and the plots in the lower row to $ {M_{{\mathrm{t} \mathrm{\bar{t}}}}} > $ 900 GeV. The vertical bars on the points represent the statistical uncertainties in the data and the shaded bands give the combined MC statistical and systematic uncertainties. The lower panels display the ratio of the data yields to the sum of the MC predictions. 
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Figure 2a:
Comparison between data and MC simulation for $ {\Delta y} $ for each of the 12 analysis channels, both before (left) and after (right) the likelihood normalization. The plots in the upper row correspond to 750 $ < {M_{{\mathrm{t} \mathrm{\bar{t}}}}} < $ 900 GeV, and the plots in the lower row to $ {M_{{\mathrm{t} \mathrm{\bar{t}}}}} > $ 900 GeV. The vertical bars on the points represent the statistical uncertainties in the data and the shaded bands give the combined MC statistical and systematic uncertainties. The lower panels display the ratio of the data yields to the sum of the MC predictions. 
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Figure 2b:
Comparison between data and MC simulation for $ {\Delta y} $ for each of the 12 analysis channels, both before (left) and after (right) the likelihood normalization. The plots in the upper row correspond to 750 $ < {M_{{\mathrm{t} \mathrm{\bar{t}}}}} < $ 900 GeV, and the plots in the lower row to $ {M_{{\mathrm{t} \mathrm{\bar{t}}}}} > $ 900 GeV. The vertical bars on the points represent the statistical uncertainties in the data and the shaded bands give the combined MC statistical and systematic uncertainties. The lower panels display the ratio of the data yields to the sum of the MC predictions. 
png pdf 
Figure 2c:
Comparison between data and MC simulation for $ {\Delta y} $ for each of the 12 analysis channels, both before (left) and after (right) the likelihood normalization. The plots in the upper row correspond to 750 $ < {M_{{\mathrm{t} \mathrm{\bar{t}}}}} < $ 900 GeV, and the plots in the lower row to $ {M_{{\mathrm{t} \mathrm{\bar{t}}}}} > $ 900 GeV. The vertical bars on the points represent the statistical uncertainties in the data and the shaded bands give the combined MC statistical and systematic uncertainties. The lower panels display the ratio of the data yields to the sum of the MC predictions. 
png pdf 
Figure 2d:
Comparison between data and MC simulation for $ {\Delta y} $ for each of the 12 analysis channels, both before (left) and after (right) the likelihood normalization. The plots in the upper row correspond to 750 $ < {M_{{\mathrm{t} \mathrm{\bar{t}}}}} < $ 900 GeV, and the plots in the lower row to $ {M_{{\mathrm{t} \mathrm{\bar{t}}}}} > $ 900 GeV. The vertical bars on the points represent the statistical uncertainties in the data and the shaded bands give the combined MC statistical and systematic uncertainties. The lower panels display the ratio of the data yields to the sum of the MC predictions. 
png pdf 
Figure 3:
Measured ${{A_{\text {C}}} ^{\text {fid}}}$ (left) and measured ${A_{\text {C}}}$ in the full phase space (right) presented in different mass regions after combining the $ \mu $+jets and e+jets channels. The vertical bars represent the total uncertainties, with the inner tick mark indicating the statistical uncertainty in the observed data. The measured values are compared to the theoretical prediction, including NNLO QCD and NLO EW corrections from Ref. [4]. The theoretical prediction in the fiducial region is obtained by fitting Asimov data that passed the signal candidate selection described in Sections 3 and 4. 
png pdf 
Figure 3a:
Measured ${{A_{\text {C}}} ^{\text {fid}}}$ (left) and measured ${A_{\text {C}}}$ in the full phase space (right) presented in different mass regions after combining the $ \mu $+jets and e+jets channels. The vertical bars represent the total uncertainties, with the inner tick mark indicating the statistical uncertainty in the observed data. The measured values are compared to the theoretical prediction, including NNLO QCD and NLO EW corrections from Ref. [4]. The theoretical prediction in the fiducial region is obtained by fitting Asimov data that passed the signal candidate selection described in Sections 3 and 4. 
png pdf 
Figure 3b:
Measured ${{A_{\text {C}}} ^{\text {fid}}}$ (left) and measured ${A_{\text {C}}}$ in the full phase space (right) presented in different mass regions after combining the $ \mu $+jets and e+jets channels. The vertical bars represent the total uncertainties, with the inner tick mark indicating the statistical uncertainty in the observed data. The measured values are compared to the theoretical prediction, including NNLO QCD and NLO EW corrections from Ref. [4]. The theoretical prediction in the fiducial region is obtained by fitting Asimov data that passed the signal candidate selection described in Sections 3 and 4. 
png pdf 
Figure 4:
The $ \pm 1 $ standard deviation ($\sigma $) impacts of the nuisance parameters corresponding to the systematic uncertainties in the full phase space $ {A_{\text {C}}} $ measurement for $ {M_{{\mathrm{t} \mathrm{\bar{t}}}}} > $ 750 GeV. The red and blue bars show the effect on the unfolded $ {A_{\text {C}}} $ values for up and down variations of the systematic uncertainty. The MC statistical uncertainties are omitted here. 
Tables  
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Table 1:
The signal event yields in data and MC simulations after the likelihood fit for each of the 12 channels ($ \mu $+jets, e+jets, 3 years; and two mass regions). The uncertainties in the MC predictions include both statistical and systematic components. 
png pdf 
Table 2:
Measured unfolded charge asymmetry in the fiducial phase space (upper rows) and the full phase space (lower rows) shown for individual channels compared with the theoretical prediction from MC. Results are shown for events with $ {M_{{\mathrm{t} \mathrm{\bar{t}}}}} > $ 750 GeV and for two invariant mass ranges, 750900 and ${>}$900 GeV. The statistical (stat) and systematic (syst) uncertainties in the data, the MC statistical uncertainty (MC stat), and the total uncertainty in the measured values (Total) are also shown. All values are in percent. 
Summary 
A measurement of the charge asymmetry in $\mathrm{t\bar{t}}$ events with highly boosted top quarks produced in protonproton collisions at $ \sqrt{s}= $ 13 TeV is presented based on 138 fb$ ^{1} $ of data collected by the CMS experiment at the LHC. The selection is optimized for top quarks produced with high Lorentz boosts that yield collimated decay products that are partially or fully merged and can result in nonisolated leptons and overlapping jets. The measured top quark charge asymmetry ($ A_{\text{C}} $) is corrected for detector and acceptance effects using a binned maximum likelihood fit. This is the first CMS measurement to use 13 TeV data and a binned maximum likelihood unfolding technique to measure $ A_{\text{C}} $ directly at parton level in the full phase space. In addition, it is the first result that focuses exclusively on the highly Lorentzboosted regime, using dedicated reconstruction techniques for the hadronically and leptonically decaying top quarks at both the trigger and offline stages. Since the relative contribution of valence quarks increases at high momentum transfer, $ A_{\text{C}} $ is especially sensitive to beyond the standard model processes in this highly boosted phase space. The resulting unfolded charge asymmetry for $\mathrm{t\bar{t}}$ events with invariant masses satisfying $ M_{\mathrm{t\bar{t}}} > $ 750 GeV is (0.42$_{ 0.69}^{+ 0.64}$)%, where the uncertainty includes both statistical and systematic components. The corresponding theoretical prediction at nexttonexttoleading order in QCD perturbation theory with nexttoleadingorder electroweak corrections from Ref. [5] is (0.94 $ ^{+ 0.05}_{0.07} $)%. Good agreement between the measurement and the most precise standard model prediction is thus observed. The result demonstrates that top quark properties can be precisely measured in the highly boosted topology. 
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