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| CMS-TOP-15-009 ; CERN-EP-2016-059 | ||
| Measurements of $\mathrm{ t \bar{t} }$ charge asymmetry using dilepton final states in pp collisions at $ \sqrt{s} = $ 8 TeV | ||
| CMS Collaboration | ||
| 20 March 2016 | ||
| Phys. Lett. B 760 (2016) 365 | ||
| Abstract: The charge asymmetry in $\mathrm{ t \bar{t} }$ events is measured using dilepton final states produced in pp collisions at the LHC at $ \sqrt{s} = $ 8 TeV. The data sample, collected with the CMS detector, corresponds to an integrated luminosity of 19.5 fb$^{-1}$. The measurements are performed using events with two oppositely charged leptons (electrons or muons) and two or more jets, where at least one of the jets is identified as originating from a bottom quark. The charge asymmetry is measured from differences in kinematic distributions, unfolded to the parton level, of positively and negatively charged top quarks and leptons. The $\mathrm{ t \bar{t} }$ and leptonic inclusive charge asymmetries are found to be 0.011 $\pm$ 0.011(stat) $\pm$ 0.007(syst) and 0.003 $\pm$ 0.006 (stat) $\pm$ 0.003(syst), respectively. These results, as well as charge asymmetry measurements made as a function of the invariant mass, rapidity, and transverse momentum of the $\mathrm{ t \bar{t} }$ system, are in agreement with predictions of the standard model. | ||
| Links: e-print arXiv:1603.06221 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Background-subtracted and unfolded distributions of $\Delta {| y_{\mathrm{ t } } | }$ (left) and $\Delta {| \eta _{\ell } | }$ (right) from data (points), normalized to unit area. Parton-level predictions from the mc@nlo simulation and calculations at NLO (QCD+EW) [12] are shown by dashed and solid histograms, respectively. The ratio of the measured bin values to the mc@nlo prediction is shown in the bottom panel. The vertical bars show the total uncertainty, the statistical component of which is marked by a horizontal tick. The first and last bins of each plot include underflow and overflow events, respectively. |
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Figure 1-a:
Background-subtracted and unfolded distribution of $\Delta {| y_{\mathrm{ t } } | }$ from data (points), normalized to unit area. Parton-level predictions from the mc@nlo simulation and calculations at NLO (QCD+EW) [12] are shown by dashed and solid histograms, respectively. The ratio of the measured bin values to the mc@nlo prediction is shown in the bottom panel. The vertical bars show the total uncertainty, the statistical component of which is marked by a horizontal tick. The first and last bins include underflow and overflow events, respectively. |
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Figure 1-b:
Background-subtracted and unfolded distribution of $\Delta {| \eta _{\ell } | }$ from data (points), normalized to unit area. Parton-level predictions from the mc@nlo simulation and calculations at NLO (QCD+EW) [12] are shown by dashed and solid histograms, respectively. The ratio of the measured bin values to the mc@nlo prediction is shown in the bottom panel. The vertical bars show the total uncertainty, the statistical component of which is marked by a horizontal tick. The first and last bins include underflow and overflow events, respectively. |
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Figure 2:
Dependence of the ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ and leptonic charge asymmetries $A_{\mathrm {C}}$ (left) and $A^{\text {lep}}_{\mathrm {C}}$ (right) obtained from the unfolded distributions in data (points) on $M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }$ (upper), $ {| y_{{\mathrm{ t } {}\mathrm{ \bar{t} } } } | }$ (middle), and $ {p_{\mathrm {T}}} ^{{\mathrm{ t } {}\mathrm{ \bar{t} } } }$ (lower). Parton-level predictions from the mc@nlo simulation and calculations at NLO (QCD+EW) [12] are shown by dashed and solid histograms, respectively. The vertical bars show the total uncertainty, the statistical component of which is marked by a horizontal tick. The last bin of each plot includes overflow events. |
png pdf |
Figure 2-a:
Dependence of the ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ and leptonic charge asymmetry $A_{\mathrm {C}}$ obtained from the unfolded distributions in data (points) on $M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }$. Parton-level predictions from the mc@nlo simulation and calculations at NLO (QCD+EW) [12] are shown by dashed and solid histograms, respectively. The vertical bars show the total uncertainty, the statistical component of which is marked by a horizontal tick. The last bin includes overflow events. |
png pdf |
Figure 2-b:
Dependence of the ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ and leptonic charge asymmetry $A^{\text {lep}}_{\mathrm {C}}$ obtained from the unfolded distributions in data (points) on $M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }$. Parton-level predictions from the mc@nlo simulation and calculations at NLO (QCD+EW) [12] are shown by dashed and solid histograms, respectively. The vertical bars show the total uncertainty, the statistical component of which is marked by a horizontal tick. The last bin includes overflow events. |
png pdf |
Figure 2-c:
Dependence of the ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ and leptonic charge asymmetry $A_{\mathrm {C}}$ obtained from the unfolded distributions in data (points) on $ {| y_{{\mathrm{ t } {}\mathrm{ \bar{t} } } } | }$. Parton-level predictions from the mc@nlo simulation and calculations at NLO (QCD+EW) [12] are shown by dashed and solid histograms, respectively. The vertical bars show the total uncertainty, the statistical component of which is marked by a horizontal tick. The last bin includes overflow events. |
png pdf |
Figure 2-d:
Dependence of the ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ and leptonic charge asymmetry $A^{\text {lep}}_{\mathrm {C}}$ obtained from the unfolded distributions in data (points) on $ {| y_{{\mathrm{ t } {}\mathrm{ \bar{t} } } } | }$. Parton-level predictions from the mc@nlo simulation and calculations at NLO (QCD+EW) [12] are shown by dashed and solid histograms, respectively. The vertical bars show the total uncertainty, the statistical component of which is marked by a horizontal tick. The last bin includes overflow events. |
png pdf |
Figure 2-e:
Dependence of the ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ and leptonic charge asymmetry $A_{\mathrm {C}}$ obtained from the unfolded distributions in data (points) on $ {p_{\mathrm {T}}} ^{{\mathrm{ t } {}\mathrm{ \bar{t} } } }$. Parton-level predictions from the mc@nlo simulation and calculations at NLO (QCD+EW) [12] are shown by dashed and solid histograms, respectively. The vertical bars show the total uncertainty, the statistical component of which is marked by a horizontal tick. The last bin includes overflow events. |
png pdf |
Figure 2-f:
Dependence of the ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ and leptonic charge asymmetry $A^{\text {lep}}_{\mathrm {C}}$ obtained from the unfolded distributions in data (points) on $ {p_{\mathrm {T}}} ^{{\mathrm{ t } {}\mathrm{ \bar{t} } } }$. Parton-level predictions from the mc@nlo simulation and calculations at NLO (QCD+EW) [12] are shown by dashed and solid histograms, respectively. The vertical bars show the total uncertainty, the statistical component of which is marked by a horizontal tick. The last bin includes overflow events. |
| Tables | |
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Table 1:
Systematic uncertainties in the inclusive values of the charge asymmetries obtained from the unfolded distributions. Uncertainties of less than 0.0005 are marked by a dash (-). |
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Table 2:
The inclusive charge asymmetry measurements obtained from the unfolded distributions and the parton-level predictions from the mc@nlo simulation and calculations at NLO (QCD+EW) [12]. For the data, the first uncertainty is statistical and the second is systematic. The uncertainties in the mc@nlo results are statistical and the uncertainties in the NLO calculations come from varying together $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ up and down by a factor of two. |
| Summary |
| Measurements are presented of the charge asymmetry in $\mathrm{ t \bar{t} }$ dilepton final states from distributions, unfolded to the parton level, of the absolute rapidity (pseudorapidity) difference of top quarks (leptons) with positive and negative charge. The data sample corresponds to an integrated luminosity of 19.5 fb$^{-1}$ from pp collisions at $\sqrt{s}=$ 8 TeV, collected by the CMS experiment at the LHC. The $\mathrm{ t \bar{t} }$ and leptonic inclusive charge asymmetries are found to be, respectively, 0.011 $\pm$ 0.011(stat) $\pm$ 0.007 (syst) and 0.003 $\pm$ 0.006 (stat) $\pm$ 0.003 (syst) when measured inclusively. The charge asymmetries are also measured as a function of the invariant mass, absolute rapidity, and transverse momentum of the $\mathrm{ t \bar{t} }$ system in the laboratory frame. Although statistically limited, all measurements are in agreement with the standard model predictions. Future measurements at $\sqrt{s}=$ 13 TeV with larger data sets are expected to have better statistical precision outweighing the dilution of the charge asymmetry from the decreased fraction of events with the quark-antiquark initial state. |
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